WHEN & HOW TO HARVEST
Harvesting, drying, curing, and storing are incredibly important processes to growers. Each contributes to the final quality of the flowers and the end-use products processed from them. A good crop followed by poor post-harvest practices is as disastrous as crop failure.
Even a small harvest requires a little bit of labor. Processing a large one adds complexity and requires planning. Determining when to cut the crop, whether to cut whole plants or to judge each bud for its ripeness, when to trim, how and where to dry and cure the buds for use or processing, and how to preserve the aroma and taste by retaining the terpenes and essential oils are important decisions for any harvest.
Growers usually plan their gardens in great detail by setting timetables and cultivation parameters for their plants and bringing the plants to the peak of maturity. However, they often spend a lot less time planning the harvest. Even large commercial operations are sometimes far behind in harvest and post-harvest operations. These final tasks need the most thought; they are the ones most likely to be improperly executed—but they needn’t be. With proper preparation the process can flow seamlessly.
For larger operations, try to divide up the tasks so that there are few bottlenecks. The task flow can be divided in many ways. For instance, if a grower has access to an area to store branches or colas but there are insufficient trimming facilities, then bringing in the buds and pulling off fan leaves should be prioritized. The colas can be placed in storage to dry before trimming or under refrigeration to keep them fresh for wet trimming. Then the material can be manicured over a longer period of time.
No matter the situation, plan ahead and prepare for the coming harvest.
The harvest consists of several tasks (not always in this order):
- Cutting the colas (tops) from the plants
- Cutting (bucking) the buds from the branches
- Trimming the buds
- Drying the buds
- Curing and properly storing the buds
Make sure to prepare for each stage. In many cases, even on large farms, not much has changed over the years. Growers tend to get set in their ways; their gardens are easily dated by the techniques they employ. Not all growers realize that new equipment is available to increase efficiency and scale back the manual labor associated with the harvest.
As soon as the plant is cut down, instead of focusing on terpene production, growers must focus on terpene retention.
If buds aren’t processed correctly, a perfectly good crop can be destroyed. Proper harvesting includes keeping the buds clean and fresh, and preserving flavors and aromas. Cannabis freshness is based on two factors: maintaining enough moisture in the material for it to be pliable without inviting mold, and, just as important, retaining the terpenes found in the trichomes. Terpenes are volatile oils that volatilize at different temperatures, some of them as low as 68°F (20°C). For this reason, at no time during the drying and curing process should the temperature be raised very much above this point.
Before it is consumed, cannabis is judged first by appearance and then by smell. Fresh, aromatic buds are the most likely to be demanded and consumed. It’s imperative to plan ahead, know the bottlenecks and schedule accordingly but, most important, as a grower, it is important to know what to do and why.
The Harvesting Process
Picking
Picking is the process of cutting the buds, branches, or whole plant from the root before trimming and/or drying.
Bucking
Bucking is the process of removing the flowers from the stems.
Manicuring
Manicuring is the process of trimming the small leaves that are growing around the buds to sculpt them into shape. This step can be done wet or dry.
Fresh, “wet” vegetation is turgid while it is being clipped, so it is easy to handle. Just as important, the trichomes that hold cannabinoids and terpenes are pliable rather than brittle and are more likely to stay attached to the plant. A downside of trimming wet is that it can require cool storage and a larger trim team or mechanization to keep up with the rate of harvest.
Allowing the crop to dry before trimming eliminates bottlenecks by allowing for a more extended trimming period. Although trichome loss will occur with either method, dry trimming may lead to a higher rate of trichome separation.
Growers sometimes manicure a bit while the plants are still standing. The plants are in a convenient position to remove fan leaves and other vegetation so that there is less damage to the bud. Use this technique only a few days before harvest.
Drying, Curing & Storing
The purpose of drying is to remove enough moisture from the buds to prevent mold developing in storage. Once the plants are initially dried, a second-stage drying process called “curing” is employed to achieve uniform drying while maintaining a slightly spongy texture and a smoother smoke. When properly cured, buds can age nicely for several months. Climate controls and processing techniques are crucial to successful drying, curing, and storing.
