When to Harvest Marijuana

SG Philosophy

We believe it is time to harvest when the hairs have reached about a 40% color switch.  What I mean here is that when about a little less than half of the white hairs have turned some other color (yellow, pink, red, blue, brown) then it is time to pick the plant.  An exception to this is the exotics that are grown outdoors which may color more quickly as part of their characteristic or conditions of environment. In general we find the highest content of THC to be at this 40% range.  If you continue to let your bud go then the THC is changing into something else called CBN. CBN is the stuff that makes you sleepy instead of the euphoric feeling you get from THC.  If you want a couch-lock high then look for a strain that produces this at the optimum THC level instead of the CBN level.  You will have better results and less hangover or after effects.

Backing Up the Stoney Girl Opinion with Science

In the following document you will find excerpts from four of the most prominent authors and scientists in the Cannabis culture. There is lots of technical stuff available for reading.  For a complete background on cannabinoids, including chemical diagrams, resin profiles and production of cannabinoids, follow the links to the authors provided.  I have included these excerpts for backing up the Stoney Girl opinion with science. Please remember that these are just parts of the writings these authors did.  They are all worth checking out to get the whole story.

The Joys of an Herb Garden at Home; v.3

by  Legal Lie Zitt

Harvesting is the reaping of the bounty, and is the most enjoyable time you will spend with your garden.
Plants are harvested when the flowers are ripe. Generally, ripeness is defined as when the white pistils start to turn brown, orange, etc. and start to withdraw back into the false seed pod. The seed pods swell with resins usually reserved for seed production, and we have ripe sinse buds with red and golden hairs.
It is interesting that the time of harvest controls the “high” of the buds.  If harvested “early” with only a few of the pistils turned color, the buds will have a more pure THC content and will have less THC that has turned to CBD and CBN’s. The lessor psychoactive substances will create the bouquet of the pot, and control the amount of stoneyness and stupidness associated with the high. A pure THC content is very cerebral, while high THC, high CBD, CBN content will make the plants more of a stupid, or hazy buzz. Buds taken later, when fully ripened will normally have these higher CBN, CBD levels and may not be what you prefer once you try different samples picked at different times. Don’t listen to the experts, decide yourself based on what you come to like yourself.
Keep in mind, a bud weighs more when fully ripe. It is what most growers like to sell, but take some buds early for yourself, every week until you harvest, and decide how you like it for yourself. Grow the rest to full maturity if you plan to sell it.
Most new growers want to pick early, because they are impatient. That’s OK!  Just take buds from the middle of the plant or the top. Allow the rest to keep maturing. Often, the tops of the plants will be ripe first. Harvest them and let the rest of the plant continue to ripen. You will notice the lower buds getting bigger and fuzzier as they come into full maturity. With more light available to the bottom portion of the plant now, the plant yields more this way over time, than taking a single harvest.
Use a magnifier and try to see the capitated stalked trichomes (little THC crystals on the buds). If they are mostly clear, not brown, the peak of floral bouquet is near. Once they are mostly all turning brownish in color, the THC levels are dropping and the flower is past optimum potency, declining with light and wind exposure rapidly.
Don’t harvest too late! It’s easy to be too careful and harvest late enough potency has declined. Watch the plants and learn to spot peak floral potency.

What is THC, CBD and CBN?

What is THC, CBD and CBN?

Marijuana Chemistry, Excerpted from Section One, Indoor Marijuana Horticulture, The Indoor Bible

