Marijuana Horticulture the indoor/outdoor medical growers bible by jorge cervantes. Here i pulled it out, to provide the complete info (i cant recall everything all the time off the top of my head, lol).
Environmental factors start influencing sex the moment the seedlings has three pairs of true leaves (not counting cotyledones). Environmental factors that influence sex determination of cannabis include but are not limited to:
Increasing the level of nitrogen makes more female plants. Lower the nitrogen level to create more male plants. Increase the level of potassium to increase male tendencies; lowering the potassium level encourages female plants. A higher nitrogen level and a lower potassium level for the first two weeks increases females.
Low temperatures increase the number of female plants. Warm temperatures make more male plants.
High humidity increases the number of female plants. Low humidity increase male plants.
Low growing-medium moisture increase males.
More blue light increases the number of female plants. More red light increases male tendecies.
Fewer hours of daylight (e.g. 14 hours) increases the number of females. Longer days (e.g. 18 hours) makes more male plants.
Stress: any environmental stress tends to yield more male plants when growing from seed.
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There ya go
