FLOWERING
Most cannabis varieties are referred to as short-day plants. They determine when to flower based on the number of hours of uninterrupted darkness they receive. When the plant receives the critical period of uninterrupted darkness, the plant flowers.
The plant measures the length of the dark period using the hormone phytochrome, which has two states. The hormone’s inactive state, Pfr, occurs when it absorbs a red spectrum of light at 666 nanometers. It also has a slight sensitivity to blue light. When the plant is in darkness, the hormone changes from its inactive form, Pfr, to its active form, Pr, over two hours. A far-red light at 730 nm after lights out converts Pfr to Pr instantly. Unfortunately, it also induces some stretching. When the Pr flowering hormone levels remain high for a critical period of time each day for several days, the plant initiates flowering.
The number of hours of darkness that plants need to initiate flowering differs by cultivar. Sativas require a longer period of darkness than indicas because they developed near the equator, where the length of the growing season is much longer and there is less variation in daylight as compared to indicas, which developed in the northern latitudes. Some sativas continue to grow vegetatively with 10 or 11 hours of darkness, which usually cues most plants to flower.
There are two notable exceptions to light sensitivity regarding flowering. Autoflowering varieties have a programmed chronological strategy, that is, they start flowering soon after germination.
Many sativas initiate flowering only when the dark cycle increases to 12 hours or more, which occurs when fall begins.
By contrast, most indicas flower with 8 to 11 hours of uninterrupted darkness (13 to 15 hours of light). In lower latitudes a few indica varieties flower as early as summer solstice, the shortest night of the year. For this reason, outdoor growers in these zones should consider sativa-indica hybrids and sativas. In mid and higher latitudes, indica and indica-sativa hybrids usually start flowering in late summer and are ready to harvest in early fall.
Some sativas and sativa hybrids require a longer dark period at the end of flowering to fully ripen their buds. Outdoors, this happens in due course as the nights lengthen in the fall. Indoors, change the lighting regimen to 14 hours of darkness/10 hours of light to promote ripening. This is especially helpful in finishing low latitude varieties that don’t reach maturity in their native lands until extremely late in the season.
Indoor Flowering
Flowering time of female cannabis plants is regulated by the length of the uninterrupted dark period, so gardens under lights can be forced to flower at any time with the flick of a timer switch.
In general, to determine when to flower the plants, look down at the canopy. When two-thirds of the floor space is hidden by plant canopy, it is time to start the flowering process. The remaining space will fill in during the flowering stage. The breadth and height of the plants may also need to be considered. If the garden has height limitations, the plants should be forced to flower before they get too tall.
To force flowering, lights must be turned on and off in a consistent manner, and the darkness must be uninterrupted. For that reason, it is essential to use a timer to regulate the lights. The timer is set so the lights stay on for 12-13 hours and then remain off for 12 or 11. Within a week, growth slows. The plants are responding to the new light regimen and are beginning to flower.
The light change does not need to be tapered. Reduce light from 18 hours daily (or continuous) down to a flowering cycle of 12 or 13 hours of light and 12 or 11 hours of uninterrupted darkness, with no intermediate steps. The change in the light regimen does not shock the plants.
Sinsemilla & Sexing
Cannabis users prize seedless female flowers, known as sinsemilla, because they are more attractive and far more convenient to use. Cannabis seeds have a noxious oily odor when they burn, so they need to be removed before smoking. Sinsemilla buds are seedless, so there are no seeds to remove. In addition, no energy is wasted on seed production.
In order for female flowers to ripen without seeds, they must remain unpollinated (unfertilized). Because cannabis is dioecious, male and female flowers appear on separate plants and the males must be separated.
If the garden is started using nonfeminized seeds, roughly half the plants will be male and must be removed from the space as soon as they are identified. This culling should be done early in the male plants’ development, before any large flower clusters appear. Even a single open male flower can release enough pollen to fertilize dozens of neighboring female buds.
Cannabis can be sexed early. There are two basic methods:
- Identifying early, premature flowers
- Forcing plants to flower by altering the light regimen
There are several ways to use the forcing method, each with pros and cons.
Visual identification of the plant gender is easiest, since it requires no intervention. Sometimes, while a plant is growing vegetatively, a single, small flower appears at the space where the leaf joins the stem (node) two to four pairs of leaves from the top. The sex of the plant is the same as that flower. Identifying the tiny, premature flowers can be challenging, and not all plants produce them. A magnifying glass or photographer’s loupe is often employed to get a clearer image.
Forcing plants to flower is a more certain method of determining sex. Flowering is regulated by the number of hours of uninterrupted darkness plants receive each day, and so it’s easy to manipulate plants to reveal their sex. Establishing a long-night regimen for a week forces them to indicate. Once this has occurred, remove the males from the garden, or separate them if they will be used in breeding, and return the garden to the vegetative growth cycle by changing the light regimen back to the long day/short night.