Cannabis is the only plant that produces chemicals called cannabinoids,
however, with gene splicing and genetic engineering, it is only a matter
of time until cannabinoids are added to other plants. Cannabinoids are
ingredients unique to cannabis; the psychoactive cannabinoids are
responsible for the mind-bending effects of marijuana. Some cannabinoids
get you high. Around 40 cannabinoids have been confirmed to exist, but
most are not psychoactive. Here is a short rundown on the six most
prominent cannabinoids.
D9 THC, the main ingredient that gets you high, is called:
D9-trans-tetrahydrocannabinol. All cannabis, whether industrial hemp or
drug marijuana, contains some D9 THC. Industrial hemp cannabis contains
infinitesimal amounts while dried flower tops (buds) of potent marijuana
can contain up to 25 percent D9 THC. In potent varieties of marijuana,
perhaps all of the mind-bending effects are derived from D9 THC.
D8 THC is found in very low concentrations in cannabis. It also gets you
high, but there is so little D8 THC in most cannabis that researchers,
breeders and growers concentrate on the more abundant and potent D9 THC.
To simplify matters we will refer to both D9 THC and D8 THC as THC.
Cannabidiol, known as CBD, also appears in virtually all varieties of
cannabis. The amount of CBD varies enormously, from a trace to more than
95 percent of all cannabinoids present in a plant. CBD generally has a
sedative effect regarding the high you experience. CBD, when combined
with THC, tends to postpone the beginning of the high, but, the good
part is that CBD can make it last twice as long. Whether CBD increases
or decreases the force of the high is subjective and must be discerned
by each smoker.
Cannabinol, or CBN, is produced as THC oxidizes or degrades. Only a
trace of CBN exists in fresh bud. Stored and cured tops or hashish have
higher levels of CBN, that has converted from THC. Marijuana with high
levels of CBN generally make the toker feel disoriented and often sleepy
or groggy, often referred to as a stupefying high. At best CBN contains
only 10 percent of the psychoactive potency of the original THC.
Tetrahydrocnnabivarin, called THCV, is the shorter three-carbon propyl
that replaces the five-carbon pentyl chain. This compound is associated
with the fragrance of the plant. In other words, very pungent smelling
marijuana normally contains THCV. Warmer temperatures bring out more
smell. THCV is found in very potent marijuana that originated in
Southeast and Central Asia as well as regions in Africa. Concentrations
of THCV usually make the high come on quicker and disappear sooner.
There is still much research to do on this cannabinoid.
Cannabichromene, or CBC, may make up to 20 percent of the cannabinoid
profile in a plant. Little study has been done on this cannabinoid. So
far, researchers believe that it may interact with THC to make the high
more intense.

Technical Stuff: For a complete background on cannabinoids, including
chemical diagrams, resin profiles and production of cannabinoids, see
the classic book, Marijuana Grower’s Guide, by Mel Frank, Redeye Press,
1997, 330 pages, $19.95.

End of Excerpt



Harvesting Marijuana

Parts of this article are small excerpts from Jorge Cervantes’ new book, Marijuana Outdoors: Guerilla Growing, 152 pages, 60 color photos, 5.5” x 8.5”, $14.95. Copyright 2000.

For complete step-by-step instructions on breeding read Indoor Marijuana Horticulture, the Indoor Bible, by Jorge Cervantes.

Available in English, German and Spanish.


“The hardest part about harvesting is waiting until it is ready. Even after growing for more than 20 years, it’s hard to wait until the buds are completely ripe to harvest,” said Felipe a homegrown Californian. “That’s why I sacrifice one or two plants for an early harvest. This way I get early smoke and don’t hack down the crop too early.”


When to Harvest

Sinsemilla Harvest

Sinsemilla, unpollinated female marijuana flower buds, are mature from 6 – 12+ weeks after flowering starts. The best time to harvest sinsemilla is when THC production has peaked, but not yet started the degradation process.

Indica and indica/sativa varieties, tend to go through 5 – 10 weeks of rapid bud formation before leveling off. The harvest is taken 1 – 3 weeks after growth slows. Harvest in most commercial indica varieties is ready all at once, in 8 – 10 weeks.

Sativa varieties (Thai, Mexican, Colombian and African) tend to form buds at an even rate throughout flowering, with no marked decline in growth rate. Buds at the top of the plant may reach peak potency a few days to a couple of weeks before buds on lower branches and require several harvests. Long season plants, such as the Thai ‘Haze’ can flower for several months.

Smoking, diminishing returns and scientific observation are three excellent techniques used to test for peak ripeness. Smoking is the most fun. Harvest an average bud, dry it at 200 degrees F for 10 – 15 minutes and smoke it. The smoke is harsh, but palatable. Test when straight and several times throughout flowering.

A point of diminishing returns is reached when the pistils on the bottom of the bud are dying (turning brown) faster than they are growing from the top of the bud. At this point, THC production has usually peaked out and on its way down hill. This is the best way to tell a ripe bud with the naked eye.

