Obviously one of the keystones to high quality cannabis is to harvest at the optimal time to achieve the desired results, be it either for use in medicinal cannabis, recreation and/or fibre & cannabis based foods.
Watching cannabis trichomes
The most accurate way to determine the optimal harvest time for a narcotic cannabis crop for use as medicine or recreation is by watching the trichomes on the cannabis plant under magnification. The image pictured here to the right is magnified at x40. As cannabis flowers it produces resin being the trichomes, this is especially presented in the female cannabis plant and as the cannabis plant matures these trichomes grow and swell from the surface producing more THC.
During the early stages of the cannabis flowering cycle the trichomes will be clear. Harvesting when the majority of the trichomes covering the cannabis plant are clear will result it less yield and when cultivating narcotic cannabis for use in medicinal cannabis or recreation practically useless.
As the cannabis trichomes begin to turn cloudy it may be harvested if the desired result is an energetic cerebral high, however this may result in a small yield depending on the selected cannabis strain. It should also be noted at this point that the high of course is also influenced by the cannabis strain.
When left to mature further the cannabis trichomes will begin to turn amber (red), and once the cannabis plant is covered with a mix of these amber trichomes of approximately 50% and 50% of the clouded trichomes, it can be harvested to produce an even body and energetic cerebral high.
The peak maturity of the cannabis plant and it's resin production is reached once the majority of the trichomes covering the cannabis plant have turned amber, and harvesting at this point will produce a heavy narcotic body high. If the cannabis is left to flower past this point quality is lost as it begins to die having served its purpose.
Watching cannabis stigmas
Another way to harvest cannabis at its peak is to watch the stigmas. The stigmas are the hairs that cover the cannabis plant as it grows and flowers protruding from the swollen calyx as seen pictured here.

Again as when watching the trichomes the stigmas begin as a clear / white colour as seen to the left, and slowly as the cannabis plant matures further they begin to turn an amber colour as seen to the right, however the stigma colours may vary from strain to strain.
The same rules apply to watching the stigmas as when watching the trichomes. Harvesting when most of the hairs are white will result in a lack of quality and yield.
When the stigmas reach a 50/50 ration of amber and white hairs, it can be harvested to produce an even body and energetic cerebral high.
When the the majority of the cannabis stigmas have turned amber and the calyx have become large and swollen either with seed or THC it can be harvested producing a heavy narcotic body high. If
the cannabis is left to flower past this point quality is lost as it
begins to die.
Once harvest is completed cannabis must be dried (see "Drying Cannabis 101") properly as it continues to under go a number of chemical changes including the conversion of THC acid to pure THC, and if treated incorrectly degraded in quality and yeild.