LIGHT

Revised by Jake Holley, with additional content on design optimization by Joey Ereñeta

Proper lighting is the foundation for productive and healthy plants. Light is important for providing plants with the energy needed for growth while also signaling the way they grow. Many different light sources exist, and properly choosing a source and using light to the best potential is a process that is evolving rapidly. This chapter covers all aspects of light: why it is important and how to make educated and calculated decisions and recommendations about how to purchase and use it.

Properties of Light

Plants use light for different purposes, including growth pattern (photomorphogenesis), growth direction (phototropism), response to day length (photoperiodism), and photosynthesis. Each of these processes significantly affects the growth and productivity of plants.

• Photomorphogenesis is a plant’s growth patterns in response to light, including leaf size, thickness, distance between leaves (internode length), and branching patterns.
• Phototropism describes the preference of a plant to grow and align leaves and chloroplasts to best use light.
• Photoperiodism is the response of plants to day length, including control of flowering (Gupta 2017).

The most amazing thing plants do with light is photosynthesis, the process that provides the foundation for most of life on Earth. Plants use photosynthesis to power the process of making sugar (C₆H₁₂O₆) from water (H₂O) and carbon dioxide (CO₂). Plants also use photosynthesis to convert the sugars they make into starches and then into complex molecules such as cellulose. Add some nitrogen atoms, and the result is nucleic acids and amino acids, the building blocks of all proteins.

Light Attributes

The three attributes of light are quantity, duration, and quality. Light is both a particle and a wave, and will be referred to in both ways in this chapter.

• Quantity of light is measured in particles and is another way to describe intensity or brightness of light. Providing plants with bright light is critical for healthy plant growth.
• Duration of light is the length of time plants experience light. Light duration is important for allowing the plant to both adequately photosynthesize and signal the plant to flower or stay vegetative.
• Quality, described as a wave, refers to the color of light and can be described by its spectrum. The spectrum of light given to a plant makes a large difference and is an important consideration when purchasing grow lights.

Light Quantity

There are two important ways to measure the quantity of light: instantaneous, which measures how much light is coming in at any given second, and cumulative, or the total amount over the course of a day.

There are several methods to measure instantaneous light, each designed to best describe a different aspect of light. Candela, lumens, lux, and foot-candles all describe how humans perceive the brightness of light. Watts per meter squared (watts-m^-2) measures the total energy of light.

The most accurate way to describe light for plants is the number, or quanta, of photons. The unit of measurement for this is micromoles per meter squared per second (µmol·m^–2·s^–1). An important aspect to note is that these units cannot be easily converted between one another, especially with different spectra of light. For example, red and blue LEDs will have very low values in lux or foot-candles, since they do not contain green light, but they are much higher in µmol·m^–2·s^–1 and can be effective for plant growth.

Candela, Lumens, Foot Candle & Lux

The candela (or candle, or candle-power) is the most basic international unit for measuring emitted light and is defined as the illumination created by a common candle. Candela is calibrated for the perception of the human eye, being most sensitive in green and less so in red and blue.

Lumens are derived from this, as a single candela light source that radiates equally in all directions produces exactly 4π (12.6) lumens.

Foot-candles and lux describe the number of lumens per unit area, with foot-candles measuring in lumens per square foot and lux being the metric equivalent, lumens per square meter. These units of measurement are accurate in describing how bright a room or lit street will appear, but do not correlate well with how plants photosynthesize and grow.

Watts per Square Meter (watts/m²)

Watts/m² describes the total amount of light energy and is often used for weather and by the solar energy industry. Shorter wavelengths, like blue, contain higher energies per photon of light in comparison with longer wavelengths like red, and watts/m² accounts for this. Watts/m² is useful to determine how much the ground will heat up for weather prediction or how much power a solar panel will produce. However, since plants use red photons more efficiently in photosynthesis, this measurement is not as accurate for estimating plant growth as micromoles (µmol·m^–2·s^–1).

Micromoles (µmol·m^–2·s^–1)

The most accurate measurement of light in relation to its usefulness to plants is micromoles per meter squared per second (µmol·m^–2·s^–1), often simply referred to as “micromoles” of photosynthetically active radiation (PAR). PAR includes the wavelengths of light from 400-700 nm. Some researchers have proposed that since far-red light affects photosynthesis, PAR should also include wavelengths from 701-750.

Sunlight at noon on a clear early summer day produces about 2,000 µmol·m^–2·s^–1 (10,000 fc or about 110,000 lux) at latitude 39° north in Maryland, which lies in a middle latitude of the US. At higher latitudes, the light is weaker; at lower latitudes, it is stronger.

Photosynthetic photon flux (PPF) is the total number of photons (µmol·s^–1) produced by the lighting source. Photosynthetic photon flux density (PPFD) measures the density of the photons (µmol·m^–2·s^–1) from the light source falling on the canopy over a given time. This measurement most accurately describes light received by plants, since it weighs all photons of light in the PAR spectrum equally, unlike lux. As mentioned earlier, this will be important in later sections because LED lights emit in many wavelengths and cannot be accurately compared with other lighting sources using lux or foot-candle measurements. Since PAR is usually expressed in moles, which measure light quanta (photons), PAR is also called quantum light. Measuring quantum light is the only way to be certain that plants are getting all the usable light they need.

In addition to measurements of instantaneous light, the cumulative total amount of light is important, especially in outdoor or greenhouse settings, where the brightness of sunlight changes throughout the day and seasonally. For indoor setups where the light output is constant, knowing a lamp’s intensity at the canopy works well enough because the total for the lit period is easily calculated. But for outside or in greenhouses, an integrated light measurement that measures instantaneous light and adds it up throughout the day is more meaningful, and daily light integral (DLI/mol·m^–2·day^–1) is a common way to express this.

Daily Light Integral (DLI) mol·m^–2·day^–1

Daily light integral (DLI) is quantified in moles per square meter per day (moles·m^–2·day^–1), or moles for short, and is a summation of how much light a space receives in a day. To convert µmol·m^–2·s^–1 to daily light integrals in mol·m^–2·day^–1, multiply by the number of seconds per hour (3,600) and number of hours of light, and divide by one million. For outdoor measurements, use the average light intensity over the entire day.

For example, if plants are growing indoors with a light that produces 1,000 µmol·m^–2·s^–1 for 20 hours per day, that would be 1,000 × 3,600 (seconds in an hour) × 20 hours for 72 mol·m^–2·day^–1.

For a 12-hour photoperiod, this would be 43.2 mol·m^–2·day^–1.

The DLI for outdoor gardens varies considerably depending on latitude, season, and weather. For example, in the middle latitudes of the US a sunny summer day produces a DLI of roughly 26 moles per day. If it’s cloudy, the DLI drops to about 12 moles per day. In midwinter, a sunny day yields a DLI of approximately 9 moles per day, and cloudy conditions reduce that to a mere 3 moles.

Faust DLI maps

DLI changes by season. Fortunately, the changes in DLI by area have been quantified, and there is a great interactive map online to see how much light can be expected outside during any month at any location in the United States (endowment.org/dlimaps/) (Korczynski 2002; Faust and Logan 2018).

The DLI chart shows how much the moles per day change from August through October. In August, the DLI ranges from 35-40 mol·m^–2·day^–1 in the eastern US to 50-55 in the west. In October the patterns change between north and south. The southwest receives 35-40 mol·m^–2·day^–1, but the rest of the country, except for the far north, receives only 20-30 moles per day. In addition to the low light levels, the sun delivers very little UVB light by mid autumn.

Light Duration

The term for the total duration of light a plant experiences in a day is “photoperiod,” usually measured in hours. Photoperiod is important, as it both affects how much photosynthesis can be done by the plant and signals to the plant whether to flower.

DLI and Photoperiod

Duration is important to photosynthesis, as plants grow more with a longer photoperiod if the DLI is consistent (Koontz and Prince 1986). With an equal DLI, lower light for longer times is better than bright light for a shorter time. During vegetative growth, most cultivators use between 16 and 24 hours of light. The plants must not experience a long uninterrupted dark period.

This chart shows how much light plants receive daily over the course of a year in the continental United States. During the fall and winter, the amount of light is based mostly on latitude. Light levels drop dramatically in September, just as the plants are ripening. By October, light has dropped in half as compared with June. By December it is down by nearly two-thirds. During the summer light intensity varies more by longitude. For example, the western half of the US receives 50% more light than the eastern half.

These are tomato plants grown at various photoperiods with the same DLI. Labeled on each pot are two numbers, the photoperiod, ranging from 12 to 24 hours, and the DLI, being 20 across all plants. Data consistently showed an increase in biomass and growth up to 21 hours, then either flattening off or decreasing depending on species. Leafy greens like lettuce were not different between 21 and 24 hours, but plants like tomatoes, cucumbers, and petunias were significantly smaller under 24 hours of light compared with 12 hours. Photo: Jake Holley

Photoperiodism

The cannabis flowering cycle is induced by short days, but the term “short-day” is something of a misnomer: what cannabis needs to flower is a sufficiently long dark period.