Looking at resin glands with an inexpensive microscope (20 – 50X) is an easy precise way to discern peak THC production. Place a small, thin, resinous portion of the bud under a microscope at 30X magnification. A flashlight or lamp will help to give you an unshadowed view of the resin glands. Portable microscopes afford a quick peek at resin glands without harvesting the bud. Look for stalked glands with the knob or ball at the top. They have the highest concentration of THC. Other hair-like and non-stalked glands or trichomes contain much less THC. Note: Resin glands rupture and decompose quickly, lowering potency, when buds are bruised. Handle buds as gently as possible to avoid premature degradation.

Draw resin glands under 30X microscope

Preference dictates harvest time. Once stalked trichomes develop a knob or a head, are fully developed and still translucent, it is the peak of an early harvest. Amber colored glands are decomposing and THC content declines slightly. Resin glands seldom develop uniformly. Therefore some glands will be changing to translucent and others decomposing. Peak harvest is when more glands are translucent than amber. Check over a period of several days and check several buds from different plants to make sure the maximum amount of trichomes are ripe for harvest.

Growers report a soaring high if buds are picked a little early, when more resin glands are translucent and a heavier more lethargic stone when buds are picked after more resin glands show amber.



Marijuana Botany

An Advanced Study: The Propagation and Breeding of Distinctive Cannabis

by Robert Connell Clarke



Cannabinoid Biosynthesis

Since resin secretion and associated terpenoid and cannabinoid biosynthesis are at their peak just after the pistils have begun to turn brown but before the calyx stops growing, it seems obvious that floral clusters should be harvested during this time. More subtle variations in terpenoid and cannabinoid levels also take place within this period of maximum resin secretion, and these variations influence the nature of the resin’s psychoactive effect.

The cannabinoid ratios characteristic of a strain are primarily determined by genes, but it must be remembered that many environmental factors, such as light, temperature, and humidity, influence the path of a molecule along the cannabinoid biosynthetic pathway. These environmental factors can cause an atypical final cannabinoid profile (cannabinoid levels and ratios). Not all cannabinoid molecules begin their journey through the pathway at the same time, nor do all of them complete the cycle and turn into THC molecules simultaneously. There is no magical way to influence the cannabinoid biosynthesis to favor THC production, but certain factors involved in the growth and maturation of Cannabis do affect final cannabinoid levels, These factors may be controlled to some extent by proper selection of mature floral clusters for harvesting, agricul tural technique, and local environment. In addition to genetic and seasonal influences, the picture is further modified by the fact that each individual calyx goes through the cannabinoid cycle fairly independently and that during peak periods of resin secretion new flowers are produced every day and begin their own cycle. This means that at any given time the ratio of calyx-to-leaf, the average calyx condition, the condition of the resins, and resultant cannabinoid ratios indicate which stage the floral cluster has reached. Since it is difficult for the amateur cultivator to determine the cannabinoid profile of a floral cluster without chromatographic analysis, this discussion will center on the known and theoretical correlations between the external characteristics of calyx and resin and internal cannabinoid profile.

Harvest Timing

With this dynamic picture of the biosynthesis and degradation of THC acids as a frame of reference, the logic behind harvesting at a specific time is easier to understand. The usual aim of timing the moment of harvest is to ensure high THC levels modified by just the proper amounts of CBC, CBD and CBN, along with their propyl homologs, to approximate the desired psychoactive effect. Since THC acids are being broken down into CBN acid at the same time they are being made from CBD acid, it is important to harvest at a time when the production of THC acids is higher than the degradation of THC acids. Every experienced cultivator inspects a number of indicating factors and knows when to harvest the desired type of floral clusters. Some like to harvest early when most of the pistils are still viable and at the height of reproductive potential. At this time the resins are very aromatic and light; the psychoactive effect is characterized as a light cerebral high (possibly low CBC and CBD, high THC, low CBN). Others harvest as late as possible, desiring a stronger, more resinous marijuana characterized by a more intense body effect and an inhibited cerebral effect (high CBC and CB]), high THC, high CBN). Harvesting and testing several floral clusters every few days over a period of several weeks gives the cultivator a set of samples at all stages of maturation and creates a basis for deciding when to harvest in future seasons. The following is a description of each of the growth phases as to morphology, terpene aroma, and relative psychoactivity.