Processing
Many plants are not destined to end their journey as simple buds. Growers often will process their harvest into concentrates and extracts, and/or infuse the oil into foods, lotions, and tinctures after harvest.
This Oakland garden harvests, processes, and dries large quantities at once. Photo: Ed Rosenthal
Strategizing for Harvest
Lay out a strategy based on the goals of the garden. (See Goals.) If the crop is to be used for its flowers, then the harvesting method may be different from that utilized for extracts and concentrates. As with any blooms, cannabis flowers should be handled gently and with care. Biomass, the entire product of the harvest, is used for extracts or concentrates and can be handled without concern for cosmetic beauty.
There are many strategies to consider. Some require planning during variety selection or before planting, while others can be decided later in the season. Numerous factors play a part in these decisions.
One of the first major decisions a grower faces is how big the garden will be. Whether the plan is for a hobby garden or a large commercial operation, the goals should be realistic. Assess the resources required in labor and capital and determine whether those resources are available to meet the expenses of production.
With that in mind, consider labor as a factor of time. The main labor factors are setup and harvest. Both of these operations can be carried out over a longer period by a smaller group, or in a shorter time by a larger group.
Personnel and Timing
A smaller group can more easily handle a crop that is harvested over several weeks versus a crop that is harvested all at once.
The biggest factor to consider when choosing a time to harvest the crop is ripening. The efficiency and rate of turnover of each round of flowering also must be considered. It is important to strike a balance between harvesting at peak ripeness and yield versus optimizing the speed and frequency of harvests each year.
Growers must make these choices about how and when to harvest based on their garden and workplace circumstances, ripening time, and desired end use.
Ripening
Ripening times vary by variety, and growers may choose to harvest at different stages of growth in order to obtain the desired effects and use. Before cutting down the plants, the grower must determine when to harvest.
The Ripening Process
Plants and varieties differ in maturation pattern. Some mature all at once, so that the whole plant can be picked. Other varieties mature from the top down or from the outside in. For these varieties, the buds on the outside mature faster than inner buds hidden from the light. Once the outer buds are harvested, the inner branches are exposed to light and quickly ripen. During a staged harvest, it can take up to two weeks of choosing mature buds before the plant is totally picked. Picking the plant a little at a time ensures that every bud is at maximum potency and quality.
A plant’s flowering cycle, and its ripening and harvesting time, are variety specific. Each variety is programmed to respond to a critical period of darkness that turns growth from vegetative to flowering. Indoors, this is accomplished when the lights are cut back to 12 hours. Outdoors, the critical dark period usually varies between 9 and 11 hours of darkness. In addition to genetics, flowering time is also affected by light intensity and total light received on a daily basis (DLI), ambient temperature, and nutrients.
The best way to determine the picking time is by watching the development of the trichomes (the stalk-like resin glands that contain the active compounds), which grow on the leaves surrounding the flowers. The flower area becomes covered with these resin glands over time. Tools are available that can measure cannabinoid content in flowers and monitor for its peak when choosing a harvest date. Growers can also use magnification to assess the size, color, and clarity of the trichomes. Combined with lab testing at various points of maturation, visual cues can be used during future harvest rounds to estimate when peak potency has been achieved for each cultivar.
Late-season and long-maturing varieties usually spend about three to five weeks in this period of heavy trichome growth.
As flowers near ripeness, their caps swell with resin and the trichomes become more prominent and stand erect. The viscous, sticky liquid that accumulates contains terpenes and cannabinoids, which are produced on the inside membrane of the trichome cap. As the resin accumulates in the cap, the flower odor becomes more intense.
Photo: Picture Fotografie
Types of Trichomes
Bulbous Trichome
Bulbous trichomes have no stalk and are much smaller than the other trichomes. They appear mostly on leaves rather than in the bud area, especially during vegetative growth, and contain cannabinoids.
Crysolith Trichome
These trichomes do not contain cannabinoids. They grow on the bottom of the leaves to deter pests.
Sessile Stalked Capitate Trichome
These trichomes appear during the vegetative growth stage and produce only small amounts of cannabinoids.
