[This is from rexresearch.com, an excellent site for anything to do with growing cannabis. I bolded the stuff i found more applicable. Feel free to +rep if you learn something, i definitely did]
The sexual expression of cannabis is determined by its genetic makeup, and by its metabolic temper, which is regulated by the male enzyme andrase and the female enzyme gynase. Environmental conditions (light, nutrients, soil and water) may suppress the formation of the dominant enzyme, and allow the opposite sex to express itself partially (hermaphroditism) or completely (sex reversal). (71, 72)
[About this next paragraph: I thought that normal levels of N with lowered levels of K were ideal for producing fems in early veg...]
Nitrogen fertilizers masculinize the phenotype by stimulating the formation of male flowers. The proportion, number and degree of monoecious plants increases with increasing N, and the total N content is always higher in monoecious individuals than it is in females. (79)
Treatment of hempseed with ethylene gas will increase the resulting number of female plants by about 50%. Ethylene is produced by certain plants (i.e., bananas, cucumbers and melons), and these can be used to treat hempseed in a simple manner. About two weeks before you plan to sprout the seeds, place them in a paper bag or envelope and put that in a plastic bag with the peels of a ripening banana or cucumber. Replace the peels after a couple of days, and change the bags to prevent mold.
Hempseed can be feminized while they are forming on the plant. Fruit peels are spread around the area for two weeks before the plants enter the flowering phase. Remove the skins when the plants begin to flower. Otherwise, treatment with Etephon will accomplish the same effect.
When hempseed is treated with the female hormone estrogen, percentage of females that are produced will increase by about 10%. Dissolve a birth control pill in water and soak the seeds overnight in the solution. After the initial soaking, continue to treat the seeds by sprouting them on a paper towel soaked in the solution. (80)
A.I. Zhatov tested the effects of ethrel on hemp:
"Treatment of hemp plants with an aqueous solution of ethrel changed the ratio of male to female flowers. The greatest effect was observed when plants were treated during flowering of male flowers." (81)
Electricity also can change the sexual expression of cannabis; B.R. Lazarenko and I.B. Gorbatovskaya reported:
"Under the influence of the electrical current, the numerical proportions between hemp plants of different sexes was changed by comparison with the control to give an increase of female plants by 20-25%... The characteristics acquired by the plants in electrically treated soils are transmitted by inheritance to the third generation..." [emphasis added] (82)
Photoperiodism is a most useful tool with which to control the sexual expression of cannabis. For example, J. Limberk made a careful study of lighttime on the sexual index of hemp, and reported thus:
"Male plants usually flowered earlier than female. Female plants flowered only when the period of daylight was shorter than 14 hours; male plants flowered even when the day was longer than 14 hours. Reduction of light intensity in the first stages of plant development lead to increases of female plants by 4.3%. Intersexual plants (22-30% of the total) were present in conditions of 11-13 hours light per day. Grafting of plants did not change sex."
The probable future sex of a pre-floral hemp plant can be guessed at by calculating the Leaf-Mass Index (LMI): Count the points (3, 5, 7) on 3 leaves in the center of a cluster. Divide that number by 3 to determine the average number of points. Repeat the process several times, and figure that average also. Multiply the two averages to determine the LMI. A high LMI indicates that the plant will be female.
The phyllotaxy changes to alternate just before the onset of flowering. Then the sex of the plant can be determined by making a close examination of the upper nodes of the main stem. The onset of flowering is indicated by the appearance of undifferentiated primordial buds behind the stipules at the nodes of the petioles (along the stem at the base of branches). Within a few days they differentiate. The male pistils are flat or knobby with a curved shape and 5 open petals about 5 mm. long; they have a single tiny stalk. Overlapping vegetation often disguises their appearance.
The female develops pairs of flowers surrounded by pointed bracts of protective leaves that will enclose the seed. The female stigma usually appear as 2 fuzzy white hairs forming a "V" that protrudes from a bract. Resinous hairs (glandular trichromes) cover the calyx (2-6 mm long).
Gibberellin will inhibit the formation of flowers on cannabis, but sometimes it will otherwise cause the growth of fertile female flowers on genetically male plants. Silver nitrate or cobalt chloride causes masculinization of flowers of female hemp, possibly due to blockage of ethylene synthesis. High levels of N salts --- and long photoperiods --- have a masculinizing effect on hemp.(74-76)
"Dioecious hemp plants were grown to an age of 20 days in a day-length of 21-22 hours, then given an inductive treatment of ten 8-hour days to initiate flowering. After return to long days and during the period of differentiation of flower buds, a total of 0.5 gr lanolin paste containing 0.5% NapthaleneAcetic Acid (NAA) was applied to leaves at the 3rd and 4th nodes. In genetically male plants, female plants were subsequently formed in sites which would normally be occupied by males, a result which appears to be regulated by the level of native auxin in the vicinity of meristems during the period of differentiation of flower primordia. Secondary effect of auxin treatment were evident in an over-all reduction in intensity of heteroblastic development, the trend towards a reduction of leaf lobing and serration which normally accompanies plants passing through a period of flowering than in untreated controls." (78)