Apart from referring to the given flowering times, one of the best indicators of a female cannabis plant’s ripeness is the colour of the hairs covering its flowers.
These hairs start out white, darkening to orange or red as the plant matures. A plant is generally at its ripest when about 75% of the hairs on its flowers have changed colour.
Observed with a high-powered magnifying glass, the resin glands on a ripening flower will undergo the same colour change, darkening from clear to opaque then usually to yellow, amber or orange. As this happens, THC is turning into the more soporific of cannabis’ active ingredients, CBD.
Some growers choose to harvest their marijuana plants when about 50% or fewer of the hairs have turned orange, reasoning that while the overall amount of resin produced will be less, a higher proportion of it will be THC.