Stalked Capitate Glandular Trichomes
These trichomes are the most abundant and contain the desired cannabinoids, terpenoids, and flavonoids that growers seek.
The odor reaches its peak at the same time that the trichomes begin to fluoresce in the light, twinkling like little crystals. In some varieties, the trichomes are so prominent that the whole bud sparkles. Using a magnifying glass, a jeweler’s loupe, or a microscope, monitor the buds’ progression to the peak of ripeness by watching the resin in the gland tops.
Under magnification, individual glands can be seen turning from clear to cloudy white or amber. This has traditionally been considered the peak harvest time. Cultivators differ in their assessment of the peak harvest period. The controversy is whether the harvest should occur when the trichomes are mostly clear, cloudy, or amber. Plants harvested at each of these stages may produce different effects. Lab testing can be used to determine peak potency and to target harvest dates by desired outcomes.
Ripening cannabis reeks of pungent terpenes, and each day brings increased intensity of odor. Rub the leaves surrounding the bud between clean fingers and inhale. This releases aroma molecules while leaving fingers sticky with resin. Inhale and smell an exotic medley of familiar and unusual odors that may range from sweet to acrid, musky, and skunky.
There are various factors to take into consideration before deciding when it’s time to harvest:
- Plants of the same variety flower and ripen at about the same time. Clones from a single plant grown under the same conditions flower and ripen at the same time.
- In most varieties, flowering time is determined by the light regimen, not a plant’s age or size.
- When all the buds on small plants get direct sunlight, they tend to ripen at the same time.
- Buds on large plants that are directly lit, whether on the top of the plant or on the sides, mostly ripen at the same time.
- Outdoors, big plants grown with large spaces between them will often get light from three different sides as the sun moves around them. In the Northern Hemisphere, light comes from the east, west, and south; in the Southern Hemisphere, light comes from the east, west, and north. This light pattern is especially prevalent in the autumn, when the sun is at an acute angle and lower on the horizon.
- On large colas, outer buds often ripen, while the inner buds are deprived of direct sunlight.
Harvesting Early or Late
Traditionally many growers have determined when to pick based on the 30/70 rule. They harvest when about 30% of the trichomes have turned amber and the remaining 70% are milky white. This may not be accurate and results are cultivar specific. More research is needed.
Some growers will harvest late, with a goal of buds with a higher cannabinol (CBN) content, which is a product of aged and degraded THC and is used to aid sleep.
Some growers harvest early to produce plants with low-enough THC levels to qualify as “hemp.” Cannabis will not continue to ripen after it has been harvested.
Weather
Cannabis flowers in the autumn. At the time when plants could use more light, sun intensity is naturally declining, beginning in the late summer. At the same time, the weather may change from balmy summer to cool autumn, sometimes with wind and rain—weather not conducive to great buds.
A grower may have to make hard decisions and compromises. Consider the following situation: the buds are ten days from early ripeness. The forecast is for cool weather followed by rain and then a long period of sun and warm weather, long enough to ripen the crop. What to do? Cut early and forfeit ripeness, or leave the plants standing and take measures to try to prevent mold attacks? (See Picking.)
Gauging Ripeness
Knowing when to pick a ripe bud has historically been more of an art than a science; however, new research is showing that traditional methods may not optimize gauging ripeness. (See The Science behind Choosing a Harvest Date later in this chapter.) Growers have their own processes and criteria for determining ripeness, but a savvy grower will watch for several signs. Most follow these steps:
Indoors, most growers induce their plants to flower using twelve hours of uninterrupted darkness. However, plants can be forced to flower at a shorter period of darkness. Outdoors, the first signs of flowering are used as an indicator. The true date of initiation for most varieties is 7-10 days before the plant indicates. The number of hours between dusk and dawn is the number of hours of darkness it takes to force flowering. That is the critical dark period. Hours for daily dusk and dawn vary by region and season and are available online. With an online “sunrise-sunset” calculator, the hours between and including morning and evening “civil twilight” make up the total light period. The hours between and including evening and morning “nautical twilight” make up the total dark period.
