Why does the sex of cannabis plants matter?  

When growing a cannabis plant to obtain cannabinoids, such as THC and CBD, it is necessary to ensure that the plants are female. Female plants produce female flowers, which in-turn produce a greater number of cannabinoids in their flowers than male plants produce. Cannabinoids, and THC in particular, are the primary psychoactive phyto-chemicals in cannabis plants.  

A grower would waste resources, including time and space, growing a single male plant because of the inferior flower produced.  Worse still, a male plant can destroy an entire crop, as it would likely fertilize all of the female plants, end their flowering cycle, thereby halting cannabinoid production. In short, male and hermaphroditic plants can wreak havoc on a crop. That is why it is critical, for the grower to know the sexes of their plants.  

What factors produce male and female plants? 

Cannabis plants like many other plants and complex organisms exist as two sexes. In the wild, two sexes in a species allow the species to preserve genetic diversity, and that genetic diversity allows it to survive and adapt to changes in its environment. For the grower, some of these adaptations can result in loss of potency in their crop. So, it is to their advantage to reduce the genetic variation in their plants and compensate for the loss of hardiness by optimizing growth conditions. 

The genetics of a plant come from physical structures in their cells called genes. Those genes in turn are made of DNA. Many genes strung together make up a chromosome. Like humans, cannabis plants have X and Y chromosomes. These are just designations. A human X chromosome looks nothing like a plant’s. The designation of a male plant is XY, and a female is XX.  

Hermaphrodites also exist. Most often they are female plants that grow male flowers, or they are genetic hermaphrodites with varying multiples of X and Y chromosomes with a mix of physical characteristics beyond flower growth.  

In the wild, pollination by male plants will consistently produce about a 50:50 mix of male and female plants. Luckily for the grower, there are simple and practical solutions to selecting for almost purely female crops.  

How can we mitigate the introduction of male plants in a crop? 

The simplest way to ensure plants will be female is to buy feminized seeds or clones of female plants, the latter being the most effective. In localities where cannabis production and consumption are legal, many supply stores and dispensaries will carry both products, and provide a wealth of information on their cultivation. 

Starting with feminized seeds or female clones is not enough. The process of making feminized seeds is only 99% successful, and female plants, as previously mentioned, can become hermaphrodites. A gambler of course, would call 99% fantastic odds. However, remember that a single male or hermaphrodite can ruin an entire crop. This can be catastrophic for a commercial grower or a hobbyist. Male and hermaphroditic plants must be culled from the onset. Luckily, plants can be sexed much like other plants and animals. If a grower can risk the wait, they can allow the plants to mature naturally and see the different, and often subtle, characteristics of each plant. Once the plants begin to mature, males will grow sturdier, woodier stalks and have relatively sparse leaf growth. Females will have more tender stalks and tender more succulent leaf growth.  

After 6 weeks, the plants will become sexually mature.  Flower buds will begin to develop earlier for male plants than females and time to flowering also depends on whether growing indoors or outdoors. The males’ flowers will appear as small balls on short stalks and relatively separate. Females’ flower buds will have a translucent filament called a pistil coming out of them. It is critical to see these subtle differences once the plants become sexually mature.  Cull all males and hermaphrodites, as soon as possible.  

Regarding hermaphrodites, these plants can develop spontaneously from female plants that have been stressed. A malfunction in the irrigation or lighting system can stress the plants.  If the growth substrate, like soil, has become depleted or infected it can stress the plants. The plants should be inspected for the appearance of male flowers on some of the previously female plants. Insect infestation or detection of a disease also causes stress and will call for inspection.  

 Alternative to identification of plant sex by physical characteristics, otherwise known as plants phenotype, plants can be genetically tested.  With genetic testing we can find the plants genetic makeup, or genotype. Various tests exist and can be purchased or ordered at increasing price points. Small scale growers can send in plant material in the form of small cuttings or a rub on a small testing substrate. The sample can be sent, where it is legal, to a facility that will perform genetic testing. Larger scale growers may opt to perform their testing in-house, and purchase relatively expensive equipment and supplies. The amount of testing and the secrecy of strain development may make this a cost-effective choice. As with buying seeds or clones, this is only a good start. Plants must still be observed for stress and male flower development throughout the plant’s productive life.  

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Written by Adrian Rubio, M.S. May 2023