A better alternative is to take a cutting from each plant and force it to flower. Each cutting is carefully tagged to identify which plant it came from. Set the clones in a grow medium and provide a light regimen of 16 hours of darkness/8 hours of light. Within a few days, the clones will indicate. Each clone has the same sex as its parent, so the parent’s sex has been identified without taking it out of the vegetative stage and disrupting growth.
The female clones can be kept under the flowering regimen to get a tiny taste of the parent’s future buds. Be careful to label cuttings and plants so they can be matched up accurately once they’ve been sexed.
Blue Light
Blue light is another option for sexing. As mentioned earlier in this chapter, cannabis flowering is very sensitive to red light of specific spectrums. Any interruption of the dark period with light that contains the red 660 nm spectrum returns the flowering hormone Pr back to Pfr, its inactive state. This prevents flowering.
Blue light at 400-450 nm also has an inhibitory effect on flowering, but its effect is weaker than red light. Plants grow some flowers when blue light is kept on during the dark period; however, they continue to grow vegetatively as well. With blue LED or fluorescent lights to provide the plants with nothing but pure blue light, they will get enough stimulation to produce some flowers for sexual identification but not go into full flowering mode.
This is a good sexing technique to use anytime, but especially when a large number of plants are involved. No cuttings need be taken and matched to their mothers, so there is no chance of a mix-up or dead, non indicative clones. As soon as a plant produces male flowers, remove it from the space. Once all the plants indicate, replace the blue light with a full-spectrum light to keep the plants growing vegetatively.
Plants use blue light to regulate flowering as well as for photosynthesis. Blue light is not as efficient a source of energy for photosynthesis as red light indoors because blue light has a higher energy value than red light and requires more energy to produce. The plant obtains the same amount of energy from both of them, however. When blue light is turned on during the dark period, plants photosynthesize, but the growth from the blue light is not significant. The stems grow a little more stocky.
The effect of blue light on flowering is more important to cannabis growers. See the Phytochrome Response chart, which shows phytochrome Pr-Pfr sensitivity across the light spectrum. The red and far-red portion shows high activity. The blue spectrum shows just a little bump. This indicates a slight activity. The result is sporadic flowering on all the plants.
Pure blue light (400 nm) can be created with LEDs and blue CFLs. Use about 10% blue light per watt of regular light. (See Light.)
Absorption spectra of phytochrome in both its active (Pr) and inactive (Pfr) forms. Pfr is the form of phytochrome that optimally absorbs far-red light (~730 nm). Pr optimally absorbs red light at ~666 nm.
Male and Female Flower Anatomy
Identifying the sex of cannabis flowers is easy once their characteristics are known. Male flower buds look like balls dangling from thin stems, with a curved protrusion at the bulb’s end that comes to a blunt point. As the male flower ripens, the head’s position moves from hanging down to upright. The petals that formed the bulb open, revealing five simple petals that range in color from cream to yellow. Each has a stamen in the middle that releases pollen to a breeze, or wind, or when touched.
Female flowers have no petals, but they are identifiable because of their pistillite structure. Each flower has noticeable stigmas, two white or sometimes pastel pink or lavender antennae-like protrusions, attached to a pistil, which is an oblong pod-like structure.
The males release wind-borne pollen. Each pollen grain contains two sperm. The stigmas capture pollen from the air and then transfer the male gamete cells via hollow tubes down to the ovary. In the ovary, the pollen tube from one stigma fertilizes the egg to form an embryo. The pollen from the other stigma is combined with portions of the ovary to form a food source for the embryo within the seed. The stigmas of fertilized flowers then dry up, beginning at the tips. Each fertilized flower produces one seed. Seed development starts as soon as the female flower is fertilized. The ovary at the base of the flower swells as the new seed grows inside it. This is typically visible within three days after fertilization.
Pollination is avoided in cannabis gardens that are well cared for. The females’ stigmas search in vain for pollen because the males have been removed. Eventually the stigmas dry up and become part of the ripe, unfertilized flowers. The leaves start growing closer together as the plants form strong stems that will hold clusters of flowers along a branch. These branches of tightly packed, ripening buds are often called “colas.”
As flowering proceeds, any plants that have not clearly indicated their sex must be closely watched. Males usually indicate earlier than females; they are less likely to be encountered later in the season.
(1) A hermaphrodite bud. Both male and female flowers are apparent. (2) Male flowers sometimes appear just as a bud is maturing. They are an indication that the bud is ripe. (3) Older male flower. These are useful breeders. Photos: Ed Rosenthal
Hermaphroditism
Some plants that are primarily female become hermaphrodites and grow male flowers in addition to female ones. This can happen indoors and outdoors; some varieties are more prone to hermaphroditism than others. Stress plays a role.
A hermaphrodite’s male flowers may be interspersed among its female buds; they may appear in clusters; or they may occupy one or more separate branches.
For obvious reasons, hermaphrodites are dangerous in any sinsemilla garden; even a single male flower can ruin many neighboring buds. Any plants with male flowers should be removed from the garden before the flowers open. This is the only safe course of action.
