"In November 1998 we introduced our “Female Cannabis Seed.†We did this after our own experiments showed that from female seed, we acquired almost 100 % female offspring.
In the meantime we are six months further on. We have received a lot of feedback from our customers. The reactions are mostly positive, from clients who have successfully produced almost 100 % female offspring. However there have been reactions from customers who found a few hermaphrodites or males plants. Apparently environmental influences affect the sex of the female seeds as well. Because of the fact that female seeds do not grow into female plants under all circumstances, we changed the name from “Female Cannabis Seeds†to “Feminized Cannabis Seeds.â€
From literature and from our own findings it appears that the growth of a male or a female plant from seed, except for the predisposition in the gender chromosomes, also depends on various environmental factors. Not only the origination of entirely male or female plants is partly affected by these environmental factors, the number of male and female flowers on a hermaphrodite plant is affected as well. The environmental factors that influence the sex of the plant (or the flower in the case of hermaphrodites), are, among other things:
the quantity of nitrogen and potassium of the seedbed
humidity and moistness of the seedbed
level of temperatures
color of the light used
length of daylight
Stress, any form of stress, causes more male individuals to originate from seed.
Even the taking of cuttings from female plants may produce male or hermaphrodite cuttings.
To optimize the result, changes in one or more of the above-mentioned environmental factors for a certain period during growth, may be applied. During this time these environmental factors will deviate from the standard growing system for maximum harvest and quality, as described in nursery literature. The desired change(s) in the environmental factor(s) are started from the moment that the seedling has three pairs of real leaves (not counting the seed-lobes). This is the moment that male and/or female predisposition in florescence is being formed. After approximately two weeks the standard growing system can be reconverted to.
Of the five above-mentioned environmental factors the first three are the most practical:
Level of nitrogen and potassium of the seedbed: a heightening of the standard level of nitrogen makes for more female plants originating from the seeds. A lowering of the nitrogen level shows more male plants. A heightening of the level of potassium tends to show more male plants, while a lowering of the potassium level shows more female plants. A combination of a higher nitrogen level for the period of a week or two and a lowering of the potassium level is recommended.
Humidity and moistness of the seedbed: a higher humidity makes for an increase in the number of female plants from seed, a lowering for an increase in male plants. The same is valid for the moistness of the seedbed.
Level of temperatures: lower temperatures make for a larger number of female plants, higher temperatures for more male plants.
Color of the light used: more blue light makes for female plants from seed, more red light makes for more male plants.
Length of daylight: few hours of daylight (e.g., 14 hours) makes for more female individuals, a long day (e.g., 18 hours) makes for more male plants."
http://www.stonerhaven.com/topic/7142-is-it-possable-to-change-the-sex-of-a-plant/?p=206165
