How to Determine the Sex of MJ Plants

The sex of a plant is determined by the appearance of pre-flowers, which usually show up after the fourth week of vegetative growth. Some strains do not display any signs of sex until they begin to flower. When you switch the lighting to a 12/12 schedule (12 hours of light and 12 hours of darkness), flowering begins. During the first two to three weeks after this switch, the plant stretches two to three times in height and reveals its sex. Some equatorial sativa varieties may not show their sex even under 12/12 lighting; these strains only start to flower after reaching a certain age, which is genetically determined. You can speed up this process by switching to a 10/14 light schedule (10 hours of light and 14 hours of darkness).

Female Plants

Female plants can be identified by pear-shaped pre-flowers from which pistils grow. Often, female plants do not show pistils until the start of flowering.

Male Plants

Male pre-flowers can be described as “a ball on a stalk.” If you’re unsure, wait a bit longer to see if pistils appear. If not, and the balls start to form clusters (so-called “bananas” or “eggs”), you’re looking at a male plant. The top of a male plant is usually covered in these “eggs.”

Hermaphrodites

A hermaphrodite, or “hermie,” is a plant that shows both male and female sex characteristics. Hermaphrodites most often develop from female plants, producing a few male flowers, usually at the very bottom. Suspicious male flowers often appear on female plants during the final stages of flowering. Male hermaphrodites are less common, as most growers do not allow male plants to reach the flowering stage where pistils could appear.

Hermaphrodites can release pollen and significantly reduce your yield by pollinating themselves and all other female plants. The resulting seeds are not particularly valuable, as hermaphrodite parents tend to pass on this trait to their offspring. Most seeds from such plants will also produce hermaphrodites, and by the F3 generation, mutations become much more pronounced.

Photos of Male and Female Traits

On the left: male; on the right: female.

Determining the Sex of Seed-Grown Plants Without Forcing Flowering

Compared to observing pre-flowers, forcing clones to flower is a foolproof method for determining sex. This method assumes you started your grow from seeds. After a few weeks of vegetative growth, the plants will be bushy enough to take cuttings. Each cutting should have at least one node and be at least 1.5 inches (4 cm) long.

• Label or otherwise distinguish each seed-grown plant.
• Take two cuttings from each plant and label them with the donor plant’s number.
• Root these cuttings and plant them in separate pots.
• Place one clone from each plant into a flowering chamber with a 12/12 photoperiod. Since these clones are only for sexing, any light source will do once they start to show sex.
• Keep the other clones in vegetative growth. By the time the flowering clones reveal their sex, the vegetative clones will be sexually mature and ready to flower directly.

For larger harvests, keep both the donor plant and the non-flowering clones in vegetative growth. Both can later serve as mother plants, providing you with clones for flowering and maximizing your yield.

If you’re a grower starting from seed without cloning (or you simply don’t have separate spaces for veg and flower), the easiest way is to wait for pre-flowers (primordia) and sexual maturity while the plant is still in the vegetative stage. In other words, if you wait until the plants are 6–8 weeks old, most will show their sex naturally without any intervention.

How to Determine Sex Using a Paper Bag

Take a small paper bag or something similar that blocks out light (a paper bag won’t work for strong lamps) and a plastic-coated paper clip. Choose a branch on the plant you want to sex, cover it with the bag, and seal it with the clip. To simulate the flowering phase, remove the bag after 12 hours of darkness and put it back on after 12 hours of light. In 7–14 days, the covered branch will start to show its sex.

Editor’s Note: When attaching the bag, take precautions. Leave a small gap for airflow to prevent overheating. Black materials tend to heat up, and the branch may wilt from excess heat.

“There’s enough air inside the paper bag because it inflates and holds its shape… it hangs on the tip of the branch like a balloon.”

This method is for those who don’t have space for clones or want to know the sex of their plants without taking and rooting early cuttings.

What You’ll Need:

• 1 can of black spray paint
• 1 box of plastic ziplock bags (sandwich size)

Spray the outside of a plastic bag with black paint. Choose one of the lower branches, farthest from the light (to avoid overheating from intense light on the black paint), and simply cover it with the bag to create 12 hours of darkness. If you’ve painted the whole bag, no light will reach the branch when the lights are on. In most cases, you’ll see the sex of this branch in 7–10 days. I’ve used this method both indoors and outdoors.