The optimal time for most growers to harvest marijuana plants is when THC production has reached its maximum. A rough guide as to when to harvest is to wait until 50%-80% of the white pistils (hairs) have turned dark (usually brown or red). But a better method of determining when to harvest is to wait until certain trichomes on the plant have matured.
Trichomes are small appendages that look like hairs, they are produced by marijuana and other plants. Some trichomes on marijuana plants develop a resin gland at the top. These trichomes are a rich source of THC, and monitoring them will allow you to best judge when the plants are ready for harvesting.
On marijuana plants, trichomes with resin glands form in the highest concentration on buds. They start out clear, turn a milky color, then turn amber (light brown). To best judge when to harvest, keep an eye on the trichomes on the buds on each plant.
The best time to harvest a marijuana plant will depend on the desired effect you want. For maximum THC content and a more cerebral and energetic high, harvest your plants when about 50% of the trichomes have turned from clear to a milky translucent color.
Waiting until about 50% of the trichomes have turned from a milky translucent color to amber will result in marijuana that produces a combination of cerebral and body intoxication.
If you harvest when a majority of the trichomes are amber colored, the marijuana will produce more of a sleepy body stone than it would have if it were harvested earlier.
It is nearly impossible to see the trichomes with the human eye, so some type of magnification is needed. An inexpensive way to effectively view them is to use a pocket microscope rated at somewhere between 20x to 70x.