Trying to control a hermaphroditic plant by removing just the male flowers is an extremely difficult task, and one mistake, lapse in monitoring, or hidden flower can cause serious damage. Even if the plant seems like a winner, it is not worth risking the rest of the buds in the garden.
There are several reasons why a plant becomes hermaphroditic. It may have a genetic predisposition to be a hermaphrodite. For instance, French hemp breeders have developed monoecious varieties; all the plants have both male and female flowers.
Female plants sometimes develop male flowers as a result of stress, including irregular light cycles and heat stress during flowering, or other drastic changes in the environment.
Some plants develop male flowers just as they ripen. This is an indicator of ripeness and is not a danger to the garden, since the plants are to be harvested shortly.
Flower Growth
Within a few days of establishing a long dark period, the plant’s growth pattern changes. First, its rate of growth, which might be as much as two inches (5 cm) a day during the vegetative growth cycle, slows. Under a flowering light regimen, indica varieties usually grow anywhere from another 25-100% taller and wider. Sativas can double, triple, or even quadruple in size before growth stops. Hybrids have growth patterns that vary between these two extremes. If the intent is to produce seeds and there are both male and female plants growing, they begin to differentiate and become dimorphic.
The males elongate and grow new shoots that hold the flowers, or they develop flowers along their existing branches. Upon ripening, the male flower sacks, which contain copious amounts of pollen, tower above the females. This difference serves the plant well, since cannabis is normally wind-pollinated, and the pollen from a tall male plant is more likely to catch the wind for a ride and drift down onto an obliging female stigma below.
Within the first week, females start to grow stockier stems with shorter nodes between the leaves. The number of fingers on new leaves decreases, and the plants no longer form leaves on opposite sides of the stem but now alternate sides. Most important, the first stigmas appear.
By the second week, the first stigmas are joined by a cascade of flower growth. The plant is now spending most of its energy on flower development.
The flowering pattern changes as the stigmas begin to wither, dry, and turn red, purple, or even a light brown, similar to the pattern of fertilized flowers. In the third week, a large number of stigmas form along the stem and on the tops of the branches. As long as it remains unfertilized, the plant continues to produce new flowers.
Flowers develop capitate trichomes all over their outer surfaces. They also develop along the small leaf parts surrounding the flower. Capitate trichomes differ from the sessile trichomes that grow on the sun leaves and stem.
Sessile glands are much smaller than capitate trichomes and are either directly connected to leaves or stems or rest on a one-cell stalk. Capitate trichomes have a much longer, four-celled stem with a large, bulbous cap at the end. When they first start growing, the caps on top are small. They swell as the resins are produced and stored. By the time they are ripe, the caps look like balloons so overinflated that they might burst. Given any stress, such as wind, rain, or touch, many of them will detach from the trichome. They are semi porous, so the terpenes evaporate. Evaporation increases with increasing temperature and vapor pressure deficit.
Over several weeks, the clusters grow thick with unfertilized flowers forming at each leaf node along the branches and main stem. The buds fill out with supporting fresh, moist stigmas reaching out for pollen. Just as the cluster looks like growth is finished, a new wave of flower growth begins, usually concentrated in a relatively bare spot. Successive waves of flowers may grow for weeks.
With most commercial varieties, flower ripening starts between the fifth and eighth week. The bracts (ovaries) start to swell. These are false seed pods; the flowers have not been fertilized and no seed can develop. The swollen bracts and underlying flower parts is one indication of ripeness. It begins about two weeks before maturation, so the timing depends on the variety.
Capitate trichomes, the tiny stalk-like resin glands that fill with THC, terpenes, and other cannabinoids, start to grow on the leaves surrounding the flowers. The flower areas will become totally covered with resin glands.
For the first 5 to 10 days after the light regimen changes to support flowering, vegetative growth slows and may stop as the plant enters the reproductive stage. Then the buds start explosive growth that lasts from four to six weeks, depending on the variety. Varieties that ripen in seven weeks usually spend about four weeks in this period of heavy flower growth. Long-maturing varieties linger in this stage for five weeks or more, maturing nine weeks after flowering is initiated.
During this time, trichomes become more prominent and stand more erect. The cap that tops each one swells with resin. The viscous, sticky liquid contains terpenes and cannabinoids such as THC, which are produced on the inside membrane of the trichome cap. As the resin accumulates in the cap, the flowers’ odor becomes more intense.
The explosive growth slows and then ends as the buds begin to ripen, that is, change form rather than grow during the 10-15 days before ripening. The buds contain the most THC and some other cannabinoids at this point, although the terpene content is probably still climbing. If the plants are being grown primarily for concentration or extraction of cannabinoids, this is the best time to harvest.
As the buds continue to ripen, the odor reaches a peak at the same time the trichomes begin to fluoresce in the light, twinkling like little crystals. In some varieties, they are so prominent that the whole bud sparkles.