During the first weeks of flowering, the stigmas become visible. Stigmas are white or pastel-colored hairs that protrude from the newly forming flower bracts. They’re hollow inside and have “brushes” on the outside through which they filter the air containing pollen. When they capture the proper pollen grain, the pollen will form a pollen tube that travels down the hollow stigma to meet the egg (ovule) located in the ovary at the base of the flower bract. If the stigmas are unpollinated, they will retain their vitality for a long time.
By week four, most varieties are in full flower, and they put out layer after layer of stigmas, so the bud gets thicker, denser, and sometimes hardens.
In most varieties the buds start to ripen around week six, and the stigmas start to shrivel and turn brown, orange, or red. The trichomes start to become more prominent; eventually the caps on these glands start filling up.
By week seven or eight, most varieties are about one to two weeks away from peak ripeness. Most of the stigmas have shriveled, and the trichomes are more prominent. The glands continue to fill up with cannabinoids and terpenes and look like mushroom caps. They begin to bulge like balloons. The odor becomes more intense, but the bud is still not ripe. It hasn’t yet reached peak intensity.
On ripe buds, the trichomes are totally erect, and the caps on them are prominent. They’re bulging with resin, the stigmas are shriveled, and the bracts are swollen, sometimes resembling seed pods.
The Science Behind Choosing a Harvest Date
By Thomas Blank, Kymron DeCesare, Reggie Gaudino, PhD, Caleb King, PhD, and Donald P. Land, PhD
Photo: Professor P of Dynasty Genetics
Cannabis cultivators have traditionally relied on visual and olfactory cues to determine “the readiness” of the inflorescences for harvest. “Cannabis lore” includes looking at the ratio of white to colored/bent stigmas, looking at the color of the trichome head, and checking whether the flower smells ripe (green smelling rather than the expected “bouquet”). Due to the diversity and complexity of cannabis, experience with the plant translates to a better product, whether for breeding or producing quality flowers. The term “results/experience may vary” has never been more appropriate.
There is still a limited understanding of the complex interactions and interplay between the different chemical production systems in cannabis. Thus, these data are a first look at the time-series production of phytochemicals during growth. Understanding this process is further complicated by the considerable variation in cultivation and fertigation practices employed by growers.
The reality is that cannabis is very responsive to its environment, and different growing regimens will produce slightly, or even drastically, different cannabinoid and terpene profiles. In the extreme, the predominant phytochemicals from what is the expected profile for that particular variety can be significantly changed.
The question “When is the best time to harvest?” has been the subject of much debate for a long time. Data presented here may not make that decision much easier. Rather, as with all things cannabis, there is no straightforward answer. “Harvesting for what?” is the only appropriate response. As the data presented here illustrate, what the grower wants, whether it be bud weight or maximum cannabinoid or terpene content, can be the deciding factor of when to harvest.
Here, an effort is made to apply the scientific method, using measurements and analytics in chemistry, biology, and environmental sciences, to identify the most important characteristics of the plant, which can help the experienced cultivator or breeder to determine optimal harvest time.
The information gained can be used to better understand and reproduce those aspects of cultivation that affect the final cannabinoid and terpene content. The data are designed to help better understand the production of cannabinoids and terpenes over the life cycle of the plant and to identify the best time, or time range, to harvest for maximum terpene or cannabinoid content.
Cannabinoids other than THCa and CBDa have become of greater interest in the industry, thanks to literature citing the potential therapeutic utility of these chemical species. Cannabinoids of interest in this study included CBGa, CBGVa, THCa, THCVa, CBDa, CBDVa, and CBCa, as well as other minor cannabinoids.
Cannabinoids were monitored during the growth cycle of several different types of cannabis (Types I, II, III, and IV), including a variety that was augmented in production of propyl (C3) cannabinoids. Both vegetative (leaf) and floral (bud) material, cured with standard USDA guidelines (strictly for determination of cannabinoid content, not with final content or bag appeal as final criteria), were tracked from preflower through harvest. The data presented closely resemble published data elsewhere in the scientific literature (Pertwee 2014; De Meijer 2003, 2005, 2009, 2009).