Premature Floral Stage

At this stage floral development is slightly beyond primordial and only a few clusters of immature pistillate flowers appear at the tips of limbs in addition to the primordial pairs along the main stems. By this stage stem diameter within the floral clusters is very nearly maximum. The stems are easily visible between the nodes and form a strong framework to support future floral development. Larger vegetative leaves (5-7 leaflets) predominate and smaller tri-leaflet leaves are beginning to form in the new floral axis. A few narrow, tapered calyxes may be found nestled in the leaflets near the stem tips and the fresh pistils appear as thin, feathery, white filaments stretching to test the surroundings. During this stage the surface of the calyxes is lightly covered with fuzzy, hair-like, non-glandular trichomes, but only a few bulbous and capitate-sessile glandular trichomes have begun to develop. Resin secretion is minimal, as indicated by small resin heads and few if any capitate-stalked, glandular trichomes. There is no drug yield from plants at the premature stage since THC production is low, and there is no economic value other than fiber and leaf. Terpene production starts as the glandular trichomes begin to secrete resin; premature floral clusters have no terpene aromas or tastes. Total cannabinoid production is low but simple cannabinoid phenotypes, based on relative amounts of THC and CBD, may be determined. By the pre-floral stage the plant has akeady established its basic chemotype as a fiber or drug strain. A fiber strain rarely produces more than 2% THC, even under perfect agricultural conditions. This indicates that a strain either produces some varying amount of THC (up to 13%) and little CBD and is termed a drug strain or produces practically no THC and high CBD and is termed a fiber strain, This is genetically controlled.

The floral clusters are barely psychoactive at this stage, and most marijuana smokers classify the reaction as more an “effect” than a “high.” This most likely results from small amounts of THC as well as trace amounts of CBC and CBD. CBD production begins when the seedling is very small. THC production also begins when the seedling is very small, if the plant originates from a drug strain. However, THC levels rarely exceed 2% until the early floral stage and rarely produce a “high” until the peak floral stage.

Early Floral Stage

Floral clusters begin to form as calyx production increases and internode length decreases. Tri-leaflet leaves are the predominant type and usually appear along the secondary floral stems within the individual clusters. Many pairs of calyxes appear along each secondary floral axis and each pair is subtended by a tri-leaflet leaf. Older pairs of calyxes visible along the primary floral axis during the premature stage now begin to swell, the pistils darken as they lose fertility, and some resin secretion is observed in trichomes along the veins of the calyx. The newly produced calyxes show few if any capitate-stalked trichomes. As a result of low resin production, only a slight terpene aroma and psychoactivity are detectable. The floral clusters are not ready for harvest at this point. Total cannabinoid production has increased markedly over the premature stage but THC levels (still less than 3%) are not high enough to produce more than a subtle effect.

Peak Floral Stage

Elongation growth of the main floral stem ceases at this stage, and floral clusters gain most of their size through the addition of more calyxes along the secondary stems until they cover the primary stem tips in an overlapping spiral. Small reduced mono-leaflet and tri-leaflet leaves subtend each pair of calyxes emerging from secondary stems within the floral clusters. These subtending leaves are correctly referred to as bracts. Outer leaves begin to wilt and turn yellow as the pistillate plant reaches its reproductive peak. In the primordial calyxes the pistils have turned brown; however, all but the oldest of the flowers are fertile and the floral clusters are white with many pairs of ripe pistils. Resin secretion is quite advanced in some of the older infertile calyxes, and the young pistillate calyxes are rapidly producing capitate-stalked glandular trichomes to protect the precious unfertilized ovule. Under wild conditions the pistillate plant would be starting to form seeds and the cycle would be drawing to a close. When Cannabis is grown for sinsemilla floral production, the cycle is interrupted. Pistillate plants remain unfertilized and begin to produce capitate -stalked trichomes and accumulate resins in a last effort to remain viable. Since capitate-stalked trichomes now predominate, resin and THC production increase. The elevated resin heads appear clear, since fresh resin is still being secreted, often being produced in the cellular head of the trichome. At this time THC acid production is at a peak and CBD acid levels remain stable as the molecules are rapidly converted to THC acids, THC acid synthesis has not been active long enough for a high level of CBN acid to build up from the degradation of THC acid by light and heat. Terpene production is also nearing a peak and the floral clusters are beautifully aromatic. Many cultivators prefer to pick some of their strains during this stage in order to produce marijuana with a clear, cerebral, psychoactive effect. It is believed that, in peak floral clusters, the low levels of CBD and CBN allow the high level of THC to act without their sedative effects. Also, little polymerization of resins has occurred, so aromas and tastes are often less resinous and tar like than at later stages. Many strains, if they are harvested in the peak floral stage, lack the completely developed aroma, taste and psychoactive level that appear after curing. Cultivators wait longer for the resins to mature if a different taste and psychoactive effect is desired.