Using a magnifying glass, a jeweler’s loupe, or a microscope, the buds’ progression is monitored to the peak of ripeness by watching the resin in the gland tops. Under magnification, the individual glands are visible as they mature and turn from clear to cloudy white to amber. When they begin to change from clear to amber or cloudy white, the buds should be harvested. This is the peak moment.
With the increase in popularity and use of cannabis concentrates, both solventless processed extracts and those made with butane, propane, and other solvents, the peak harvest times for specific uses of the plant may differ. (See When & How to Harvest.)
Research conducted by David Potter at GW Pharmaceuticals is instructive. Potter found that in plants that ripen at nine weeks, peak THC occurred between weeks six and seven. High THC potency rather than terpene production is sought by producers of distillates, so harvesting at seven weeks is more productive than waiting for ripening.
After week seven the plants continued flower production and weighed a slight bit more at nine weeks after inducing flowering and had a much higher terpene content. For growers of cannabis bud for medical or recreational use, the nine-week marker for peak terpenes is important.
Variations on Light and Flowering Regimen
The flowering response to different light cycles is a graduated one. Plants that initiate flowering at a particular light-to-darkness ratio put more energy into flower growth when the length of darkness is increased. This response is more pronounced in plants such as indicas that originated at higher latitudes where the light cycle has more seasonal variation.
All varieties respond to a longer dark period by hastening ripening. Shortening the light regimen down to 10 hours of light and 14 of darkness forces all plants, indicas as well as long-flowering sativas, to ripen faster.
Fig. 1 Average flower biomass per plant between weeks 3 and 9 of flower production. Fig. 2 Average THC concentration between weeks 4 and 9 of flower production. Weeks 3 through 6 of harvesting show a steady increase in yield with the final three (7, 8, 9) weeks not showing substantial increases. The THC % is not substantially different between weeks 6 and 7. According to these data, the optimal time to harvest this 9-week cultivar is at 7 weeks, when THC content and biomass production are no longer increasing. SOURCE: David Potter, Handbook of Cannabis, edited by Roger G. Pertwee
A consistent, uninterrupted dark period is key to good results for cannabis and other long-night flowering plants. Chrysanthemums and poinsettias are examples of plants with similar growth patterns and flowering behaviors that have been studied extensively by the greenhouse industry. Researchers found that the largest flowers with the highest total weight are grown when the dark-cycle routine is provided each night. When the plants were in darkness only six nights a week, there was a slight diminution of flower size and total weight. With each additional lost night, flower size and weight dropped.
Without consistent dark periods of sufficient length, cannabis buds elongate and grow looser. Every time the dark period of the flowering cycle is interrupted, there is a slight loss of flower-growing time and thus of yield. A spate of irregular start and finish times may also stress the plant to the point that it becomes hermaphroditic.
Some cannabis varieties have a flowering trigger that they respond to under normal growing conditions, but when they are accustomed to an unusual light regimen, they may respond to the change in the light conditions in unusual ways. For instance, early-flowering indicas normally trigger when they receive a minimum of 10 hours of darkness, but when they are grown under continuous light, a regimen of just 8 hours of darkness, they initiate flowering.
Once indicas are triggered, the light cycle has little effect upon them. The developing flowers are not as sensitive to occasional interruption of the dark cycle. Indica-sativa hybrids, early-flowering indicas, and South African varieties react similarly. These plants don’t revert to vegetative growth as easily as some sativa-indica varieties, so the plants are harder to regenerate.
Cold may hasten sexual expression but not flower development of some northern varieties. Cold weather slows growth, lowers yield, and delays ripening. In autumn, gardeners often protect their northern-variety plants from bad weather, waiting for a few days of warm, sunny weather so the buds will ripen.
Males of most varieties indicate under continuous light in three months. Some equatorial sativa males are exceptions and require a dark period to flower. Most cultivars will show male flowers sooner than their female counterparts, given the same light schedule.
Autoflowering Plants
Males and ruderalis varieties from the far north are not photosensitive at all. Both age and development play a role in determining when these plants flower. Ruderalis develops flowers under continuous light within a few weeks of germination.
This trait has been bred into several varieties that are available commercially as “autoflowering” plants. No matter what light regimen they are growing under, they will germinate, grow, and flower in a predetermined pattern. Flowers ripen between 60 and 100 days from germination. For the most part, these varieties produce small plants. They are very difficult if not impossible to treat as clones, so feminized seeds are helpful when growing a garden of autoflowering plants. Clones are set to flower at the same growth stage as the mother plant they were cut from, and ruderalis plants have already initiated flower when they are large enough to take cuttings.
Critical Light Period
The 12/12 light-dark period formula has been accepted without question by growers all over the world. Probably one of the reasons was that this advice was common to most early grow books, including Marijuana Grower’s Guide (1974), the first serious cannabis cultivation book ever published in the US, written by Ed Rosenthal and Mel Frank. Subsequent books adopted this advice over the decades since. The 12/12 regimen was arrived at based on the reasoning that no matter what critical period a specific variety might have, 12 hours of darkness was sufficient to induce flowering. It was an easy and reliable heuristic; however, most cannabis varieties need fewer than 12 hours of darkness to initiate flowering.