Flower Initiation

Most cannabis cultivars (except autoflowering) flower only if they have a daily regimen of dark period that varies tremendously depending on variety. Setting the lights to a 12-hour light and 12-hour dark cycle is most commonly used to induce a strong flowering response. This time period is based on tradition; however, a shorter critical period of darkness can be used. When this critical dark period occurs for five to seven consecutive days, the plant changes its growth from vegetative to flowering.

Photoperiod Interruption

Cannabis is a “short-day plant.” It measures the number of hours of uninterrupted darkness to determine when to initiate flowering. It does this by constantly producing a group of light-sensitive proteins collectively called phytochrome. They are inactivated by red light, which the plants receive all day. When darkness occurs, they become active over about two hours or immediately in the presence of more far-red than red light, which occurs at sundown. When the level of phytochrome reaches a certain level, flowering is induced. Each cultivar has a critical period that is the minimum number of hours of uninterrupted darkness that it requires to flower.

Cannabis is sensitive to light during the dark period, and too much light exposure at any time during the flowering cycle negatively impacts, or even inhibits, flowering. Red light (660 nm) most effectively inhibits flowering.

A very low intensity of light, only two micromoles, is required for only a few seconds to interrupt the dark cycle. Think of a water spray that wets all the vegetation. That is the light “spray.” The entire plant must be “sprayed” with light, because the flowering reaction is localized. Parts of the plant that are not sprayed will flower, while the sprayed part will be inhibited.

Using this information, gardeners can manipulate when the plants flower outdoors as well as indoors. Indoors, the gardener turns the lights off and blocks all light from the flowering room.

Outdoors, gardeners can use light sprays to prevent flowering.

Light deprivation, covering the plants with an opaque covering so no light reaches them, is used to induce and maintain flowering during periods of short nights such as late spring and summer. (See Flowering.)

Beware of using green light: it is not as “safe” as often believed (Baskin and Baskin 1978). Although green light is not as powerful at inhibiting flowering as red, too much green light for too long of a duration does inhibit flowering (Park and Jeong 2019).

If there is a need to do any work or view plants during the dark period, it is important to be brief and to use the least amount of light possible. An occasional green light will not stop the plants from flowering for two reasons:

• The interruption of the dark period preventing flowering must take place each evening (Borthwick and Cathey 1962). An occasional light foray won’t stop the flowering process; however, it can reduce yield or slow down maturation time, producing fewer flowers and taking more time to ripen if it occurs frequently.
• Plants are much less sensitive to green than red light, so they won’t be affected as much.

Plants are much more affected by light interruption during the first few weeks of flowering than in later stages. Turning on lights during the last weeks of flowering will have far less effect on the process than light pollution during early flowering.

Light Quality

The quality of light is its color, or spectrum. Light spectrum is useful for directing the plant’s growth habits, as well as contributing to healthy photosynthesis. Comparing this to a car, if light intensity is the gas pedal to increase speed of photosynthesis, light quality would be the steering wheel. Using different colors of light allows growers to affect the plant in terms of yield, flavor, color, growth, flowering, and even the severity of pests and diseases (Davis and Burns 2016).

Light color, as described in this text, is part of the electromagnetic spectrum that lies between wavelengths of 280 and 800 nanometers (nm) and is influential for plant growth and development. In horticulture, the rainbow of colors is abbreviated to only a few: blue (400-499 nm), green (500-599 nm), and red (600-699 nm). White light is created by combining all wavelengths between 400 nm and 700 nm. Ultraviolet light is between 200 nm and 399 nm, with UVC (200 to 279 nm), UVB (280 to 315 nm), and UVA (315 to 399 nm). In addition, far-red light lies from 700 to 750 nm, although most far-red light sources emit at 730 nm (Both et al. 2017). The reality of defining light by color is far more complex, as wavelengths within a color (425 vs. 450 nm) can affect growth differently, and different wavelengths are also known to have synergistic effects with one another.

Photosynthetically Active Radiation (PAR)

Several defined regions of the spectrum and their associated acronyms are used to describe the wavelengths of light for plant growth, specifically photosynthetically active radiation. PAR defines light that plants can use to gain energy for photosynthesis, from 400 nm (blue) to 700 nm (red), although wavelengths of light up to 752 nm can still positively affect photosynthesis (Cathey 1980; Sager 1984; Hogewoning et al. 2012; Zhen et al. 2019). Light from 280-399 nm and 700-750 nm can elicit responses in plants, such as the production of terpenes and cannabinoids, or affect flowering, while not necessarily contributing to photosynthesis. These wavelengths are sometimes included in a wider spectral range than PAR, which is referred to as photo-biologically active radiation (PBAR) because the wavelengths can affect more than just photosynthesis. Understanding the differences in light spectra and the effect they have on plants can help the careful cultivator ensure that the plants are getting everything they need to thrive.

This chart represents relative absorbance of various plant pigments across the spectrum of photosynthetically active radiation. These pigments primarily absorb light in the blue and red spectra, however there is still absorbance in the green/yellow/orange spectra that also drives photosynthesis.

The McCree Curve depicts the photosynthetic quantum efficiency across the spectrum of photosynthetically active radiation. It shows a plant’s real world photosynthetic efficiencies. This curve provides insight when designing spectral output from artificial lighting. SOURCE: Smart Grow Technologies

Light Spectrum Effects

White (400-700 nm)

White light is a combination of all wavelengths of visible light, sometimes referred to as broad spectrum. The sun emits white light, and plants have evolved to use this light effectively. Since plants have evolved to use a broad spectrum of light, they require some variation in light color for healthy, optimal growth.

Red (600-699 nm)

Red light is the most effective light for promoting photosynthesis (McCree 1971). Plant lighting favoring high ratios of red light grows plants more effectively than other spectra of light; however, it is not an effective light to use by itself (Massa et al. 2008; Hernandez et al. 2016).

Far-Red (700-750 nm)

Far-red light has important implications for photosynthesis, photomorphogenesis, and flower induction in plants. Far-red light increases the photosynthetic rate of other light sources in a synergistic way that is more than the addition of these light sources independently, despite being outside the PAR range (Zhen and van Iersel 2017; Hogewoning et al. 2012). In addition, far-red light has also been shown to be beneficial for inducing flowering responses in short-day plants (Cathey and Borthwick 1957).

Red to Far-Red Ratio

One of the best-known spectral effects comes from the ratio of red to far-red light. A low ratio (i.e., high amounts of far-red light) causes plants to stretch and leaves to expand. Using incandescent bulbs for night interruption will cause plants to stretch for this reason. Having a high ratio of red to far-red light will help keep plants stout (Franklin 2008). Using far-red at the start of the dark period in an attempt to promote flowering causes stretching and taller plants (Lund 2007).

Phytochrome

The main active photoreceptors for red light are phytochromes. Phytochromes come in two interchangeable forms: “red absorbing” P_r and “far-red absorbing” P_fr. If P_r absorbs light, it will shift to P_fr and vice versa. Although P_r absorbs most strongly at 660 nm and P_fr absorbs most strongly at 730 nm, both states absorb some light anywhere from 300 nm to at least 750 nm (Butler et al. 1964; Stutte 2009). Phytochromes can be induced to P_fr with green and yellow wavelengths (Shinomura et al. 1996; Wang and Folta 2013). The balance of P_r to P_fr is known as the phytochrome photostationary state (PSS) and is altered anytime light is present. This is important for some of the differences in growth habits observed with different spectra. In addition to absorbing light, P_fr will naturally transition to P_r in darkness, and this forms the basis of dark periods being required for flowering (Borthwick and Cathey 1962).

Blue (400-499 nm)

Blue light has been shown to be critical for producing healthy plants, as chlorophyll development needs blue light and will not properly occur under only red light (Massa 2008). High ratios of blue light help keep plants stout (Hernandez 2016a, 2016b). In addition, blue light helps induce secondary metabolite production similar to having a higher light intensity (Hogewoning 2010). Although research has not been validated with cannabis, more blue light may increase THC and CBD production.

Cryptochromes, Phototropins & Light-Oxygen-Voltage Sensors

Blue light is effective for promoting photosynthesis while also activating a myriad of critical photoreceptors, including cryptochromes, phototropins, and light-oxygen-voltage sensors (LOV). These photoreceptors are important for chloroplast development, stomatal opening, circadian rhythm, and secondary metabolite production (Cashmore 1999; Ouzounis 2015; Pocock 2015; Kopsel et al. 2015).

Green (500-599 nm)

The perception and use of green light by plants is a commonly misunderstood topic. Photosynthesis driven by absorbing green light is almost as efficient as blue light and may in fact contribute more to full-plant photosynthesis, as it has greater leaf and canopy penetration than red and blue light (McCree 1972; Smith et al. 2017). There is even a green-light-specific photoreceptor theorized to exist (Pocock 2015; Mccoshum and Kiss 2011; Folta and Carvalho 2015; Zhang and Folta 2012). Green light is not wasted light, or invisible to plants, and plays an important role in both photosynthesis and signal transduction in plants.

