Hermaphroding is a survival trait for cannabis. If there is no pollen available from a male plant many strains will throw a few male flowers in order to continue/preserve their genetics. When a plant impregnates itself (usually referred to as selfing) the recessive chromosomes for herming are reinforced, so that the offspring from that breeding are much more likely to herm. There are quite a few factors which can trigger the phenotypical expression of this genetic potential. The most common cause of herming is light. More herms come from someone disrupting their dark period, even if it’s just for a tiny amount of time, than anything else. Second is probably someone deciding to change their light cycle once it’s been established. If a plant doesn’t get a rest period of at least 2 hours of darkness per day it is much more likely to herm. Likewise if you force flowering by giving them an extended dark period (24-72 hours) you run a significantly greater chance of them growing male flowers. This happens way more at 72 hours than it does at 24. In general, the more stress a plant is exposed to, especially during the flowering stage, the greater the probability is that it will hermie.
If a plant hermies naturally it is out of the program as far as I am concerned. But according to Robert C. Clarke’s Marijuana Botany:
“Remember that a selfed hermaphrodite gives rise to more hermaphrodites, but a selfed pistillate plant that has given rise to a limited number of staminate flowers in response to environmental stresses should give rise to nearly all pistillate offspring.”
“If the proper pistillate hermaphrodite plant is selected as the pollen parent and a pure pistillate plant is selected as the seed parent it is possible to produce an F1, and subsequent generations, or nearly all pistillate offspring. The proper pistillate hermaphrodite pollen parent in onewhich has grown as a pure pistillate plant and at the end of the season, or under artificial environmental stress, begins to develop a very few pistillate flowers.
Everyone please remember that this is in reference to those female plants which develop a few male flowers- not your naturally occurring full blown hermies. You must draw the distinction between a “selfed hermaphrodite” and a “selfed pistillate” plant.
The below picture is of a plant that turned Hermaphrodite on us. The little yellowing things you see are male flowers. Those flowers were filled with pollen that was released, pollinating itself and a couple other plants around it.