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Marijuana is a short-day plant. As the days get shorter, marijuana determines when to flower based on the number of hours it receives of uninterrupted darkness. It measures the length of the dark period using the hormone phytochrome, which has two states. The hormone’s inactive state, Pfr, occurs when it absorbs red spectrum of light at 666 nanometers. It also has a slight sensitivity to blue light. The hormone changes to its active form, Pr, over a period of two hours when the plant is in darkness. When the Pr flowering hormone levels remain high for a critical period of time over several days, the plant changes from vegetative growth and initiates flowering.
The number hours of darkness plants need to initiate flowering differs by variety. Sativas require a longer period of darkness than indicas because they developed near the equator, where the length of daylight is much more consistent than at northern latitudes, where indicas developed. Some sativas continue to grow vegetatively with 10 or 11 hours of darkness, which usually cues most plants to flower.
Many sativas initiate flowering only when the dark cycle increases to 12 hours or more, which occurs September 22 in the Northern Hemisphere.
By contrast, most indicas flower with 8 to 11 hours of uninterrupted darkness (13 to 15 hours of light). In southern tiers of Europe and the U.S., a few indica varieties flower as early as June 22, the shortest night of the year. For this reason, outdoor gardeners in these regions should consider sativa-indica hybrids and sativas. In the central and northern tiers, indica and indica-sativa hybrids usually start flowering in August and are ready to harvest in September through mid-October.
Some sativas and sativa hybrids require a longer dark period at the end of flowering to fully ripen their buds. Outdoors, this happens in due course as the nights lengthen in the fall. Indoors, change the lighting regime to 14 hours of darkness/10 hours of light to promote ripening. This is especially helpful in finishing low latitude varieties that don’t reach maturity in their native lands, where there is no winter, until the middle of the short-day season.