This is the point of optimum harvest for some strains, since most additional calyx growth has ceased. However, a subsequent flush of new calyx growth may occur and the plant continue ripening into the late floral stage.

Late Floral Stage

By this stage plants are well past the main reproductive phase and their health has begun to decline. Many of the larger leaves have dropped off, and some of the small inner leaves begin to change color. Autumn colors (purple, orange, yellow, etc.) begin to appear in the older leaves and calyxes at this time; many of the pistils turn brown and begin to fall off. Only the last terminal pistils are still fertile and swollen calyxes predominate. Heavy layers of protec tive resin heads cover the calyxes and associated leaves. Production of additional capitate-stalked glandular trichomes is rare, although some existing trichomes may still be elongating and secreting resins. As the previously secreted resins mature, they change color. The polymerization of small terpene molecules (which make up most of the resin) produces long chains and a more viscous and darker-colored resin. The ripening and darkening of resins follows the peak of psychoactive cannabinoid synthesis and the transparent amber color of mature resin is usually indicative of high THC content. Many cultivators agree that transparent amber resins are a sign of high-quality drug Cannabis and many of the finest strains exhibit this characteristic. Particularly potent Cannabis from California, Hawaii, Thailand, Mexico, and Colombia is often encrusted with transparent amber colored instead of clear resin heads. This is also characteristic of Cannabis from other equatorial, subtropical and temperate zones where the growing season is long enough to accommodate long term resin production and maturation. Many areas of North America and Europe have too short a season to fully mature resins unless a greenhouse is used. Specially acclimatized strains are another possibility. They develop rapidly and begin maturing in time to ripen amber resins while the weather is still warm and dry.

The weight yield of floral clusters is usually highest at this point, but strains may begin to grow an excess of leaves in late-stage clusters to catch additional energy from the rapidly diminishing autumn sun. Total resin accumulation is highest at this stage, but the period of maximum resin production has passed. If climatic conditions are harsh, resins and cannabinoids will begin to decompose. As a result, resin yield may appear high even if many of the resin heads are missing or have begun to deteriorate and the overall psychoactivity of the resin has dropped. THC decomposes to CBN in the hot sun and will not remain intact or be replaced after the metabolic processes of the plant have ceased. Since cannabinoids are so sensitive to decomposition by sunlight, the higher psychoactivity of amber resins may be a secondary effect. It may be that the THC is better protected from the sun by amber or opaque resins than by clear resins. Some late maturing strains develop opaque, white resin heads as a result of terpene polymerization and THC decomposition. Opaque resin heads are usually a sign that the floral clusters are over-mature.

Late floral clusters exhibit the full potential of resin production, aromatic principles, and psychoactive effect. Complex mixtures of many mon oterpene and sesquiterpene hydrocarbons along with alcohols, ethers, esters, and ketones determine the aroma and flavor of mature Cannabis. The levels of the basic terpenes and their polymerized by-products fluctuate as the resin ripens. The aromas of fresh floral clusters are usually preserved after drying, as by the late floral stage, a high proportion of ripe resins are present on the mature calyxes of the fresh plant. Cannabinoid production favors high THC acid and rising CBN acid content at this stage, since most active biosynthesis has ceased and more THC acid is being broken down into CBN acid than is being produced from CBD acid. CBD acid may accumulate because not enough energy is available to complete its conversion to THC acid. The THC-to-CBD ratio in the harvested floral clusters certainly begins to drop as biosynthesis slows, because THC acid levels decrease as it decom poses, and at the same time CBD acid levels remain or rise intact since CBD does not decompose as rapidly as THC acid. This tends to produce marijuana characterized by more somatic and sedative effects. Some cultivators prefer this to the more cerebral and clear psychoactivity of the peak floral stage.

