Trichomes are small appendages that look like hairs. They are produced by marijuana, and other plants. Female marijuana plants produce certain trichomes that are a rich source of THC. These trichomes can be found in their largest concentration on the buds. They start out clear, turn a milky color, then turn amber (light brown).
The trichomes in picture 1 are clear. After the plant has flowered for a few weeks, the trichomes start to turn a milky color (picture 2). After a few more weeks, they will be totally milky in color. In the later stages of flowering, trichomes will turn to a light brown color (picture 3). The amount of time required to get to this point depends on the marijuana strain and the growing conditions.
In picture 2 you can see the stems have started to turn from a clear color to a milky translucent color. For maximum THC content and a more cerebral and energetic high, harvest your plants when a majority of the trichomes on the plants in your garden are a fully milky translucent color.
You can wait until most of the trichomes have started to turn amber, but the resulting marijuana will produce more of a sleepy body stone than it would if plants were harvested earlier. The trichome in picture 3 is about 90% amber, with just a trace of the milky translucent color it previously possessed.
After the trichome is fully amber in color, the THC starts to degrade. This makes it very important to harvest marijuana at the time before a majority of the trichomes have attained a total amber color. If not, the marijuana will not be as potent as it could have been.
On your first harvest, if you are having a hard time judging when to cut the plants down, a good rough guide as to when to harvest a plant is to wait until 50%-80% of the white pistils (hairs) have turned dark (usually brown or red) and about 10% of the trichomes start to turn amber.
Do not be in too much of a rush and harvest when you see the first amber trichome. It is normal for a small number of trichomes to mature several weeks prior to optimal harvest time. But when 10% or more of the trichomes are turning amber and 50%-80% of the white pistils (hairs) have turned dark, you should consider harvesting your plants.
Note that trichomes are too small to be seen properly with the human eye. To see them properly, use a pocket microscope rated at somewhere between 20x to 70x. The greatest concentration of trichomes can be found on the growing buds.