If cannabis plants grown outdoors required a 12-hour dark period to flower, they would not be induced to start flowering until the autumn equinox, when day and night are of equal length. They would ripen six to eight weeks later. Most modern varieties ripen in the early fall. Budding was triggered six to eight weeks earlier.
For instance, under natural light an eight-week variety that requires 55 days from forcing to maturity starts flowering in late summer and ripens in early fall. In San Francisco on August 5, sunrise occurs at 5:14 a.m. and sunset at 7:18 p.m., a total of 14 hours 4 minutes. Dawn and dusk add another 15 minutes of red light. Plants use the absence of this spectrum to measure the dark period. The total lit period comes to about 14 hours and 20 minutes, leaving 9 hours and 40 minutes of darkness. Thus the critical period for this plant was 9 hours 40 minutes. Indoors, if it was given just 10 hours of dark period daily, rather than 12 hours, it would still flower.
Cannabis responds to a lengthening dark period while it is flowering by hastening ripening. Buds ripen faster when they receive an extra hour of darkness after the first three or four weeks of flowering.
Using Critical Time Period Indoors
Outdoor growers who discover the critical time needed to induce flowering can use this information to make more efficient use of their indoor garden. Currently, plants grown under a 12/12 cycle spend half their time in the dark. If the plants have a critical flowering time of, for instance, 10 hours of darkness daily to induce flowering, they can be provided 14 rather than 12 hours of light each day so that they receive almost 17% more energy with which to produce sugars used for more and faster growth.
Two stigmas are attached to each female flower. Both must be pollinated. One becomes the seed and the other the endosperm, food for the embryo.
During the first week of flowering, stigmas begin to appear. If stigmas are unpollinated they remain white for a long time. These plants have been flowering for 24 days.
By day 38 the plants are in full flower and put out layer after layer of stigmas. The bud gets thicker and thicker, and eventually quite hard.
By day 55 the buds start to ripen and the stigmas start turning brown.
All photos by Phil Sullivan / Team Terpene
This Garlic Breath plant has trichomes at varying stages of ripeness. The clear trichome heads are underripe, the milky ones are ripe and the amber ones are overripe. Photo: Kandid Kush
Manipulating Light Outdoors
Perhaps nature didn’t get it quite right on cannabis ripening, as far as humans are concerned. If ripening were determined by human needs, the buds would ripen in late spring and be ready to be enjoyed in early summer, the social season. If they ripened at the best time for the farmer, it would be in late summer when the weather is warm and the sun is still shining but it has lost a bit of intensity. Nature has chosen the fall. The harvest can be good, but there’s more of a chance of bad weather.
With a bit of effort, the outdoor light cycle can be manipulated to grow and flower the plants at the grower’s convenience. To get the most potent and cosmetically beautiful flowers, it is best to harvest during the summer rather than the fall. Buds that ripen in mid or late summer experience more intense light and much more UV spectrum light than fall-ripened buds. The intense light gives the plant energy to grow a bigger bud. The UV light increases its potency. At the same time, stretches of bad weather (cold, rain, snow, and wind) are much less likely to occur in summer than in the fall in northern latitudes.
The further a region is from the equator, the greater the seasonal variations in light period.
The easiest way is to choose a cultivar that naturally flowers early in the season. For instance, Ed Rosenthal Superbud begins flowering at 37° N around July 16, when there are only 10 hours of darkness.
During the late summer, through winter and early spring, the dark period is long enough to force almost all cultivars to flower. Sativa-dominant hybrids continue to grow even after they have been transitioned to flowering. Indicas can grow from as low as 30 to 100% larger after they are forced to flower.
It is difficult to grow some popular hybrids and indicas outdoors without light extension in many parts of the southern US and regions south toward the equator because the variation in light duration between summer and winter is slight. The dark period is never short enough for the plants to grow vegetatively. Sativa and sativa hybrids produce a better yield in those conditions. Growing these plants at low latitudes requires extending the light period.
The technique is to manipulate flowering so plants continue to grow vegetatively until they reach the desired size before letting them flower. To do this, the dark period is interrupted with light for as little as a few seconds. Think of it as a water spray. All the leaves should be “wet” with light, but once they have been sprayed, they need no more light. The light has reset the phytochrome to the nonflowering mode, and it will require about two hours to return to the flowering position, but the plant will not be in the uninterrupted dark period mode long enough to switch the plants to the flowering mode.
This short interruption of the dark cycle is enough to reset the time count of uninterrupted darkness. By lighting the plants once or twice during the dark period each night, the plants continue to grow vegetatively rather than initiate flowering. When the light interruption stops, the plants immediately start to flower.
