We all know that these books are to be taken with a grain of salt but here is what it says in Marijuana Botany...
The cannabinoid balance between CBC, CBD, THC, and CBN is determined by genetics and maturation.
THC production is an ongoing process
as long as the glandular trichome remains active. Variations in the level of THC in the same
trichome as it matures are the result of THC acid being broken down to CBN acid while CBD acid
is being converted to THC acid. If the rate of THC biosynthesis exceeds the rate of THC
breakdown, the THC level in the trichome rises; if the breakdown rate is faster than the rate of
biosynthesis, the THC level drops. Clear or slightly amber transparent resin is a sign that the
glandular trichome is still active. As soon as resin secretion begins to slow, the resins will usually
polymerize and harden. During the late floral stages the resin tends to darken to a transparent
amber color. If it begins to deteriorate, it first turns translucent and then opaque brown or white.
Near-freezing temperatures during maturation will often result in opaque white resins. During
active secretion, THC acids are constantly being formed from CBD acid and breaking down into
CBN acid.
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 CBD), 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.