Somewhere over the Rainbow: The Argument to Include Far-Red in PAR

The electromagnetic spectrum is the range of frequencies of electromagnetic radiation, from the high-energy, short-wavelength, ionizing radiation in gamma rays to low-energy, long-wavelength radio waves. There is a range in the spectrum where the wavelengths are between 400 and 700 nm that is especially important to plants. The radiation within this range is called photosynthetically active radiation (PAR). Radiation with a wavelength at 400 nm is blue light, whereas at 700 nm, it is red. Everything in between is part of the visible spectrum of light, seen when sunlight shines through mist or rain as a rainbow.

PAR is also the range of light that plants can absorb for photosynthesis; however, chlorophyll primarily absorbs red and blue light. Historically, chlorophyll’s absorption spectrum was measured by extracting chlorophyll from a leaf using specific solvents and then passing visible light through a solution of the extracted pigment. This does not mean that accessory pigments are not absorbing other colors in the PAR spectrum. Plants reflect a lot of green light, which is why leaves appear to be green, but they also utilize green light to drive photosynthesis quite efficiently (Terashim et al. 2009).

In 2020, Drs. Shuyang Zhen and Bruce Bugbee published two papers that suggest PAR may need to be extended to include more than just light within the 400-700 nm range (Zhen & Bugbee 2020). Far-red light (700-750 nm) is just above the PAR spectrum.

Historically, plant scientists have described far-red light as integral to regulating photoperiodism in plants such as cannabis, but not as part of the spectrum that drives photosynthesis. This new research suggests that this may not be the case. Drs. Zhen and Bugbee designed experiments to measure photosynthetic rates in various crops when provided with PAR or PAR with far-red light. The treatment with the far-red light produced an increase in photosynthesis equivalent to providing extra light in the 400-700 nm spectrum. Far-red light alone increased photosynthesis, but only minimally. This suggests that far-red light, when paired with light in the 400-700 nm range, is just as effective at driving photosynthesis as PAR.

Far-red light increases the rate of photosynthesis, but does that translate into increased yields? Drs. Zhen and Bugbee tested that as well with lettuce. They treated lettuce with lights within historic PAR range and then others with PAR plus far-red. The PAR plus far-red had 15% more far-red added and 15% PAR removed, so the treatments had the same light intensity. Treatment with far-red light increased net photosynthesis and carbon capture, and increased yield by 29-31%.

UVA & UVB

UVA light is at the wavelength of the invisible portion of emissions from blacklights. It helps reverse damage done to plant DNA by UVB light.

UVB light affects the potency of high-quality plants. The amount of cannabinoids and terpenes that a plant produces increases as it receives more UVB light. This light can be provided to indoor plants with proper lighting. Outdoors, the amount of UVB light is highest at the beginning of summer and begins to decline in late summer, depending on the latitude. By early fall, the amount is a fraction of summer levels. (For more information on using UVB light to increase potency, see Flowering.) In humans, UVB causes tanning and sunburn.

UVR8

UV sensing by plants is governed by the UVR8 photoreceptor. This receptor has been linked to the reactions seen with UV light, including pigmentation, leaf expansion, and tolerance to UV-B light (Jenkins 2014). Activation of UVR8 stimulates the production of terpenes, anthocyanin, and other flavonoids (Folta and Carvalho 2015).

The weather app on iPhones provides UV level readings on a scale of 1-11+.

Other Light-Capturing Plant Pigments

Chlorophyll

For photosynthesis, light energy is captured by chlorophylls A and B, primarily from the red and blue portion of the spectrum. Light absorption by chlorophyll A peaks at 430 nm in the blue band and 663 nm in the red, and chlorophyll B peaks at 453 nm in the blue and 642 nm in the orange-red bands. Total chlorophyll absorbance peaks at 435 nm and 445 nm in the blue spectrum and 640 and 675 nm in the red wavelengths. Although chlorophyll is not the only plant pigment that absorbs light to drive photosynthesis, it is the pigment most closely related to the plant’s photosystems. Thus, supplemental and indoor lighting designers have focused much of their energy on these specific wavelengths, which is why purple LEDs are abundant in the industry. This is not necessarily the best path, especially since there are other pigments that absorb light in different spectra and also drive photosynthesis.

Carotenoids

Chlorophyll is not the only light-sensitive part of the plant. Carotenoids are a group of orange pigments that capture light in the blue portion of the spectrum, at about 448 nm in the blue spectrum and 475 nm in the blue-green range. Carotenoids not only contribute to photosynthesis but also protect the chlorophyll from excess light that could have destructive effects.

When a photon is absorbed by a chlorophyll, carotenoid, or another light-harvesting molecule, its energy is transferred to an electron and it ceases to exist. Several things can happen to this excited electron. The energy absorbed can be lost either as heat, when the excited electron drops back to its ground state, or as light, causing fluorescence at longer wavelengths. Most important, it can be transferred to another molecule nearby, causing a chemical reaction that results in photosynthesis.

Other Pigments Related to Light

Anthocyanin and other flavonoid pigments absorb blue and UV light to protect chlorophyll from photo-destruction. Another pigment that appears to play a role in plant health is xanthophyll. This yellow pigment captures light in the range from 400-530 nm, violet (400-450 nm), blue (450-490 nm), and cyan (490-520 nm), but is usually hidden from human view by the green of chlorophyll. If a leaf loses its chlorophyll—because of a nitrogen deficiency, for instance—xanthophyll’s bright yellow color becomes apparent.

Xanthophyll has several functions. First, it acts as a light and heat regulator. At dawn, it is in its low-energy form, violaxanthin, which has peak reactions to light at 480 and 648 nm. As the light increases to levels that might hurt the thylakoids and lead to photo-oxidation of the chlorophyll molecules, violaxanthin siphons off the photons, excess energy of using them to create its high-energy form, zeaxanthin. When light intensity decreases, the zeaxanthin returns to its low-energy state, violaxanthin, in a cycle that can take anywhere from a few minutes to several hours.

These chemical processes enable plants to cool themselves during lighted periods and stay warm during cool nights. Plants bank energy during the day and release it at night by shifting xanthophyll to its low-energy form, releasing heat. During the day, some of the light energy may also be transferred to chlorophyll by releasing an electron to be used for photosynthesis. Other plant pigments also gather energy from spectrums not used by chlorophyll. Neoxanthin, lutein, and zeaxanthin each transfer more than half the energy they gather to chlorophyll.

Light Meters

Most growers have not used a light meter because they have grown great gardens with excellent buds just by calculating the wattage input of the lights used. However, light meters are used to identify “hot spots” and dimmer areas of the garden, providing readings on how much light the plants are receiving and have received cumulatively. Light meters are well worth the investment because the information provided can be used to determine “hot” or dim spots so that the space is lit evenly, to control supplemental lighting, and to use energy most efficiently. Some models use phone apps. Rather than estimate the light reaching the garden, the meters provide an accurate reading.

A light “hot spot” is a localized area in the plant canopy that is getting significantly higher light intensity than surrounding areas. This is especially undesirable in large canopies, as it disrupts canopy uniformity.

Purchasing a Quantum Sensor

Many different tools can be used to measure light, but quantum sensors that accurately measure light can be difficult to find. Although quantum sensors can be more expensive than other lux and foot-candle meters, they are the most accurate and reliable tool to use to ensure that the plants are getting enough light and that it is evenly distributed. The main companies producing quantum sensors are LI-COR and Apogee. LI-COR is known for its accuracy and has an unrivaled history of producing quality plant-measuring tools, but it’s more expensive and less user-friendly than Apogee. Apogee Full Spectrum quantum sensors are an excellent alternative that can interface with laptop computers easily via USB and free software, and can be used to log data without any additional equipment. In addition, they are easy to recalibrate using the Clear Sky Calculator website, ensuring years of quality use. For serious growers, these are a must-have for determining the life of fixtures and checking for dead spots. The record-keeping capability can ensure that the timers are functioning properly.

Apogee’s MQ-500 is a full-spectrum Quantum PAR meter that accurately measures the photosynthetically active radiation of all grow lights, including LEDs, with a spectral range of 389 to 692 nm ± 5 nm.

Properly Lighting Plants

Cultivar Light Recommendations

Proper plant light is based on the variety being cultivated. Here are the general lighting requirements on different lines of cultivars.

Sativas require the most light because they evolved near the equator, between the 30th parallels, and are adapted to long periods of intense sun. They are followed by sativa-indica hybrids, indica-sativa hybrids, then indicas. Sativa-indica hybrids need less intense light than sativas, but still do best in more intense light. Indica-sativa hybrids are more light forgiving than sativa-based plants. They can function in the mid-to-low light range. Indicas need the least intense light of any of the varieties. They evolved at the 35th parallel in the Tibetan plateau and are the best bet for lower-light gardens.