Harvesting Cannabis at the proper time requires information on how floral clusters mature and a decision on the part of the cultivator as to what type of floral clusters are desired. With harvesting as with other techniques of cultivation, the path to success is straightened when a definite goal is established. Personal preference is always the ultimate deciding factor.


Factors Influencing THC Production

Many factors influence the production of THC. In general, the older a plant, the greater its potential to produce THC. This is true, however, only if the plant remains healthy and vigorous, THC production requires the proper quantity and quality of light. It seems that none of the biosynthetic processes operate efficiently when low light conditions prevent proper photosynthesis. Research has shown (Valle et al. 1978) that twice as much THC is produced under a 12-hour photoperiod than under a 10-hour photoperiod. Warm temperatures are known to promote metabolic activity and the production of THC. Heat also promotes resin secretion, possibly in response to the threat of floral desiccation by the hot sun, Resin collects in the heads of glandular trichomes and does not directly seal the pores of the calyx to prevent desiccation. Resin heads may serve to break up the rays of the sun so that fewer of them strike the leaf surface and raise the temperature. However, light and heat also destroy THC. In a drug strain, a bio-synthetic rate must be maintained such that substantially more THC is produced than is broken down. Humidity is an interesting parameter of THC production and one of the least understood. Most high-quality drug Cannabis grows in areas that are dry much of the time at least during the maturation period. It follows that increased resin produc. tion in response to arid conditions might account for increased THC production. High-THC strains, however, also grow in very humid conditions (greenhouses and equatorial zones) and produce copious quantities of resin. Cannabis seems not to produce more resins in response to dry soil, as it does to a dry atmosphere. Drying out plants by with-holding water for the last weeks of flowering does not stimulate THC production, although an arid atmosphere may do so. A Cannabis plant in flower requires water, so that nutrients are available. for operating the various bio-synthetic pathways.

There is really no confirmed method of forcing increased THC production. Many techniques have developed through misinterpretations of ancient tradition. In Colombia, farmers girdle the stalk of the main stem, which cuts off the flow of water and nutrients between the roots and the shoots. This technique may not raise the final THC level, but it does cause rapid maturation and yellow gold coloration in the floral cluster (Partridge 1973). Impaling with nails, pine splinters, balls of opium, and stones are clandestine folk methods of promoting flowering, taste and THC production. However none of these have any valid documentation from the original culture or scientific basis. Symbiotic relationships between herbs in companion plantings are known to influence the production of essential oils. Experiments might be carried out with different herbs, such as stinging nettles, as companion plants for Cannabis, in an effort to stimulate resin production. In the future, agricultural techniques may be discovered which specifically promote THC biosynthesis.

In general, it is considered most important that the plant be healthy for it to produce high THC levels. The genotype of the plant, a result of seed selection, is the primary factor which determines the THC levels. After that, the provision of adequate organic nutrients, water, sunlight, fresh air, growing space, and time for maturation seems to be the key to producing high-THC Cannabis in all circumstances. Stress resulting from inadequacies in the environment limits the true expression of phenotype and cannabinoid potential. Cannabis finds a normal adaptive defense in the production of THC laden resins, and it seems logical that a healthy plant is best able to raise this defense. Forcing plants to produce is a perverse ideal and alien to the principles of organic agriculture. Plants are not machines that can be worked faster and harder to produce more. The life processes of the plant rely on delicate natural balances aimed at the ultimate survival of the plant until it reproduces. The most a Cannabis cultivator or researcher can expect to do is provide all the requisites for healthy growth and guide the plant until it matures.

