This article explains photoperiodism, and how it relates to vegetative and flowering growth phases in marijuana plants.
Photoperiodism refers to developmental responses in plants due to changes in light cycles. The chemistry of a marijuana plant changes according to the percentage of light vs. darkness it is exposed to in a 24 hour period. Specifically, marijuana transitions from a vegetative (grow) phase to a flowering (bloom) phase when kept in the dark for between 9-11.5 hours, for 5-7 days consecutively.
Phytochrome (PR) is the hormone in the marijuana plant that regulates the transition from vegetative growth to flowering. The hormone is only active when the plant is in darkness. Exposure to light renders the hormone inactive. This hormone must be active for prolonged, regular periods to build up enough and reach a threshold that triggers a transition from vegetative to flowering state. A high amount of light per day keeps the hormone inert, and the plant in stays in vegetative growth. Low daily light causes a build up of the hormone and triggers flowering.
Vegetative (grow) growth is when plants are growing new shoots and branches. Plants will continue to get bigger in vegetative state. They will not produce any reproductive organs. Grown indoors, with control over lighting, you could maintain a plant in a vegetative growth state indefinitely. Plants will grow massive with prolonged vegetative growth. When grown outdoors cannabis will switch to bloom in the fall when days become shorter.
During the flowering (bloom) stage, the plant shifts its energy from vegetative growth to producing reproductive organs. For approximately the first 3 weeks of flowering the plant begins to grow flowers, while also continuing vegetative growth – growing 25-50% larger than when flowering started. After that, vegetative growth stops completely and the flowering process fully takes over. This is a dramatic shift as the physical appearance of the female plant completely changes. Eventually, female flowering plants will produce ripe buds, end their life cycle, and start to deteriorate.
Cannabis is an annual plant. Grown outdoors, it stays in a vegetative state in the spring when the sun is in the sky longer, and switches to a flowering state in the fall when the nights become longer. To take advantage of both cycles an outdoor grower plants in the early spring and harvests in the late fall.
When growing indoors, artificial lights are kept on adjustable timers to regulate the duration a plant is kept in vegetative (grow) vs. flowering (bloom) growth phases. When you want your plants in the vegetative stage, leave the lights on 18-24 hours per day. When you want to switch to the flowering phase, set the light timers to 12 hours on and 12 hours off. During the 12 hours off the plants should be kept in complete darkness. If there is any light contamination during the switch to flowering, phytochrome buildup can be neutralized and interrupt the transition to flower.
The exact amount of time a plant is kept in vegetative growth is up to you. The longer plants are kept in vegetative growth the bigger they will get. Once you switch to flowering, it takes approximately 7-9 weeks for buds on female plants to fully ripen. Some strains flower faster than others.