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Friday February 18, 2022

By Trevor Ross

Image of a cannabis plant leaf with pollen on it. Growing

Knowledge of the reproductive process of cannabis plants is fundamental for any grower. Even hobbyist home-growers should be aware of what their plants are capable of in case, for example, a hermaphroditic plant appears in the grow room. But amateur growers can also use this knowledge to breed their own plants, or even invent new strains!

In this article, we review how to pollinate cannabis, and offer a primer for cross-breeding marijuana.

How to Pollinate Cannabis

Cannabis is a dioecious plant, meaning the sexes are represented separately in male or female plants, and, generally speaking, both sexes are required to produce a fertilized seed (as opposed to monoecious plants like tomato vines, which can reproduce with themselves). Male plants produce pollen which fertilizes the female pistils, which in turn produces seeds.

In the wild, this process usually consists of a male plant “dropping” its pollen, which is then carried on the breeze to surrounding plants.

Cannabis pollen can remain viable on the wind for up to ten miles before its efficacy is significantly diminished. Female plants grow dozens of flowers each reaching out with pistils looking for this pollen, and once the female plant has been fertilized, much of its energy is diverted from growing flowers to growing seeds.

This is why we grow feminized plants for marijuana flower, and why male plants in the grow room pose such a threat. Unless, of course, you want to breed some cannabis.

Pollinating a grow room can be as easy as shaking a male plant in the middle of the room, but most growers are more meticulous, especially when crossing new strains. In these cases, pollen is harvested from male plants and collected. This pollen is then brushed directly onto female pistils. You’ll know the plant has been successfully fertilized when the bracts at the base of the stems begin to swell, or the tips of the pistils turn amber while the rest of them remain white.

How to Cross-breed Cannabis

Cross-breeding is the process of combining genetics from two or more parent plants into a single new plant. And it’s not a process unique to cannabis — farmers and researchers have been cross-breeding fruits, vegetables, and other plants for millennia, usually to improve farm yields. For instance, a drought-resistant tomato may be crossed with a high-yielding tomato to produce tomato plants that will not suffer shortages in dry climates. Similarly, cannabis has been cross-bred to produce strains that are more mold-resistant, have higher THC content, and of course to blend a myriad of therapeutic and psychotropic effects.

Small brush with cannabis pollen on it applying to a flowering marijuana plant.
Example of pollen being applied to a female cannabis plant for cross-breeding.

Cross-breeding cannabis can be done by anyone with patience and a penchant for ruthless note-taking. To understand why, let’s begin with a vocabulary lesson: genotype and phenotype.

Genotype refers to an organism’s genetic makeup, the whole spectrum of possible traits — dominant and recessive — as defined by the organism’s DNA.

Phenotype, then, is the individual expression of those potential traits. Much the way two parents can have five children that all look and act a little different, so can two cannabis plants produce seeds that all express a little differently, albeit from the same pool of potential differences. Narrowing down that pool of potential differences to produce a reliable, or stable, phenotype is a lengthy process.

Cross-breeding begins with two genotypes, A and B, which are bred to produce seeds. Those seeds will not be the best traits of both parents, but a mishmash of dominant and recessive traits randomly smashed together. So, all of those seeds must then be grown and meticulously observed — How tall did they grow? How much water did they consume? Were they prone to mold or pests? What was the yield? And those questions carry through the curing and smoking process — How did the bud taste? What were the effects? All of this must be recorded to reliably track the genetics.

Cannabis seedlings growing side by side.
Cross-breeding cannabis requires extensive phenotype hunting. photo credit

The first round of seeds is usually a buckshot of phenotypes, with desirable and undesirable traits in every plant. While one may be a perfect blend of traits, it is still considered “unstable” because it cannot yet be relied on to consistently pass on that same collection of traits. To stabilize new phenotypes with desired dominant traits, they are often bred again with a parent plant in a move called back-crossing. Back-crossing reintroduces the desired effects to the genetic pool, relegating less desired effects to recessive genes, and ultimately narrowing the spectrum of possible effects.

After several rounds of breeding over several grow cycles (likely a few years), those phenotypes can be crossed again and again until a new, stable genotype is established.

Tips for Breeding Cannabis

When breeding cannabis plants, keep the males and females separate if possible. Plants can be left to pollinate naturally, but the approximate nature of this method will blur your records. Manually pollinating cannabis plants minimizes guesswork and makes for tighter records over the years it may take to produce a new strain.

Another tip that’s obvious but bears repeating is: care for your mother plants like they were a pregnant woman, because they are. While those individual plants may not produce flowers, generations of offspring may, so you want those seeds to be as healthy as possible, and healthy seeds start with a healthy plant. Don’t slack on nutrient regimens or compromise with watering, and track the pH in your growing medium to avoid nutrient lockout.

Woman in an outdoor cannabis grow taking notes.
Keeping records is a great way to track the history of cross-bred strains. photo credit

Finally, be patient, and if it hasn’t been said enough: keep records. The process of producing a whole new strain may take years, and over that time you will want a meticulous record of how each plant grew, strengths and weaknesses, resources consumed, yields produced, and of course the medicinal effects. None of this will happen in fits and starts, but must be a dedicated process with acute interest all the way through. Patience may take practice, but those who stick with it will earn the supreme pleasure of naming their own strain.

Do you have any tips for cross-breeding cannabis? Let us know in the comments below!

Photo Credit: Shutterstock


Trevor Ross Trevor Ross

Trevor Ross is a writer, medical marijuana patient and cannabis advocate. He holds an MFA in writing from the School of the Art Institute of Chicago. He has previously worked as a copywriter, a teacher, a bartender, and followed Seattle sports for SidelineBuzz. Originally from Washington state, you can find him now working in his garden or restoring his house in Scranton, PA, and he can be reached through LinkedIn.

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