As long as there isn’t excessive heat, rain, or wind in the forecast, plants can be started any time of the year. The denser they are planted, the less time it takes for them to fill the canopy. Then the lights that interrupt the dark cycle can be turned off. The plants will immediately start to flower. As long as the weather is warm and won’t be lower than a night temperature in the 40s F (5°C), the next crop can be planted. Winter harvests are not as large, potent, or attractive as summer crops, but they are worth the effort, especially if the harvest is to be used for concentrates.
The upper line shows sunlight reaching the top of the canopy. The lower line shows light reaching the under-story. Plants absorb or reflect most of the blue, green, yellow, and red light. Some of it is used for photosynthesis. Far-red light (730 nm) is mostly transmitted or reflected, so there is a much higher ratio of far-red to red light under the canopy than in full sunlight.
A. If the plants receive 18 hours of light, they do not flower. B. If the light cycle is interrupted by a dark period, the plants will not flower. C. When the plants receive 10-12 or more hours of uninterrupted darkness, they flower. D. When the dark period is interrupted, the plants won’t flower.
In areas that are not quite as warm or as bright, the growth and yield can be increased and the season extended by enhancing natural light with reflective material and electric lights. This is easier to accomplish in small gardens.
To get the best possible harvest, force plants to flower in the late spring to early summer. They will be ready to harvest in mid to late summer. This is the time when the UV and light intensity are strongest, so photosynthesis is occurring at a fast pace, and at the same time the UV light is at its strongest, stressing the plant and increasing its production of THC and terpenes.
Early Harvests & Photoperiod Manipulation
Some zones experience a mild winter, and the light is intense enough and the temperature sufficiently warm to start the garden. In other areas a greenhouse can extend the season by a month or two. The only problem facing the gardener is providing the 16-18 hours of light the plant needs for strong vegetative growth.
Plants started indoors or in a greenhouse with extended light hours in the winter will immediately start to flower when placed outdoors or in a greenhouse using only natural light in the early spring, when there is a long dark period of 10 or more hours of darkness.
Autoflowering plants are excellent choices for early harvest gardens, since they are genetically programmed to ripen in 75 to 90 days and are not affected by light periods.
Late-Summer Crops
A late-summer crop can be generated by moving starter plants, clones, or indoor plants outside late in the season. Many varieties start to flower when they are exposed to 10 to 11 hours of darkness daily.
Plants placed outdoors in midsummer have a chance to grow a bit during the waning days of summer before longer autumn nights force them to flower.
Sativas continue to grow a bit even as they transition to flowering. As flowering progresses, they put all their energy into reproductive development, producing long colas filled with buds. They often require 90 days or more to ripen, so they can only be grown where the growing season extends into the fall.
During the intense days of summer, indoor plants moved outside require conditioning in shade before being placed in bright light. An antitranspirant can be used to help the plants withstand sunburn and adjust to UV light, which is mostly absent indoors under HPS lamps. Plants grown under lights that include UVB spectrum require less adjustment to outdoor light.
Forever Flowering greenhouses have been designed around the specific needs of cannabis growers for over 15 years. Light-deprivation greenhouses are weather resistant and come with automated darkening curtains to control the photoperiod at the touch of a button.
Winter Crops
Plants placed outdoors from late summer through the beginning of spring are triggered to flower by long nights. This technique can be used in subtropical and low-latitude mild-climate zones such as Florida, the Gulf Coast, Hawai’i, Southern California, the Mediterranean zone, parts of South Africa, New Zealand, Australia, Chile, and Argentina, all of which receive sunlight intense enough to support fast growth in winter and rarely drop below 45°F (7°C).
There are many advantages to growing winter crops:
- They require less irrigation than summer crops.
- Non summer crops grow in a better temperature range. Temperatures lower than 85°F (29°C) support rapid growth and dense bud development. Temperatures above 90°F (32°C) stress the plant, decrease growth, and result in looser buds.
- Plants are less likely to be attacked by insects because fewer of them are around, and they are less active in lower temperatures.
- The plants stay much smaller and are more controllable.
Greenhouses
Greenhouses are ideal environments for extending the garden season. Even unheated greenhouses add three weeks of growing time to both the beginning and end of the season. Two weeks of growing time can be gained back using passive heating techniques such as blackened water containers, which absorb sunlight and radiate heat at night. Depending on the zone, heated greenhouses can be used all winter.
Jump-Starting Flowering
Long periods of uninterrupted darkness are nature’s trigger to cannabis to begin flowering. This is measured chemically by phytochrome, which is deactivated by red light. The inactive form is referred to as Pr. In the absence of red light, with a peak at 666 nm and effects from about 500-700 nm, Pr drifts into its active form Pfr over about two hours. This begins at dusk or when the lights are turned off or during dusk outdoors. This transition period, during which Pfr has limited effectiveness, can be dramatically shortened.
Pr is sensitive to far-red light with a peak at 730 nm and is affected in a range of about 700-750 nm. In its presence it changes almost immediately to the active form, Pfr. This effect may be useful for shortening the two-hour time it takes plants to switch from the inactive to active form.