Terpene profiles are strongly influential in entourage effects and the end users’ experience. Cannabis cultivars can be grouped into seven familial clades evolved from a common ancestor and thus share common characteristics based on the most abundant primary terpenes or combinations of the most abundant primary terpenes in the plant oil profile. Plant genes that enable the synthesis of terpenes include highly active terpene synthases responsible for producing the most abundant terpenes in a cultivar.
Clades can involve a single dominant primary terpene or up to three primary terpenes of relatively similar co-abundance. Terpene gene variation in cannabis leads to substantial diversity in cultivar offerings. The potential to breed for specific terpene profiles has sparked new, or perhaps renewed, interest in cannabis terpenes.
Terpene measurements presented here were carried out on flower material that was cured using a procedure that preserves volatile terpene content or “bag appeal.” In this study, terpene content was acquired only at weeks five and eight of flowering due to the time and cost of the preparation and testing. The overlap of sampling for both cannabinoids and terpenes at those two time points allows the comparison of terpene content relative to cannabinoid accumulation.
While the data presented here are scientifically robust, some limitations on what can be interpreted must be outlined so that the data can be viewed with the proper perspective. The data represented here are not a comprehensive survey of all varieties but just a representative sampling of several types of varieties currently available: high THC (Type I), mixed ratio (Type II), high CBD (Type III), and high CBG (Type IV). The information presented here can help make it easier to decide what week to harvest after flower initiation, for best terpene and/or cannabinoid content.
Materials and Methods
Cannabis plants producing predominantly THC, CBD, both THC and CBD, or CBG were vegetatively propagated from mother seed stock. All plants, apart from Type II, were grown indoors in one- or two-gallon (7.5 l) plastic pots supplemented with high pressure sodium vapor lighting, filtered water, and macro- and micronutrients.
Type II plants were grown outdoors in Colorado under summer and autumn field conditions with overhead watering and fertigation. All flower samples were obtained from the top third of each plant, with each plant’s final sample collected from the most apical flower.
Leaf tissue was obtained within the top third of each plant and from three to four different branches. About one gram of dried leaf and two to five grams of dried flower samples were collected weekly from the same plant. Plant tissues were dried in darkness at 25°C (77°F) with dehumidified air between 30 and 50% relative humidity for 7 to 10 days until a consistent dry weight was obtained. Any residual moisture was neither measured nor subtracted to account for dry weights.
Dried samples were milled to a powder, with stems and seeds mechanically removed. Cannabinoids and terpenoids from 0.5-gram powder aliquots were solvent extracted using ultrasonication. A validated reverse-phase high pressure liquid chromatography (HPLC) method was employed to quantify the amount of CBD, CBDa, Δ9-THC, Δ9-THCa, CBC, CBCa, CBG, and CBGa, presented as % weight cannabinoid per mass of herbal material, in the original sample. The sum of acid and neutral cannabinoid percentages do not account for decarboxylation reactions/constants. Terpenoids were quantified using a liquid injection technique on a validated gas chromatography mass spectrometry (GC-MS) method, presented as % weight terpenoid per mass of herbal material.
Results and Discussion
Data were collected for both vegetative (leaf) and floral (bud) material. However, it is important to note that the results show that, in each tissue, the production of cannabinoids and terpenes is not the same, with respect to either the compounds made or the timing in which they are produced.
Fig. 1 - Type III leaf and flower total cannabinoids from the same plants throughout female floral development.
All cannabinoid data were taken after flowering was initiated, unless otherwise indicated. Leaf total cannabinoid data is presented in Figure 1. “Total cannabinoid” is defined as the sum of the “major” cannabinoids (THCa, CBDa, CBCa, and CBGa, as well as their less abundant neutral forms).
Figure 1 compares the cannabinoid production in leaf versus floral (bud) tissue. The data here can be misleading, because they do not take into account the change in mass of the leaf tissue between day 21 post initiation and day 49 post initiation. The downward trend could be simply due to the fact that the leaves increase in size while the level of cannabinoid production stays constant (or drops). A steady rise of cannabinoids in floral tissue until weeks five and six is clearly seen in the data presented, not only in Figure 1, but also in Figures 2-4 as well, regardless of whether the variety predominantly makes THC, CBD, or both THC and CBD. No CBCa-dominant plants were ready for testing at the time these data were being collected. Thus the CBCa values presented in Figure 6C are the natural levels found in the varieties used in this study.