Minimum Light Requirements for Indoor Cultivation

During the vegetative growth cycle, most varieties will do well with a minimum of 400-500 µmol·m^–2·s^–1 (2,000-2,500 fc, 21,500–27,000 lux), although, with long-length photoperiods, the plants can efficiently use 1,000 µmol·m^–2·s^–1 (5,000fc, 54,000 lux) or more. The more total light (DLI) they receive during vegetative growth, the faster their growth and the sturdier their stems. When grown under low light, under a leafy canopy, or when shaded by trees or other tall plants, all varieties develop long internodes (spaces on the stem between the leaves) due to the enhanced far-red light. Plants with equatorial genetics are more affected by this.

Apogee’s DLI-500 can spot-measure PAR levels and also log and display the daily light integral for the current day and last seven days. It also records and displays the photoperiod in hours of light that the area received each day.

During the flowering cycle, equatorial sativas need intense light and a minimum of 800-1,000 µmol·m^–2·s^–1. With less light, the buds will be loose and lanky. Sativa-dominant hybrids require bright light. They will produce luscious buds when illuminated with as little as 700-800 µmol·m^–2·s^–1. Indica-dominant hybrids require less light and can produce well using a minimum of about 600-700 µmol·m^–2·s-1. Indicas need the least amount of light to thrive. Some indicas produce well starting at about 550 µmol·m^–2·s-1, though others need higher levels to produce nice, tight buds. With more light (800 µmol·m^–2·s^–1), the indica and indica-dominant hybrid buds will be larger, tighter, and more potent.

Outdoor Growing

Why Use the Sun?

The best source of light is the sun. It requires no electricity or expense. During the summer, the sun is brighter during parts of the day than indoor lighting and is self-regulating. Outdoors, on a clear day at the beginning of summer, when sunlight hits Earth at the most direct angle, the light’s intensity can reach upward of 2,000 µmol·m^–2·s^–1 (15,000 fc, or 161,000 lux) at noon, the brightest part of the day. Plants may not be able to process all the light at its peak, since cannabis plants are probably not able to use more than 1,500 µmol·m^–2·s^–1 (7,000-7,500 fc, or 75,000-80,000 lux). Only about 50% of sunlight is PAR (Boyle 2004). The rest of the light spectrum is not used by plants. The excess light is converted into heat and then dissipated through transpiration, reradiated as infrared heat, or dissipated using biochemical processes.

Cannabis plants do best under full light all day. Gardeners can use the sun as the primary source of light if they have a garden, greenhouse, terrace, patio, roof, skylights, or even a directly lit window. Bright spaces that are lit from unobstructed sunlight at least five hours a day usually need no supplemental light during the summer.

Autumn light can be more problematic. If the garden continues to receive direct sunlight, there is usually enough light for the buds to mature. However, if the direction of the light changes in the fall so that the plants get little direct sun, they will need artificial light to supplement the weak sunlight, overcast conditions, and oblique angles that create shadows. Without the additional light, buds do not develop properly. They grow loose and airy and are not particularly potent. Natural light can be supplemented using the same kinds of lights used for indoor production.

Checking Adequate Sunlight

To find out exactly how much light plants are getting, use a light meter. If the garden receives a minimum of 900 µmol·m^–2·s^–1 (4,500 fc, or 48,375 lux) of light for five hours or more, the space is bright enough for a moderate harvest. Lower light levels result in less growth, slower ripening, and lower yield and quality. The converse is also true.

Cultivar Choice

In higher latitudes, plants must be harvested early in the fall to accommodate climatic conditions. This is due to the natural lengthening of darkness to the threshold to induce flowering. Unfortunately during autumn, when the plants are finishing flower growth and ripening, both the length and intensity of light diminishes, which reduces yield potential. There are three possible solutions: grow early-maturing or autoflowering plants, force the plants to flower early, or supplement the sunlight with electric lighting.

Supplementing Natural Light in Autumn

One rule of thumb is to supplement autumn’s low light levels to a minimum of 900 µmol·m^–2·s^–1 for at least five hours a day during daylight. White LEDs or CMH lamps can be used safely. Both light sources supply heat to the plants, which can be helpful in autumn.

Greenhouses

Greenhouses provide a middle ground between outdoor and indoor production. Many allow for year-round production in any climate, yet take advantage of natural sunlight. Others extend the growing season. This is extremely beneficial, as one of the largest single costs of indoor production is electricity to run lights. Greenhouses are still affected by season, and proper lighting controls should be used to both reduce electricity consumption and produce consistent, quality products.

Greenhouses benefit from three systems to control light: blackout curtains, shade cloth, and supplemental lighting.

Blackout curtains are opaque curtains used to block all light to allow for the necessary dark period for flowering during the long days of late spring to late summer. Blackout curtains can be pulled manually or using an automated, motorized system. They are closed and opened at the same time each day to block natural sunlight and lengthen the dark period.

Shade curtains are necessary with crops that can be scorched by sunlight. Although cannabis grows well under full sun, shade cloth can be used to acclimate young plants started indoors and to reduce temperature in summer.

With cannabis they are used only on the hottest days, when other systems are inadequate to handle the heat load. Supplemental light using LEDs or high intensity discharge (HID) lamps such as high pressure sodium (HPS) and ceramic metal halide (CMH) is necessary for consistent, year-round production in all climates. They can be used to supplement natural sunlight on cloudy days or seasonally. They can also be used to break up the dark period to delay flowering during long nights.

The most sophisticated greenhouses contain all three systems working together to produce the most consistent crops.

Light inside a Greenhouse

Despite being covered in glass or other clear glazing materials, greenhouses reduce the amount of sunlight reaching crops anywhere between 10 and 50%. Even if the outdoor sunlight provides a DLI of 60 mol·m^–2·day^–1, inside the greenhouse may only receive a DLI of 30 mol·m^–2·day^–1. During winter months, especially at higher latitudes, supplemental lighting will be necessary for healthy plant growth.

Proper Lighting Intensity

Supplemental lighting has been common in vegetable and floriculture greenhouses for several decades. Typical intensities of supplemental lighting in these greenhouses are between 100 and 200 µmol·m^–2·s^–1. In greenhouses growing cannabis, the most important time to provide supplemental light is during flowering, which is often limited to just a 12-hour photoperiod. This has pushed many growers to install lighting intensities of 300 to 400+ µmol·m^–2·s^–1, which provides an additional 13-17 mol·m^–2·day^–1 of light. This is especially important in higher latitudes, where natural lighting may provide only 5-10 mol·m^–2·day^–1 of light on the darkest days of winter. Since lights are one of the largest purchases a greenhouse owner will make, it is important to understand the climate, electrical costs, and expectations of production to make the best decision on light intensity.

Seasonal & Daily Differences

Perhaps the largest challenge to greenhouse lighting is the variable nature of sunlight. Seasonally, the amount of sunlight increases during summer, with longer days where the sun is higher in the sky, while the winter experiences short days, with the sun’s zenith much lower. Seasonal impacts should not be underestimated. During the winter or on cloudy days, DLI values can be less than half, compared with a sunny summer day. This is especially true in higher latitudes such as Canada, Tasmania, and New Zealand.

In addition, weather affects light intensity on any given day, often more so than seasons. Days with complete cloud cover in the summer often have lower DLIs than bright sunny days in the winter. With the goal of consistent production, greenhouses need to be equipped with a light control system that measures and responds to light-intensity changes throughout each day.

Clear Sky Calculator

Clear Sky Calculator (clearskycalculator.com) is a website used to determine the light intensity at any given time in any region, given the skies are cloud free. This is a great tool to see how much light can be expected on a sunny day or to calibrate light meters to ensure more accurate measurements.

Controlling Greenhouse Lighting

Greenhouses are best maintained through control systems to manage all aspects of the growing environment. Lighting control systems come in three forms: time clock, threshold control, and sophisticated computer-controlled systems.

Time clock systems run the same as in indoor production: circuits are wired to be powered on and off based on an internal clock. Lights turn on and off at the same time every day. These systems are good to have as a backup when plants are flowering, but they may not take into account season or weather and need to be adjusted based on the time of year and daily sunlight to properly light plants without wasting power. More sophisticated controllers use sensors to control lighting. If not automated, timers must be adjusted manually to prevent both overlighting and underlighting.

Threshold control offers a step up in control in comparison with time clock systems. Threshold incorporates timer control to set a photoperiod while monitoring the incidental sunlight to turn lights on and off. If sunlight is below a preset intensity decided by the grower, lights turn on; if sunlight is above that intensity, lights turn off. This is a step above time clock systems because lights automatically turn on and off based on the current weather. Better threshold systems track DLI through changing seasons and weather conditions.

There are many greenhouse control systems available that maintain a consistent DLI within set photoperiods. Some also control CO₂ and take electric rates into account to provide the cheapest and most consistent growing environment possible given seasonal variability and weather. Some computer-controlled systems have the goal of never needing to be adjusted, making them “set it and forget it” tools.

The level of lighting and control utilized depends on the location, size, and level of investment of the greenhouse. Small, backyard greenhouses can be run automatically with lights on a timer, an air conditioner or ventilation system, heater, humidifier, and dehumidifier, all set by thresholds, and watered using either automated irrigation or a passive system. Adding a blackout curtain and shade cloth increases the flexibility of the greenhouse.