Indoors, after the grow lamps are turned off, when the plants are exposed to far-red (730 nm) light, the Pr is turned into Pfr much faster and induces flowering within a shorter dark period. Far-red lighting can be provided using LEDs or some fluorescents. Far-red light can also be used to restore the active form of the hormone if the dark is interrupted by light. This may somewhat ameliorate the consequences of interruptions in the dark period.
Outdoors, growers have no control over dawn and dusk, but can force flowering out of season by using far-red lighting to increase the time the plants are under Pfr’s flower-inducing influence by 2 hours. Plants receiving 15 hours of light and 9 hours of darkness react as if they were under a lighting regimen of 11 hours of darkness because of the additional 2 hours of active hormone. Most plants initiate flowering under 11 hours of darkness, which is shortened to 9 after exposure to 730 nm far-red light. The plants should be exposed to far-red light each evening at the end of dusk. They need only a few seconds of the light each night.
Unheated greenhouses are often used to grow late-flowering sativas that need some protection from the changing climate. They can also be used to produce a harvest through late planting. By supplementing the weak sunlight of late autumn using lamps during the lit period, the yield can be increased substantially. The amount of light used during the daytime can be adjusted to take into account the sunlight’s intensity. For more information on light supplementation, see Light.
Yield is also increased by keeping the plants on a 12-hour schedule by lighting the plants at full intensity throughout the lit period. This can be done outdoors.
UVB Light & Flower Forcing
Cannabis has evolved a very successful survival strategy. It domesticates easily and escapes domestication just as easily. To make a change, there has to be an advantage for at least one species. Humans have always respected cannabis as a fall flowering and ripening crop. However, it turns out that human interests and the plant’s natural proclivities have diverged.
John Lydon published his PhD thesis in which he reported on experiments he performed on cannabis that showed the amount of THC that a cannabis plant produces goes up in a direct ratio to the amount of UVB light that the plant receives (Lydon 1985).
The relevance of this information to this discussion is that the angle of the sun to the Earth is most perpendicular on the first day of summer, when UVB light is most intense. As the angle of the Earth and sun becomes more oblique, there is a higher proportion of light from the red spectrum and less blue and UV. By the fall, when sinsemilla normally ripens, not only is the light far less intense, but the amount of UVB being delivered is a small fraction of the amount that is received on the first day of summer.
Cannabis buds that are ripening under the intense sun of early summer grow bigger and denser and are more potent than when they ripen under the waning sun and variable weather of the fall.
To induce flowering, plants should be placed under 11-12 hours of darkness each day. There are many ways to accomplish this. They range from moving the plants to and from a dark place daily, to covering the garden using an opaque plastic sheet over tunnels, to automated blackout gardens.
To force plants to flower, timing must be punctual and take into account the daily changes in dawn-dusk times to make sure that the plants are getting enough darkness. There are automated systems available that regulate greenhouse curtains.
There are two ways to approach restricting light: darken in the morning or at night. Morning darkening is the preferred method. The shade material is placed over the garden each morning before dawn. The plants need to be shaded counting forward 11-12 hours from dusk. If dusk is at 9:00 p.m. and dawn is 6:00 a.m., curtains should cover the garden before dawn, anytime before 5:30 a.m. They should be removed at 8 a.m. after receiving no light for 11-12 hours.
The other method is to install the curtains in the evening. Count back 12 hours from dawn, which occurred at 6 a.m. in the above example. The curtains are placed over the garden at 6 a.m. Then at 9 p.m., after dusk ends and night begins, the curtains are removed so that any condensed moisture can evaporate into the night air.
To force the plants to flower during the summer, they need to receive the critical dark period each day. Plants are covered with a blackout cloth each day so that they receive 11-12 hours of darkness and are forced into flowering.
In late stages, mold prevention measures should be taken because moisture levels often build up when the plants are covered. Spray the plants with potassium bicarbonate, diluted milk, or other organic fungicides to prevent fungal attacks.
The advantage of placing the curtain over the garden early in the morning rather than before or after dusk is that dew settles after dusk because the temperature drops, increasing relative humidity. Plants stay wet under the tarp. When the tarp is spread in the morning, there is less moisture to deal with. The farther from the plants that the coverings are placed, the less of an effect that condensation has on the garden.
Some of the very best outdoor cannabis is grown using this technique. The reasons are more intense light, more UVB light, and better weather conditions with less chance of cool weather that slows growth and development.
UVB Light Facts
Ultraviolet B (UVB) light is a spectrum of light that is invisible to humans but is visible to insects and many other organisms. On humans it causes suntan and sunburn and is implicated in the formation of eye cataracts. Tanning bulbs emit UVB light.