Fig. 2 - Type I total floral cannabinoid development (N = 7 plants)
The Type I cannabis plants described in Figure 2 are atypical of most high-THC-producing varieties. This particular variety produces a significant proportion of minor cannabinoids and matures later. In this particular example, maximum cannabinoid content is more closely tied to maximum flower size (later in the flower cycle means additional accumulation of biomass), and thus the decision process is much simpler.
This sample illustrates the differences between cannabis varieties and further highlights that there is no “one size fits all” solution. Harvest time decisions need to be made differently for the various cultivars and for different growing regimens as they are applied to those cultivars. The variety presented here would be considered a “mid-maturity” variety.
Fig. 3 - Type II total floral cannabinoid development (N = 1 plant)
Type II cannabis plants (mixed ratio, producing both THC and CBD) showed a similar cannabinoid maximum at around six weeks post flower initiation in outdoor trials. A number of environmental factors may have influenced these results, such as an early September snowstorm, which occurred in Colorado at about 28 days post outdoor flower initiation. Total cannabinoid content in Type III plants grown in the same outdoor field plots responded similarly to the environmental perturbation (data not shown).
Fig. 4 - Type III total floral cannabinoid development for two genetically distinct varieties.
Two CBD-dominant varieties are presented in Figure 4. Note that one variety peaks at about day 42 (week six) post flower initiation and drops off sharply thereafter, while the other peaks at about day 49 (week seven) post flower initiation before falling off.
Fig. 5 - Type IV total floral cannabinoid development for two genetically distinct varieties.
Typically, Type IV cannabis plants do not produce significant amounts of “major cannabinoids” (THC or CBD), with the exception of CBC. Thus CBG, the precursor for all major cannabinoid synthesis, accumulates to varying degrees. The data presented here indicate that the peak of CBGa production seems to fall between three and five weeks post flower initiation, which is different from the peak of CBGa production shown in the data for Type III cannabis plants presented in Figure 6D. (See CBG peak in Fig. 6D that seems to be delayed compared with CBG peak in Fig. 5.)
The combined cannabinoid data presented here show that, for THC-dominant, CBD-dominant, and mixed-ratio varieties, there is a general trend for cannabinoid production to reach its peak by about week six after flower initiation (around day 40-42 of flower). For the limited number of CBGa-dominant and CBCa-producing varieties presented here, the data illustrate that the timing of maximum production for CBGa and CBCa may vary.
Terpenes
A key aspect of identifying therapeutic utility of cannabis varieties involves measuring and assessing the plant terpene and terpenoid concentration profile. Terpenes are organic molecules based on 5-carbon “isoprene” units, while terpenoids are terpenes containing oxygen.
Volatile terpenes in cannabis consist of mostly monoterpenes (10-carbon molecules) and the somewhat less volatile sesquiterpenes (15-carbon molecules). For simplicity, the whole group of terpenes and terpenoids is referred to as “terpenes.”
Few plants produce the numbers and diversity of terpenes found in cannabis. Over 100 terpenes have been identified in cannabis, though only a maximum of about 20-25 volatile terpenes are found at significant levels in any cultivar. Each cultivar can have 6-15 volatile terpenes present above trace levels in its profile. Volatility is important in delivery as a vapor. Larger di- and tri-terpenes are not volatile and, hence, cannot be efficiently delivered by vapor.
Fig. 6 - Type III individual cannabinoid time series for two genetically distinct varieties.
A. CBD Development (top left) B. THC Development (top right) C. CBC Development (bottom left) D. CBG Development (bottom right): Variety 1 (square), N = 10 Plants; Variety 2 (circle), N = 10-20 Plants]
Here the discussion is focused on the smaller volatile terpenes that have 10 (2 isoprene units) or 15 carbons (3 isoprene units). Terpenes are believed to be the most critical factor in the “entourage effects” of a cultivar, as they largely determine the cultivar-specific effects experienced by the consumer.