Using Artificial Light in Windows & Terraces

During autumn, gardens in windows and on terraces may receive direct light, as the light comes from a more oblique angle. Sometimes window spaces that are shaded in the summer get direct light in the fall as the sun’s angle changes seasonally. If the plants receive direct sunlight, they are probably getting enough light. If plants get only indirect light, even bright light, they require supplemental lighting. Using artificial lights to supply light to plants on a patio or terrace, or in a greenhouse or a window, need not cause suspicion. Use a ceramic metal halide (CMH) lamp or white LED, which both emit a clear light that blends in with natural light. High-powered LED security lights may be a solution for backyard gardens.

Indoors

Plants in indoor gardens require very bright light to grow well and yield a good crop. However, cultivars differ in the amount of light they require to support fast growth and high-performance flower development.

Gardeners have a wide selection of lights to choose from. These include LEDs, fluorescents, ceramic metal halide, and high-pressure sodium lamps. Growers rarely use incandescent or quartz halogen lights. These lamps are inefficient, converting only about 10 to 20% of the energy they use to light while wasting the rest by creating heat.

The recommendation table and next section help a grower decide how many lights are needed to grow an average hybrid indoors. Once the lights are set up and running on timers, no more thought may be needed for indoor gardens in terms of lighting.

Cultivar-Specific DLI Requirements

Optimal PPFD and DLI requirements vary by cultivar and stage of development. Despite evolutionary adaptations to high- or low-light environments, most hybrid cultivars grown indoors under consistent light intensity optimize illumination around 1,000 PPFD (µmol m^–2 s^–1).

Excessive light saturation can lead to photo-inhibition (reduced photosynthesis) and photo-oxidative stress, which causes damage to chloroplasts, plant pigments, lipids, proteins, and nucleic acids, and can lead to suboptimal CO₂ assimilation rates (Pintó-Marijuan and Munné-Bosch 2014).

Determining the optimal light levels for each cultivar may require some trials. Start with the genetic lineage of each variety and then examine its phenotypical and morphological expressions. Adjust light levels and monitor for growth response and any signs of stress, including chlorosis or leaf burn.

Most commercial cultivars are highly hybridized, so their “native” genetic lineage is less indicative of their light needs than morphological growth traits. Wide-leaf varieties tend to have darker green coloration due to a higher concentration of chlorophyll. These traits indicate a higher photosynthetic efficiency and lower PPFD and DLI requirements. Thin-leaf varieties tend to have a lighter green coloration, indicating lower chlorophyll density. This may lead to lower photosynthetic efficiency and higher PPFD and DLI requirements.

Optimal Indoor Light Conditions

This is an example of how PPFD and DLI might be adjusted weekly throughout the life cycle of many hybrid cultivars. Other light formulas vary from these recommendations. These higher levels of illumination require all other conditions for growth to be optimized. (See The Limiting Factors.) These light intensities can be achieved by using dimmable LEDs. HID lights have deleterious spectrum changes when dimmed.

Lighting Design

To calculate lighting design, the light-intensity goal (PPFD) is determined based on the DLI and photoperiod requirements of the cultivar in each cultivation zone. The amount of canopy to be illuminated determines the number and strength of lights needed to achieve this goal based on the lamps’ wattage and efficacy.

Each lamp’s or fixture’s horticultural light intensity is rated in photosynthetic photon flux (PPF), or the total number of PAR photons emitted by the fixture. When the concentration of PAR photons from the lights is measured in a certain area of the canopy, lighting can be optimized based on the PPFD goal.

By adjusting the height and spacing of the lights, the PPFD is adjusted, because the photons are spread apart or concentrated, decreasing or increasing the density of the finite number of photons received by the plant canopy. Once the light spacing and height is established, light controllers are used to dim or ramp up the light intensity for the particular cultivars in each zone.

The height of the lights has less impact in larger spaces. In a large space with good light reflectors, virtually all the light gets to the garden. Although the density from an individual light decreases over a given area, as the height increases, the cumulative density of all the lights remains the same because almost all the photons reach the canopy. The cross illumination achieved with higher mounting height also creates a more uniform light intensity across the canopy, minimizing illumination “peaks and valleys.” This is why it is common for large grow rooms to utilize a high-bay lighting design. In smaller gardens more of the light strays outside the garden’s perimeter.

Unless the light is directed back by reflective material, it is lost to the canopy.

Different lighting technologies have different efficiencies at converting electricity (electrons) into light (photons). Calculating the power draw of the fixture (wattage) versus the output of the lamp (PPF) is essential to choosing the right light. Using the PPF rating of the lights, the number of lamps required and their configuration can be determined.

The PPFD goal for each cultivation zone is dictated by the DLI requirements of the cultivars being grown, the stage of growth, and photoperiod provided.

Area is calculated by multiplying length and width of the space. For example, in a cultivation zone with dimensions of 12 by 8 feet (3.6 × 2.4 m), the total area of the canopy is 96 square feet (9 m²).

12 × 8 feet (3.6 × 2.4 m) = 96 square feet (9 m²) = total area

It will vary by application and cultivar, but the goal is to provide approximately 1,000 µmol m^–2 s^–1 PPFD of lighting in a flowering room. To determine the total lighting (PPF) requirements for the space, the total canopy square footage is converted to square meters and multiplied by the PPFD target.

9 m² × ~1,000 µmol m^–2 s^–1 = ~9,000 µmol s^–1 = approximate lighting goal in PPF

Lighting fixtures are rated in PPF. After calculating the total PPF lighting needs for the canopy (~9,000 µmol s^–1), choose from available lighting options to create a configuration that will achieve or exceed the light-density goal of the cultivars in that zone, and then spread the lights evenly over that canopy area.

In this example, six 631-watt LED fixtures rated at a PPF of 1,550 µmol s^–1 would provide ~9,300 µmol s^–1. Or, four double-ended HPS fixtures (1150 watts), each rated at a PPF of 2,400 µmol s^–1, provide ~9,600 µmol s^–1. This slightly exceeds the calculated PPF target. The LEDs can be adjusted down using a light control module. The HPS lamps’ spectrum will change adversely when they are dimmed. Placing the six LED fixtures in two rows of three lights per row, or the HPS fixtures in two rows of two lights per row, will provide the best coverage across the 12 by 8 foot (2.6 × 2.4 m) canopy, slightly exceeding the 1,000 µmol m^-2 s^-1 PPFD target.

This approach can be used to measure and convert an area of any size of plant canopy into a lighting goal. Use the highest-desired PPFD, measure the canopy to be illuminated, and calculate the total PPF required for that space. Then, using the rated intensity output (PPF) of various lighting solutions, create a lighting design (lighting fixture type and how many rows/lights per row) that will properly and evenly illuminate the canopy. It doesn’t always work out perfectly, so it is common to slightly oversize canopy illumination potential. With LEDs, use light controllers to adjust intensity based on the plants’ needs. With HIDs, adjusting light is more complex. With high bays, it is best to shut off a percentage of lights instead of dimming. Another solution is to give the plants a little more light than originally planned.

Choosing a Light

All electrical work should be permitted and done only by licensed electricians. Input voltage (volts), current (amperage, or amps for short), and power consumption (wattage) are important factors for selecting a light source. Most household voltages operate around 120 volts; however, there are often outlets to 240 volts for appliances that require more power. Greenhouses and industrial warehouses may have access to 277 or higher voltage. The advantage of higher voltages is better electrical efficiency. When buying lights, ensure the voltage matches the available power service, as lights manufactured in Europe or Japan may not run using lower than 200 volts. (See Electricity Basics.)

The amperage of lights is an important factor to consider when determining how to set up multiple lights. For example, many home circuit breakers operate on a 20-amp limit on a 120-volt circuit. For safety consideration, the circuit should only be run continuously at 80% capacity, meaning only 16 amps of power should be run. This places the safe limit on a single circuit at 1,920 watts, which wouldn’t even allow for two 1,000-watt fixtures. As mentioned before, if the lamps were to run at 240 volts, a 20-amp circuit would allow for double the wattage capacity of a 120-volt circuit.

The next factor to consider is the wattage, or overall power draw of the light. Higher wattages result in brighter lights. For instance, a 1,000-watt HPS produces more than double the light of a 400-watt HPS.

The last important factor to consider is the efficacy of the lighting source. For lights, the term “efficacy” is used instead of “efficiency,” although they mean similar things. Efficiency is measured as a percentage, while efficacy is an independent value. For example, an LED may have an efficiency converting energy to light of 40%, and the efficacy may be 3.0 µmols/J. Despite these values being two ways to describe how much energy is needed to create light, efficacy allows for more direct calculations to estimate lighting.

The measure of lighting efficacy for plant use is µmols/J. This measures the lighting output (µmols of PAR photons, or PPF) and the power required to make the light (J, joules, or watt-second). Some lighting manufacturers will list this as µmols/watt; however, µmols/J is the most correct unit. A higher µmol/J rating produces more light with the same power.