UVB light affects cannabis potency. The potency of high-quality cannabis increases in direct ratio to the amount of UVB light it receives. This is very significant. In commercial markets, many dispensaries reject much of the fall-harvested outdoor material as inferior. They have found it lacks the potency of indoor crops and is a harsh smoke. However, cannabis grown outdoors that was forced to ripen in the summer was accepted—because it had a high potency and lacked the “aesthetic qualities” of outdoor crops harvested later in the season.
Indoors, under fluorescent and high pressure sodium (HPS) lamps, gardens receive little UVB light. Metal halides (MH) without glass reflectors emit a bit. However, there are ways of supplying the garden with UVB light. Tanning lamps work, that is, lamps that tan people, because of the UVB light they emit. Using tanning lamps increases the THC content of the crop. Reptiles and lizards require the UVB spectrum to stay healthy, so 10% of the output of “reptile fluorescent” lights is UVB. Tanning lamps and UVB-emitting LEDs are available on the internet. UVB supplementation needs to only be a small percentage of the total illumination used in the flowering space. This technology works, but is still in development. Start by using tests in the garden.
Green Light at Night
Plants have evolved for hundreds of millions of years and have never encountered a separation of light spectrums or unusual lighting regimens. When they received light, it came from the sun in a mixture of spectrums from which they could pick and choose. With the advent of gas and then electric lighting, plants encountered unusual regimens and splintered spectrums.
Plants measure day length using the red and far-red light spectrums. While they use other spectrums, they are not sensitive to most of them as far as flowering is concerned.
They use less green light than other spectrums and reflect much of it while absorbing most other spectrums. Plants’ insensitivity to green light can be used to a gardener’s advantage. Turning a light containing red or blue on in the middle of the dark cycle disturbs the plants’ flowering paradigm. HPS, fluorescent, and MH lamps all emit red light. Green fluorescent and LED lights contain no red light and will minimally disturb the flowering plants, allowing growers to enter the garden during the dark period.
Extreme Defoliation aka “Schwazzing”
In most conventional cannabis gardens, the cultivator removes leaves to improve air flow and increase light penetration throughout the canopy, which is meant to increase yields. The goal is to remove leaves (especially those that are being shaded or shading other leaves) during the early part of the flower cycle. Deleafing, or defoliation, provides the added benefit of directing carbon allocation in the plant.
Leaves are either carbon sources or carbon sinks depending on how much light they are getting. If a leaf is getting plenty of sunlight and is actively photosynthesizing, it becomes a carbon source. A carbon source provides sugar to other parts of the plant. A carbon sink is a part of the plant that accepts sugar from the carbon sources. Shaded leaves end up being carbon sinks, as they photosynthesize minimally. Removing leaves that are carbon sinks increases the carbon use efficiency by allocating more sugar to the flowers. Optimizing carbon allocation to the flowers increases yield and potency. This is one of the primary reasons why proper deleafing can increase yield in cannabis canopies.
This is a schwazzed plant. To sustain healthy growth during the schwazzing process, growers should provide plants with an optimal feeding regiment and nutrients specially formulated to provide macro and micronutrients delivered at exactly the right time during the plant’s life cycle. Courtesy of Joshua Haupt / Three a Light
Schwazzing is one method of extreme defoliation. First made popular by Joshua Haupt, author of Three A Light, a good schwazze requires intense defoliation during two separate times within the first three weeks of the flower cycle. The first defoliation occurs on day 1. Removing all the fan leaves on the first day of flower seems counterintuitive, but it increases light penetration to the lower bud sites and allows for more air flow through the canopy. On top of that, the plant is now forced to allocate carbon to producing new leaves. Instead of allocating resources to elongating the stem during this stage of growth, the plant must build new leaves, resulting in shorter stems and more compact nodal stacking.
Schwazzing may feel uncomfortable and unnatural to many cultivators, but the science behind it is sound. More light penetration on more bud sites and allocating more resources to those flowers as opposed to stem elongation are the fundamentals behind schwazzing.
After the first schwazze, it is important to temporarily increase the amount of nutrients the plants receive. The big fan leaves, which are removed during the schwazze on day 1, store nutrients. Removing those leaves also removes that nutrient storage pool that the plant can rely on if it needs an added boost of nutrients when actively growing. From days 6 through 10, the nutrient solution concentration should be increased by about 20%, primarily focused on added phosphorus and potassium.
The second schwazze occurs on day 20 of the flowering cycle. It is important not to remove any of the buds, as this is where the flowers develop. Again, the removal of the leaves forces the plant to allocate resources to new leaf production as opposed to elongating the stems. By the time the new leaves have been fully developed, the plant is now in full flower development mode and has bypassed the stage at which stem elongation occurs. Shorter stems mean that more of the buds will develop into each other, creating larger buds.
Extreme defoliation, as opposed to removing lower branches to increase air circulation and light penetration, has the added benefit of not removing potential flower sites. Each bud will develop into a grade A flower if it gets enough light. The increased light penetration provides energy to bud sites lower in the canopy, so that the total volume of high-quality flowers increases, thus increasing yield.