Here's some info:
Marijuana Grower's Handbook - Part 3 of 33 - Ed Rosenthal
"Growth and Flowering" The cannabis plant regulates its growth and flowering stages by measuring the changes in the number of hours of uniterrupted darkness to determine when to flower. The plant produces a hormone (phytochrome) begining at germination. When this chemical builds up to a critical level, the plant changes its mode from vegetative growth to flowering. This chemical is destroyed in the presence of even a few moments of light. During the late spring and early summer there are many more hours of light than darkness and the hormone does not build up to a critical level. However, as the days grow shorter and there are longer periods of uniterrupted darkness, the hormone builds up to a critical level.
Flowering occurs at different times with different varieties as a result of the adaptation of the varieties to the environment. Varieties from the 30th latitude grow in an area with a temperate climate and fairly early fall. These plants usually trigger in July or August and are ready to harvest in September or October. Southern African varieties often flower with as little as 8 or 9 hours of darkness/15 to 16 hours of light. Other 30th latitude varieties including most indicas flower when the darkness cycle lasts a minimum of 9 to 10 hours. Jamaican and some Southeast Asian varieties will trigger at 11 hours of darkness and ripen during September or October.
Equatorial varieties trigger at 12 hours or more of darkness. This means that they will not start flowering before late September or early October and will not mature until late November or early December. Of course, indoors the plants' growth stage can be regulated with the flick of a switch. Nevertheless, the plants respond to the artificial light cycle in the same way that they do to the natural seasonal cycles. The potency of the plant is related to its maturity rather than chronological age. Genetically identical 3 month and 6 month-old plants which have mature flowers have the same potency. Starting from seed, a six month old plant flowers slightly faster and fills out more than a 3 month old plant.