Different lighting technologies (LED, fluorescent, HID) have various efficacies, and not all lighting within the same technology has equal efficacy (some LED fixtures are more efficacious than others). If the power of µmol/J and the footprint of a lighting fixture is known, an estimate (with some error due to light escaping or diminishing due to reflectance) of the light intensity of the grow area can be made.

Calculating Average Light Intensity (PPFD)

If there is one light fixture that operates at 1,000 watts with an efficacy of 2.0 µmols/J, it will produce 2,000 µmols·s^–1. If this fixture lights four square meters (43 ft²), it would light the space at an average intensity of 500 µmol·m^–2·s^–1. Again, this is only an estimate, as some light will be lost, so the actual intensity will be lower and the hang height of a fixture and optics will affect the lighting footprint. Reflectors help reduce light loss and make this calculation more accurate. Larger areas and more fixtures also improve accuracy.

Calculating the Number of Lights Needed

This equation can be rearranged to determine how many lights are needed based on the area of the cultivation space and the target light intensity. At least 30 lights are needed to illuminate a 6 by 10 meter (20 × 32 foot) space, or 60 m², to 1,000 µmol·m^–2·s^–1 using 1,000-watt lights with an efficacy of 2.0 µmols/J.

Again, this equation assumes 100% of the light reaches the target area. Factors like hang height, light footprint, reflectors, and even the color/reflectivity of walls affect this. Designers and manufacturers of large greenhouses or indoor grows use software, such as AGi 32, to most accurately quantify and implement light to ensure proper coverage at the right intensity.

Horticultural Lighting Labels

Choosing a light requires knowing as many details about its output, spectrum, and electrical requirements as possible. Unfortunately, a standard horticultural light label has not yet been agreed on. With lights for human use, several important measurements are included on package labels: lumen output, color rendering index (CRI), and correlated color temperature (CCT). These measurements are important to know the impact of the bulb on the brightness of a room, how well colors can be differentiated, and overall mood of the appearance of a room for people, but are not particularly useful for describing how effective lights are for growing plants.

As referenced in the light measurement section earlier in the chapter, the perception and use of light by plants does not correlate well with the human eye. Research is ongoing about ideal spectrums. However, horticultural lighting labels typically include power consumption parameters (volts, amps, wattage), spectral output of the lamp, and the µmols/J efficacy or PPF of the fixture.

Most manufacturers of non-LED lights did not provide information requested for this chart. This sort of data comparison on lighting specifications explains why LEDs are quickly becoming the industry standard. Most companies provide data when asked; many more are found online.

Lighting Technologies

Many technologies exist today for providing light for plant growth. LEDs are the most-recommended light source for growing cannabis, but there are a variety of lamps that can also be used; high intensity discharge (HID), which include high-pressure sodium (HPS) and ceramic metal halides (CMH), and fluorescent bulbs. These light sources all differ in initial cost, efficacy, and spectrum.

The heat produced by a light (or any piece of electrical equipment) is directly related to the wattage the fixture consumes, regardless of the technology. A 1,000-watt LED produces as much heat as a 1,000-watt HPS. A 500-watt LED can be equivalent in light intensity to a 1,000-watt HPS, so there is less of a need for cooling fixtures. Heat from the fixtures is based on their wattage.

LED Lights

Price per light: High

Efficacy: Low to excellent (based on model), 0.9-3.2 µmols/J (Nelson and Bugbee 2014)

Spectrum: Customizable and can be excellent for either growth or secondary metabolite production. Some LEDs are adjustable and give growers the option to change spectral ratios or intensities.

HLG 650R commercial indoor horticulture LED LAMP is designed to replace a double-ended 1,000-watt HID lamp. This lamp uses full-spectrum, high-efficiency Quantum Boards® powered with Samsung’s latest LM301H and Deep Red 660nm LED. This unit is dimmable, with wattage output from 60 to 630 watts.

HLG 100 RPSEC® produces over 15,000 lumens with just 95 watts of power and is equivalent to a 200-watt T5 or a 300-watt fluorescent or metal halide. This fixture is ideal for vegging, supplementing, or powering a small garden.

HLG Scorpion is designed for commercial indoor gardens with low ceilings or vertical racks. HLG Scorpion uses six Quantum Boards® for an even light spread at just 12 inches (30.5 cm) from the canopy. This lamp uses Samsung’s latest LM301H and Deep Red 660nm LED. This unit is dimmable, with wattage output from 60 to 630 watts.

Light-emitting diodes (LEDs) come in many configurations, including floodlights, panels, bars, circles, and rectangular fixtures. LEDs are highly efficient, using less electricity than HIDs and fluorescents. As a result, LEDs create less heat, and fixtures are only three or four inches (7-10 cm) deep, so they make great lamps for closet cultivation or vertical, tier-based production, where height is an issue.

LEDs are unique among lighting systems because they can be made with diodes that emit light in an unusually narrow spectrum. In addition, white LEDs can be made by phosphor-coating blue LEDs to provide a broad emission of wavelengths within the PAR range. LED fixtures are designed to provide plants with exactly the spectrum intended. These factors make LEDs an excellent source of plant lighting (Morrow 2008).

LEDs have a wide range of efficacies due to many different factors, including the generation of diode, spectrum, cooling strategy, adjustability, and additional features on the fixture. Diode manufacturers are continually improving on efficacy, following what is called Haitz’s law (similar to Moore’s law for computer chips): “The brightness of LEDs increases 20-fold per decade and decreases in price 10-fold per decade” (Steigerwald et al. 2002; Haitz and Tsao 2011). LEDs will eventually reach a theoretical maximum of 4.6-5.1 µmols/J in the coming years, depending on diode spectrum. Diode spectrum is important to determining efficacy, as red diodes are the most energy efficacious, currently followed by blue diodes. Other colors are not nearly as efficacious as red and blue, particularly monochromatic green, UV, and far-red. When LED fixtures are manufactured with these diodes, they have lower µmols/Js. “White” LED light is a combination of red, green, and blue, which is an effective way to get green light to the plant.

The cooling strategy of the fixture also affects efficacy. LEDs contain a large aluminum heat sink to cool the diodes. These can be actively cooled by fans or passively by ambient air. When a fixture uses fans to cool the heat sink, they must be powered, lowering fixture efficacy. LEDs have the ability to be adjusted for light intensity. Some fixtures have the option of adjusting specific channels of diodes, changing spectrum. Unfortunately, the circuitry that allows adjustability adds inefficiency to the fixture and lowers efficacy.

Currently, the most efficacious fixtures are passively cooled fixtures with fixed spectrums that are manufactured with the latest, most efficacious LEDs and use red, blue, or white LEDs. Still, the addition of other colors, including far-red, may be worth the minor loss of efficacy, as it may have a dramatic impact on plant growth and flowering.

Light from LEDs can be combined with HPS and CMH lamps, so there is no reason to scrap digital HID lamps. Adding LEDs to the other lights increases the amount of light delivered to the garden. For instance, a garden using a 400-watt HPS is increased to the equivalent of 600 watts using 100 watts of LEDs. Adding 400 watts of LEDs can create the equivalent of a 1,000-watt HID system. An advantage of using a combination of HPS and LED lights is that the plants will receive all the spectrums needed to thrive.

Using Scynce LED’s “full power spectrum tuning,” the grower has complete control over the spectrum delivered to the plants and can perfect cultivar-specific light recipes. Light intensity is not compromised as the spectrum is adjusted, so there is no need to swap out bulbs or fixtures. Secondary optics help focus this fine-tuned light energy over and deep below the canopy, eliminating the “pin-point” intensity that plagues most LEDs and some traditional lights.

EYE HORTILUX’s LED 700-ES extends the standard LED growing spectrum by providing energy outside the PAR range to include ultraviolet (UV) and far-red, which promotes stronger plant growth with heavier flowers. Far-red light spectrum promotes increase in plant biomass, triggers flowering, enhances flower size, and improves the efficiency of the overall spectrum.

LED fixtures cost more than HID lights, but save money in the long run.

LED lamps are capable of using significantly less electricity compared with HIDs per unit of PAR light produced. LEDs can save significant electrical costs per year when compared to the cost of running a 1,000-watt HID for 12 hours a day. To calculate the overall cost of purchasing and running a light for a period of time, add the cost of the fixture, any cost of maintenance over the running period (bulb replacements), and then multiply the cost of electricity by the hours run. This formula compares the cost of owning an LED fixture with an HPS lamp with similar light output for a year:

Alternatively, this equation can be modified to include two fixtures to determine how long a fixture must be owned in years when comparing two fixtures:

With

Below are the parameters and fixture specifications compared within the equations. The two fixtures selected have similar light output and should be comparable to each other in terms of plant growth and yield.

