"The trichome withering process does not occur rapidly. It can take up to two weeks before the plant has withered its trichromes and pistils entirely, but even then new trichromes and pistils can be found growing among the faded pistils. Eventually, the old pistils will die, to be replaced by the new pistils. The process will continue until there is a change in the photoperiod or a cellular breakdown in the plant and it dies.
As the plant reaches its peak bloom, many of the pistils will change in color. Using a magnifying aid, you can observe this change in the trichromes themselves, which then affects the overall look of the pistils. By checking these trichomes daily you will be able to detect when the plant is nearing peak potency. The more trichromes change color, the more some will start to wither. The ones that wither first are usually in the minority because they were the first trichomes to form on the first pistils during the early days of flowering.
As some of these older pistils wither, the other pistils begin a visible transformation in color from white to orange, red, or brown. At this point, you should be anticipating the "harvest marker" of 50 to 70 percent. When 50-70% of the pistils change color, the plant will have stopped producing new trichromes and resin, the smell will have reached its peak and the bud mass will not have increased in a few days. At this stage, any of the major bud masses on the plant can, theoretically, be observed as follows:
-less than 5% pistils are withering
-approx 90% of the pistils have reached maturity
-less than 5% of pistils appear to be in early stages of growth"
The Cannabis Grow Bible by Greg Green, second edition