Flowering in Cannabis may be forced or accelerated by many different techniques. This does not mean that THC production is forced, only that the time before and during flowering is shortened and flowers are produced rapidly. Most techniques involve the deprivation of light during the long days of summer to promote early floral induction and sexual differentiation. This is sometimes done by moving the plants inside a completely dark structure for 12 hours of each 24-hour day until the floral clusters are mature. This stimulates an autumn light cycle and promotes flowering at any time of the year. In the field, covers may be made to block out the sun for a few hours at sunrise or sunset, and these are used to cover small plants. Photoperiod alteration is most easily accomplished in a greenhouse, where blackout curtains are easily rolled over the plants. Drug Cannabis production requires 11-12 hours of continuous darkness to induce flowering and at least 10 hours of light for adequate THC production (Valle et al. 1978). In a greenhouse, supplemental lighting need be used only to extend daylength, while the sun supplies the energy needed for growth and THC biosynthesis. It is not known why at least 10 hours (and preferably 12 or 13 hours) of light are needed for high THC production. This is not dependent on accumulated solar energy since light responses can be activated and THC production increased with only a 40-watt bulb. A reasonable theory is that a light-sensitive pigment in the plant (possibly phytochrome) acts as a switch, causing the plant to follow the flowering cycle. THC production is probably associated with the induction of flowering resulting from the photoperiod change.

Cool night temperatures seem to promote flowering in plants that have previously differentiated sexually. Extended cold periods, however, cause metabolic processes to slow and maturation to cease. Most temperate Cannabis strains are sensitive to many of the signs of an approaching fall season and respond by beginning to flower. In contrast, strains from tropical areas, such as Thailand, often seem unresponsive to any signs of fall and never speed up development.

Contrary to popular thought, planting Cannabis strains later in the season in temperate latitudes may actually promote earlier flowering. Most cultivators believe that planting early gives the plant plenty of time to flower and it will finish earlier. This is often not true. Seedlings started in February or March grow for 4-5 months of increasing photoperiod before the days begin to get shorter following the solstice in June. Huge vegetative plants grow and may form floral inhibitors during the months of long photo-period. When the days begin to get shorter, these older plants may be reluctant to flower because of the floral inhibitors formed in the pre-floral leaves. Since floral cluster formation takes 6-10 weeks, the initial delay in flowering could push the harvest date into November or December. Cannabis started during the short days of December or January will often differentiate sex by March or April. Usually these plants form few floral clusters and rejuvenate for the long season ahead. No increased potency has been noticed in old rejuvenated plants. Plants started in late June or early July, after the summer solstice, are exposed only to days of decreasing photoperiod. When old enough they begin flowering immediately, possibly because they haven’t built up as many long-day floral inhibitors. They begin the 6-10 week floral period with plenty of time to finish during the warmer days of October. These later plantings yield smaller plants because they have a shorter vegetative cycle. This may prove an advantage. in greenhouse research, where it is common for plants to grow far too large for easy handling before they begin to flower. Late plantings after the summer solstice receive short inductive photoperiods almost immediately. However, flowering is delayed into September since the plant must grow before it is old enough to flower. Although flowering is delayed, the small plants rapidly produce copious quantities of flowers in a final effort to reproduce.

Extremes in nutrient concentrations are considered influential in both the sex determination and floral development of Cannabis. High nitrogen levels in the soil during the seedling stage seem to favor pistillate plants, but high nitrogen levels during flowering often result in delayed maturation and excessive leafing in the floral clusters. Phosphorus and potassium are both vital to the floral maturation of Cannabis. High-phosphorus fertilizers known as “bloom boosters” are available, and these have been shown to accelerate flowering in some plants. However, Cannabis plants are easily burned with high phosphorus fertilizers since they are usually very acidic. A safer method for the plant is the use of natural phosphorus sources, such as colloidal phosphate, rock phosphate, or bone meal; these tend to cause less shock in the maturing plant. They are a source of phosphorus that is readily available as well as long-term in effect. Chemical fertilizers sometimes produce floral clusters with a metallic, salty flavor. Extremes in nutrient levels usually affect the growth of the entire plant in an adverse way.

Hormones, such as gibberellic acid, ethylene, cytokinins and auxins, are readily available and can produce some strange effects. They can stimulate flowering in some cases, but they also stimulate sex reversal. Plant physiology is not simple, and results are usually unpredictable.

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