The paper bag
You can usually tell the gender of your marijuana plants relatively early, enabling you to remove your male marijuana plants from the growing location before pollination occurs. All you have to do is cover the lowest branches of the plant for 12 hours each day during a 24-hour light phase. Using something like a black bag (paper preferably) to cover these branches will still allow for ample air circulation and keep the light from reaching the leaves. You want to make sure the routine is solid because if there is still light hitting these branches, they will not likely show you which gender they will turn out to be. While using a magnifying glass, you will need to look at the end of these branches. Males will have what looks like a club on a playing card at the end with a little stem protruding underneath, while females will have what looks like a singular or double pistil coming out of an immature calyx.
Removing the males
Some growers use light to force early flowers from plants only reaching 8 inches (20 cm) so they can remove the males. Most growers also do not want to waste any of their time, energy or other resources on dealing with male plants so they just put their marijuana plants into a 12 hour cycle of light and dark for two weeks, enabling them to see which plants are male and which are female. Once the males are removed, these growers then put the plants back into an 18-24 hour light cycle to help them grow vegetation. This can put your harvest back by the two weeks if you use this method, however, so we recommend you do not force your marijuana plants to flower early. Covering the lower branches works well for us and it keeps the plants healthy.