Run time: 20 hours per day

Cost of electricity: $0.10 /kWatt·hr

LED Lamp

Price of LED: $1,500

Wattage of LED: 630 watts

Maintenance: $0

HPS Lamp

Price of lamp: $350

Wattage of lamp: 1000 watts

Maintenance: $55 for new bulb every 5,000 hours of run time

*LEDs do not need bulb replacement or maintenance

The final equation is

Although the initial cost of the LED fixture is more than HPS, it pays for itself in a short period of time. There are more savings with the LED that are not included in this. For example, since there is less cooling equipment required for the LED fixture, this reduces the cost of equipment needed to run the lights and the operating cost of running cooling equipment. There are also efficiency rebates offered by many power companies for using LED lighting instead of conventional light technologies. As LEDs continue to drop in price and increase in efficacy, it will become even more advantageous to choose them over other lighting sources. LEDs have such a long life that they will not be replaced until more efficacious lamps are available. For these reasons, LEDs are often the best option.

High Intensity Discharge

High Intensity Discharge (HID) is a significant step forward in brightness and efficacy of light compared with fluorescent. These lights occupy less space and offer intensities bright enough for greenhouse supplemental lighting or as a sole source of light. HID fixtures come in two different bulb types that vary in efficacy and spectrum: HPS lamps, which are more popular, and CMH lamps.

TOP: gGRO^™ Ceramic Metal Halide (CMH) by Genesys Global lamps offer full-spectrum output with +14% more red and +104% more far-red spectrum, promoting growth and higher yield. The halides present are optimized to provide UV content for higher THC, broad spectrum for higher growth quality, and enhanced reds for more growth volume during flower stage.

BOTTOM: gGRO^™ can be tailored to combine gLED^™ and gHID^™ to optimize around variables, including but not limited to business location, climate, and facility architecture, through every stage of plant growth.

Ceramic Metal Halide Lamps (CMH)

Price per light: Moderate

Efficacy: Moderately good, 1.5 µmols/J (Nelson and Bugbee 2014)

Spectrum: Good for growth, excellent for secondary metabolites and overall quality Ceramic metal halide (CMH) lamps are the type of lamp used outdoors to illuminate sports events because they emit a white light. They were originally promoted as the best light to use during the vegetative stage of plant growth, before the plants are forced to flower, but now it is generally recognized that plants also grow well vegetatively under HPS and LED lamps as under CMH. CMH lamps do have the advantage of minimizing internodal stretching during vegetative growth due to their high blue and low far-red light output.

CMH lamps, like fluorescents, come in many spectrums. This is very important for figuring how effective a lamp is. In general, 15% of the energy used by CMH lamps is emitted as PAR, as compared with 13% for HPS, but the crisp white light emitted by standard CMH lamps is low in the red spectrum. Since plants need red spectrum light for photosynthesis and flowering, its absence is felt. Nonetheless, under metal halides, plants grow quickly and flowering is profuse, with heavier budding than under fluorescents.

CMH lamps may be the solution to plant lighting problems indoors and out. During the fall, ceramic metal halide lamps can be used in backyard gardens to supply the extra energy boost needed to ripen. Run the lights during the day to supplement the ambient light. Although the light they emit is very bright, it is white, not the unusual amber color emitted by HPS lamps, so it is not as likely to look unusual.

CMH lamps can be used during the vegetative stage. They come in 315-, 630- and 1,000-watt sizes. The 315-watt lamps can easily illuminate a three by three (90 by 90 cm) to four by four foot (1.2 × 1.2 m) garden. The 630-watt lamp can illuminate a garden up to five by five feet (1.5 by 1.5 m). The 1,000-watt lamp can illuminate a garden up to six by six feet (1.8 by 1.8 m). This varies by cultivar. Additional light can be provided to increase intensity during flowering.

CMH lamps are convenient to use. The complete unit consists of a lamp (bulb), fixture (reflector), and ballast. The fixture and lamp are lightweight and easy to hang. A chain or rope is used to suspend the fixture, which takes up little space, making it easy to gain access to the garden.

Lights become dimmer as they age, but may need to be replaced after a year or less. A light meter is required to properly determine if the lights are still viable. Smartphone applications are available to perform light metering.

Ceramic Metal Halides & Ultraviolet Light (UV)

Ultraviolet (UV) light, as discussed earlier in the chapter, is composed of spectrums beyond blue that are invisible to humans but are visible to many animals. UV light is divided into three bands—UVA, UVB, and UVC. UVB is critical to the development of THC. The potency of cannabis is dependent on the amount of UVB light it receives. Ceramic metal halides emit UV light and can be used for this purpose.

Even if plants are being grown under HPS lamps, potency increases significantly if they are replaced with ceramic metal halides during the last two weeks of flowering. Some CMH fixtures are air-cooled and use a protective glass that absorbs UV light before it gets to the garden. The only way to benefit from the UV output is to remove the glass and take other steps for cooling. However, it is not recommended to remove the glass in an air-cooled hood, especially when there are so many newer options that do not require it. Modern CMH fixtures require less air-cooling, so they are no longer manufactured with the protective UV-light blocking glass.

The EYE HORTILUX CMH 315 grow lamp was designed specifically for horticulture, rather than for use in retail spaces, like most ceramic metal halide lamps. These CMH lamps allow an effective amount of UV to pass through, more than 50% compared with other lamps, increasing cannabinoid content of cannabis flowers.

High-Pressure Sodium Lamps (HPS)

Price per light: Moderately low

Efficacy: Good, 1.7 µmols/J (Nelson and Bugbee 2014)

Spectrum: Excellent for growth, good for secondary metabolites and overall quality

High-pressure sodium (HPS) lamps emit an orange or amber-looking light. HPS lamps are commonly used as street lights. Their spectrum is heavily concentrated in yellow, orange, and red, with only a small amount of blue. HPS lights are usually used for flowering because they supply more orange and red light than CMH lamps, and are slightly more efficacious. The increased red and yellow light seems to promote more flower production.

Gardeners usually use HPS sizes of 400, 600, or 1,000+ watts, all of which are sold in indoor garden shops. The 600-watt HPS lamps are about 7% more efficient than the other sizes.

HPS lamps support fast growth during both vegetative and flowering stages. They need no supplemental lighting during any stage of growth, though they may produce more internodal stretching in vegetative plants than their CMH counterparts. HPS lamp brands and models differ in both the amount of light emitted per watt and in the spectrum that is emitted. Some HPS lamps emit enhanced levels of blue light, which encourages stout short stems and branches. Since light of each particular spectrum is processed differently by plants, some lamps produce more growth and flowering than others.

Fluorescent Lights

Price per light: Low

Efficacy: Fair, 0.8-0.9 µmols/J (uncited 1.1) (Nelson and Bugbee 2014) (Wallace and Both 2016)

Spectrum: Moderate for growth, excellent for secondary metabolites and overall quality

Growers have used fluorescent tubes to provide light since the early years of indoor cultivation in the 1960s. They are inexpensive, easy to set up, and moderately efficient. Plants grow and bud adequately under them. However, fluorescents do not create the intensity of light emitted by other technologies, so they usually don’t produce the large, tight buds the more powerful lamps do.

Fluorescents can produce various spectral outputs, which are determined by the type of phosphor used to coat the surface of the tube. Each phosphor type emits a different set of light colors, rated in kelvin, and identified as “warm white,” “cool white,” and “daylight,” or “natural white.” These names signify the kind of light the tube produces, with daylight or natural white coming closest to approximating the sun’s spectrum. Lamps of different spectrums can be used in the same fixtures.

Cool white fluorescents emit more blue light than warm white. They are useful during the vegetative stage because the blue light promotes stout, compact stem growth.

Warm whites emit more red light than cool whites and are often used during flowering because the red spectrum promotes flower growth.

Several brands of special “plant growth” lamps are also available. They concentrate their light emissions in the red and blue spectrums to provide plants with more energy, but they produce less total light.

T12s are the widest fluorescent lamps and consume less power, produce dimmer light, and emit lower heat. High output (HO) fluorescents are supercharged. They use almost 60% more power than standard T12 fluorescent lamps. They can illuminate a smaller garden very brightly and are an alternative light source, using a high-intensity discharge lamp. They are readily available in T-5 tube size.

T-5 Straight-Length Tubes

T-5 tubes are 5/8 inches (1.5 cm) wide and are available in a variety of lengths. Their more compact size means it is possible to fit four T-5 tubes in each foot (30 cm) of width. T-5 tubes are available in high output models, which are the ones usually sold in shops. A 4-foot (1.2 m) high output tube uses 54 watts and emits roughly 5,000 lumens, almost twice as much output as a conventional T-5. A bank of eight, four-foot (1.2 m) T-5 high output tubes emits 40,000 lumens and uses about 435 watts, as compared to a 400-watt HPS that actually uses 440 watts and produces about 50,000 lumens. T-5 HO fluorescents use about 60% more electricity than regular T-5s.

Compact Fluorescents (CFLs)

Compact fluorescents are often the most convenient lamps to use in small gardens. Unlike other fluorescent bulbs, they have ballasts built into the bulb assembly, so they can screw into standard incandescent sockets. They are available as floodlights, twisted tubes, and straight mini-tubes. Another advantage of CFLs is that they deliver a lot of light from a small point. Unlike tube fluorescents that deliver their light over a large area, often spanning several feet, the compact “point of light” emissions make it easier to increase light intensity by grouping them close to each other. Large-wattage CFL lamps are available in sizes of 25, 50, 100, 150, 200, and 250 watts. LEDs have largely supplanted the use of CFLs.

