Cannabis is relatively unusual in the world of flowering plants, in that it has separate male and female plants. For purposes of growing high-potency herbal cannabis, the male is unnecessary and is in fact a hindrance. Breeders have therefore poured time and resources into developing seeds that will produce only female plants on germination.
When male pollen comes into contact with the female stigmas, fertilisation occurs and the calyxes begin to form seeds. If male pollen can be excluded from the grow room entirely, the calyxes will fill with sticky, cannabinoid-rich resin instead. It is thought that this overproduction of resin may be a last-ditch attempt to attract pollinating insects, although cannabis is generally a wind-pollinated plant.
Dioecious & monoecious cannabis
Cannabis is dioecious, meaning that it has two sexes, male and female. Most flowering plants are hermaphrodite, meaning that male and female flower structures are combined into “perfect” flowers. Others are monoecious, meaning that they have separate male and female flowers on the same plant. Cannabis is usually dioecious and sometimes monoecious, but true hermaphrodite flowers are extremely rare to non-existent (the few documented examples are disputed).
Occasionally, isolated populations of cannabis will become monoecious in response to environmental pressures. These monoecious plants have the ability to self-fertilise in the absence of fully male plants, to produce offspring that are genetically identical to the parent.
When breeders observed this tendency in wild plants, they realised that if the technique could be applied to indoor varieties, resulting in feminised seeds. One way to achieve this result involves crossing monoecious plants with dioecious to achieve increasingly monoecious results, while still retaining the desirable qualities of the dioecious parent.
However, this method can be unpredictable and take generations to produce useful results. Another technique is light-poisoning, which involves disrupting the daylight hours with random hours of darkness. After a few weeks of such stress, the female plant will begin to produce male flowers.
Another method involves treating a dioecious female with a substance that induces production of male flowers. Gibberellic acid is the most widely-used chemical, but other options include silver thiosulfate and 2-chloroethanephosphonic acid. The chosen chemical is sprayed on a healthy female two weeks prior to flowering, and as the male flowers emerge they pollinate any female flower in the vicinity, ultimately producing all-female flowers.
Feminised cannabis seeds are suitable for most indoor growers, except those that specifically wish to breed or cross strains to create new varieties. For these growers, feminised seeds are useless as without male DNA to increase diversity, viability is lost in a few generations.