Terpenes are the main constituents in essential oils derived from many plants and have been used around the world for centuries to treat many ailments orally, topically, or by aromatherapy (Dagli et al. 2015; Ali et al. 2015).
Terpenes found in cannabis can also be found in the essential oils of many other plants. Over the last few decades, activities of the individual terpene contents of essential oils have been studied and have demonstrated that there is a wide range of therapeutic activity of individual terpenes.
Individual terpenes have been shown in animal and human testing to produce cognitive benefits, sedative effects, and pain relief, and to influence EEG patterns in the human brain (do Vale et al. 2002; Gertsch et al. 2008; Cho et al. 2017; Russo 2011; Sowndhararajan 2016; Kei et al. 2016; Teixeira et al. 2017; Lim et al. 2011). Some terpenes introduce a calm focus in the mind, while others can cause the mind to race, and still others act on pain, anxiety, and depression.
Fig. 7 - Concentrations of the 10 most abundant terpenes at five weeks and final harvest. The β-myrcene value for the final harvest sample shows anomalous results, and the low value is likely the result of poor handling during the preparation process for analysis by GCMS. β-myrcene is a volatile compound that has poorer solubility in certain extraction solvents and thus is sensitive to extraction methodology.
Each unique variety contains a terpene profile that acts with cannabinoids to produce an entourage effect (Russo 2011). To a large degree, terpenes are believed to be the compounds primarily responsible for the differences in the effects experienced with different varieties.
Investigation into leaf terpenes showed little correlation to terpenes in flower tissue and are predominantly sesquiterpene in content. Therefore, to focus on the “harvest” aspect, only terpenes during the flowering phase are considered.
Data presented in Figure 7 are from a mixed ratio (~1.2:1 CBD:THC) variety grown in a tent using LED lights. The data were collected at five weeks (mid-flower) and final harvest (around week eight or nine). Note that, as early as week five, terpene production seems to have peaked and remains constant through final harvest.
For comparison, production of several cannabinoids also seems to peak around the five- or six-week mark, regardless of whether they were grown indoors, in a greenhouse, or outdoors. Again, it must be reiterated that, while many varieties follow the timeline of peaking between five or six weeks, Figures 2 and 3 are presented to illustrate that some varieties take much longer to finish and thus would require different harvest considerations.
Conclusions
Trichome Color & Harvest
Some growers use color changes in the trichomes as a way to determine a harvest time. As trichome oil loses monoterpenes to volatilization, or as the ratio of percentage of THCa to percentage of terpenes increases in the oil, THCa can be expected to precipitate as a white solid in the oil, giving a milky appearance.
If THCa crystals are contaminated with some other chemicals, they may turn off-white in color, and then amber, as contaminants rise to higher levels. This color change can be expected because some phytochemicals, including terpenes, have a high susceptibility to oxidation and can turn amber or precipitate out of the trichome oil.
A very small amount of highly colored material can have a large effect on visual appearance. Varieties that do not have abundant oxidizable monoterpenes (e.g., White CBG, White Buffalo, White 99, Super Silver Haze) may not show much of the amber color at peak potency. Thus, color change of trichomes is not a reliable or universally applicable methodology on which to decide when to harvest.
Due to the diversity of cannabis, the likelihood of developing a single methodology to determine peak harvest time is unlikely. The combination of finish times, environmental response, and genetic makeup (responsible for the terpenes and cannabinoids produced) combine to make each variety, or at least groups of similarly classified varieties (based on structure, chemical content, photoperiod, etc.), distinct enough that they may need very specific decisions on why and when to harvest, as well as how to cure.
Curing itself is fraught with several different decision points based on what the final application of the flower will be.
As stated previously, the only valid answer to “When is the right time to harvest?” is “What is the end goal of the harvest?” Cannabinoids and terpenes seem to peak around the same time frame, generally two or more weeks before maximum flower weight. Chemically, the plant is ready for harvest before the flower is fully mature. Thus, as with all things agriculture, perhaps maximum yield is ultimately the real deciding factor.