By Daniela Vergara
When you think about cannabis plants, you probably don’t think about them having “sexes”, but they do. Just like most animals (including humans), Cannabis sativa plants can be male, female, or sometimes a mix of both. Figuring out how a cannabis plant becomes male, female, or both is a fascinating story about DNA, the environment, and evolution.
Today, I want to tell you a little about a paper I published recently about sex determination and expression in the Cannabaceae family (which includes C. sativa and its close relative Humulus lupulus, an important ingredient in beer). The paper focuses especially on C. sativa.
https://acsess.onlinelibrary.wiley.com/doi/10.1002/agg2.70050
A review paper is a type of scientific article that brings together and critically examines the existing research on a specific topic.
What Is Dioecy?
First, let’s talk about dioecy. This term simply means a species has separate male and female individuals. In plants, this is rare, only about 5–6% of flowering plants have completely separate sexes.
Cannabis sativa is one of the few that do. Some plants grow male flowers, others grow female flowers. (And if you are a grower aiming for buds rich in cannabinoids like THC or CBD, you are usually looking for female plants.)
Interestingly, dioecy has evolved many times in different plant groups. Plants that aren’t even closely related, like papaya, asparagus, and cannabis, have developed separate sexes. Plants originally started as hermaphrodites (having both male and female parts) and over millions of years evolved into species with separate sexes through different evolutionary pathways.
The Sex Chromosomes of Cannabis
In humans, biological sex is determined by X and Y chromosomes: XX for females and XY for males.
Cannabis plants work the same way:
- Females are XX
- Males are XY
So, when a cannabis seed sprouts, whether it will grow into a male or female plant is mostly determined by its DNA.
But it’s not quite that simple. The environment and hormones can also influence how sex traits are expressed. Things like temperature, stress, and certain chemical treatments can sometimes shift the plant’s sex characteristics.
In C. sativa, the situation is even more interesting because not every plant fits neatly into “male” or “female” categories.
There are three main sexual strategies in cannabis:
- Dioecious plants: separate male and female plants (the most common in cannabis).
- Monoecious plants: a single plant with separate male and female flowers in different places. Monoecy is common in hemp grown for fiber or grain.
- Hermaphrodite plants: a single flower that has both male and female parts. Although people talk about true hermaphrodite flowers in cannabis, I have yet to see one personally.
Distinguishing hermaphroditic flowers from monoecious plants can be tricky because both forms produce a single-seed fruit called an achene, but that’s a longer story for another time.
Because some cannabis populations include both monoecious and dioecious individuals, it’s possible that C. sativa is in an intermediate evolutionary stage called paradioecy. This means the species might be balancing between mixed sexual strategies.
The Cannabis sativa Genome?
These are important things about the cannabis genome:
- Cannabis sativa has 10 chromosomes: 9 regular chromosomes (called autosomes and are those that are not related to sex) and 2 sex chromosomes (X and Y).
- The Y chromosome is larger than the X and is packed with repetitive DNA and “jumping genes” (transposable elements), commonly known as “junk DNA”.
- About 70% of the Y chromosome does not recombine with the X during reproduction.
- There is a small region, the pseudo-autosomal region (PAR), where the X and Y chromosomes can still exchange some genetic material.
The full assembly of the Y chromosome still isn’t finished. It’s a difficult task because the Y chromosome is packed with repetitive DNA, making it like trying to complete a massive jigsaw puzzle where millions of the pieces look almost the same.
Can You Tell If a Plant Is Male or Female Early On?
If you’re a grower, it would be great to know early on whether a plant will be male or female. But unfortunately:
- Male and female seedlings look exactly the same.
- Sex differences don’t become visible until the plants start flowering.
- Male plants usually show their flowers a little earlier than females.
- Male flowers release pollen; female flowers catch the pollen and develop the cannabinoid-rich buds.
Other physical (phenotypic) traits such as leaf shape or growth speed are not reliable to predict sex.
However, there are now DNA tests that can identify the sex of a plant early on. Several private companies offer these services, and they are quite accurate at detecting whether a plant will be genetically male or female.
However, these tests can’t predict whether a plant will produce flowers of the opposite sex (become monoecious) later on due to environmental factors. So even if your plant tests as female, it could still produce male flowers under particular conditions.
Environment Matters Too
While genetics plays the biggest role, environmental conditions can cause changes in how a plant expresses its sex:
- Stress factors like too much light, heat, drought, or certain chemicals can cause a female plant to develop male flowers.
- Hormones like ethylene and gibberellins influence sex traits.
- Growers can intentionally spray chemicals like silver thiosulfate to make female plants produce male flowers. This method is widely used to create feminized seeds, which are almost guaranteed to grow into female plants. (But that’s a whole different story!)
Why Does This Matter?
Understanding sex determination in C. sativa isn’t just an academic question, it’s important for anyone growing the crop:
- If you want cannabinoid-rich buds for medical or recreational use, you need female plants.
- If you want to breed new varieties, you need both male and female plants.
- If you are producing hemp for fiber and particularly for grain, you might prefer monoecious plants that can self-pollinate.
Knowing how sex is determined and influenced helps farmers improve their crops, avoid unwanted surprises, and optimize plant yields.
The Big Picture
Cannabis sativa shows just how complex and amazing plant biology can be. DNA, environment, and even random chance all come together to shape whether a seed grows into a male, female, or mixed plant.
Even after thousands of years of cultivation, researchers are still discovering new things about how cannabis grows, flowers, and evolves. Ongoing research into cannabis genetics and sex expression is shaping the future of farming, medicine, and science.
If you’re interested, I have many more stories to share about sex determination and expression in C. sativa, let me know if you would like to hear more.