HID Ballasts

HID lamps have electrical systems that require conversion to higher voltage than is delivered through the electrical grid. The ballast converts incoming current to the appropriate voltage. Some ballasts are remote from the light, connected by a long electrical cord. The convenience of this is that the heavy ballasts do not hang from the ceiling or other supports, while the lighter and less cumbersome reflector and bulb are stationed above the plant canopy. In most large commercial indoor gardens and greenhouses, the ballast and lamps are configured together.

Old-style magnetic ballasts are dedicated to a particular type and wattage lamp and a single grid voltage, such as 110 or 220. They are very heavy and use almost 200 watts to power a 1,000-watt lamp.

Most digital ballasts can be used for both CMH and HPS lamps of the same wattage and with both 110 and 220 voltage. Some can power different wattage lamps.

Solid state digital ballasts are even more efficient and produce a steadier light that increases growth. They are used extensively by commercial growers.

Compared with magnetic ballasts, digital ballasts are more convenient. They are more electrically efficient, using only 50-100 watts, are lighter weight, and do not make any sound. They are also gentler on the bulb during start up and regulate current more precisely. Some are also capable of running both CMH and HPS bulbs. They are also safer than magnetic ballasts, as they incorporate safety features that ensure shutdown if problems occur.

Lighting Accessories & Light Reflectors

Light doesn’t become weaker or disappear with distance. It appears to dim as the light beam widens over a larger area. As it spreads, its intensity dissipates. Just think of a flashlight. If the beam is tightly focused, it will have the same intensity at the focus point as it had at the point of emission. The larger an area of the garden that a lamp illuminates, the less intense the light that plants receive.

Reflectors are a way to keep the light focused where it is needed. The three other types of lamps other than LEDs discussed—fluorescents, CMH, and HPS—all emit light in all directions. Even a portion of LED light, although low in comparison with the other light sources, is emitted sideways, causing light to be lost. Only a portion of the light shines directly on the garden. Unless it is redirected, the light illuminates the wall and ceiling.

Any light emitted by the lamps that doesn’t reach the plants is wasted and might as well not have been produced. There are many solutions, and all but a few involve the use of light reflectors.

Fluorescent Light Reflectors

The shape of a fluorescent reflector determines, to a great extent, how much light the plants receive. Use the best reflector available. Almost all fixtures with reflectors place the tubes too close to one another, so that only about 60% of the light is actually transmitted out of the unit. The rest is trapped between the tubes or bounces back and forth between the tubes and the reflector. This light may as well not be emitted, since it is not hitting the plants.

Like other fluorescents, CFL bulbs emit light in all directions. Inexpensive clamp-on fixtures with bowl reflectors help direct the light to the garden. Commercial reflectors are available for larger size CFLs. Good reflectors can double the light intensity the garden receives from CFLs.

The fluorescent tubes that fit inside shop lights can be fitted using LED tubes. They will produce more light cheaper. New fixtures with 4-foot (1.2 m) LED tubes are dimmable and come with remote controls.

CMH & HPS Reflectors

Both CMH and HPS bulbs emit most of the light along their length, so it comes out of the sides of the bulb. Many fixtures orient the bulb horizontally to take advantage of this. The reflector must direct the rest of the light downward.

There are many reflector models, but they can be classified into two general types depending on which position the bulb is held: horizontally or vertically. For most gardens, horizontally held lamps are preferred.

Horizontal Reflectors

Horizontal lamps deliver more light directly to the garden because of the position of the bulb and the direction that light is emitted. Manufacturers have created many designs, and each leaves its own illumination footprint. Some focus light in a small area; others are designed to distribute it over a large space. The best reflector for a particular garden depends on the garden’s dimensions and design.

Small one-light or two-light gardens do better with focused reflectors. The light is directed downward, so most of it goes directly to the garden. Focused beam reflectors minimize the light that goes off to the sides. Larger gardens grow more vigorously when the reflectors in the center spread the light over a larger area. The plants receive light coming from different directions, minimizing shadows and giving a larger portion of the plant the opportunity to actively photosynthesize. Reflectors closer to the perimeter should still be close focused so that light remains in the garden.

Vertical (Parabolic) Reflectors

When a bulb is held vertically, almost all the light comes out the side and follows a horizontal path to the walls. This light must be directed down to the garden. None of the vertical reflectors are designed to keep the light in the area directly below the reflector, so lots of light leaks out of the garden if not directed back using additional reflectors. The light they redirect is broadcast over a very wide area. Some reflectors have adjustable bulb positioning, so the light is controlled somewhat, but most reflectors are too shallow and miss a large portion of light, and it is lost to the garden. Because of their poor design, vertical reflectors are very inefficient at directing light to the garden. They are considered old technology.

Air-Cooled Lights

Lights emit a lot of heat. Each 1,000 watts of light input creates about 3,412 Btus of heat, regardless of whether lights are fluorescent, HID, or LED. Just three 1,000-watt lamps release the same amount of heat as two standard electric space heaters. If all this heat is released into a room temperature garden, it has to be removed either through ventilation or air-cooling.

Air-cooled light reflectors solve part of the problem by removing the hot air before it gets into the room. The fixtures have a glass bottom, so the lamp is totally enclosed, trapping much of the heat. They come with four-, six- or eight-inch (10, 15, or 20 cm) flanges on either side of each reflector. An equally sized inline fan is attached to each row of air-cooled reflectors using flexible ducting. This inline fan draws cool air, typically from outside the room, and supplies it to each reflector in its row. Ducting attached to the other side of the reflectors exhausts much of the heat from the light out of the room. The cooling air travels through the sealed ducting and reflectors and never has contact with the air in the garden. It picks up the lamps’ heat but absolutely no odor from the room. The air can be safely exhausted from the room or used to heat another interior space.

Depending on the quality of the air-cooled reflector, up to half of the lamp’s heat is removed from the room, simplifying temperature management. The one problem with air-cooled CMH lights is that the bottom glass in the fixture absorbs UVB light and other spectrums, so the heat is eliminated at the cost of light, which is more expensive.

Another strategy is to use air-cooled lights with no enclosing glass. Air is drawn from the garden space through the tubing and then cooled and returned to the room or cleaned of odor using an inline carbon filter so that it can be exhausted outside or used for heating another interior space. Air above the canopy is pulled up into the reflector, drawing the heat with it. This keeps the canopy cool, and the CMH’s UV light is directed to the garden.

Light Movers

No matter what kind of light reflector is used for HID lamps, they deliver light in an uneven pattern. Usually the center area, directly under the lamp, receives the most light. The intensity of the light tapers off as the distance from the center increases. Light movers help eliminate differences in light intensity by moving the center around so the angle of light changes. As the lamps move, each plant section comes directly under the light repeatedly. Instead of plants in the center receiving more light than those on the edge, the light is distributed more equally throughout the garden.

Shuttles move lights back and forth in a straight line along a track. The movers have a standard-issue 6-foot (1.8 m) movement area. However, some models have shorter lengths, and the distance they travel can be adjusted. Even if the light is only being moved a single foot (30 cm), the angle of light to the plant changes. Areas that were in constant shadow see the light and respond with increased growth. Using attachments, light movers can move several fixtures at the same time. They use little electricity, so they won’t add much to the electrical budget.

These units increase the efficiency of the light and garden in several ways.

Moving lamps distributes light evenly. There are fewer hot spots, so plants grow more uniformly. Plants of a single cultivar grow to the same size and ripen at the same time.

In smaller gardens, lights can be placed closer to the plants, so they receive more intense light with less directed outside the plant canopy.

Total garden growth and yield increases. Getting light to the formerly light-deprived areas of the garden increases their growth. Expect a 10-20% increase in yield using light movers in small gardens. Even in larger spaces, moving the lights increases yield.

Reflective Materials

To maximize the light, it should be directed to the canopy, rather than illuminate spaces outside of it. White paint or reflective materials are used to ensure more light is directed to the plants. This is especially important with smaller garden spaces as compared with larger spaces, where there is a higher ratio of perimeter to plant canopy. Unless the light is reflected back from the perimeter, it is wasted.

Many self-contained units or tents have built-in reflective materials. Painting the walls, floor, and ceiling is the most common option in indoor spaces. Black or dark walls absorb light; white reflects it. Highly reflective white epoxy paint is an easy option to ensure light is reflected back onto the plants from the walls, floor, and ceiling, but other techniques are also used:

• Mylar is a commonly used wall liner, although it can become crinkly and is harder to clean.
• Panda (white and black) polyethylene plastic is highly reflective, easy to clean, and reusable.
• Aluminum foil can be used. It is inexpensive and recyclable.
• Aluminized bubble wrap doubles as insulation and is easy to clean, reusable, and can be recycled as packing material.