Cross breeding cannabis seeds

Cross-Breeding and Pollinating Cannabis: An Overview

K nowledge 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.

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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.

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.

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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.

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!


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.

Cannabis Breeding: How Are New Strains Created?

While browsing Leafly’s strain database, you may wonder what a cross of this and that strain is, what a hybrid or a backcross is, or what a parent strain is. All of these have to do with plant breeding—essentially, breeding a male and female plant to combine or refine the genetics of two plants or strains. Breeding two different strains often results in a new strain, or hybrid.

Cannabis breeders typically breed to purify and strengthen strains, combine strain traits, or enhance specific characteristics.

Cannabis breeders typically breed to purify and strengthen strains, combine strain traits, or enhance specific characteristics like higher yields, specific aromas, potency, and many other things.

When growing and breeding, it’s important to know where your seeds come from and what kind of genetics they have. If the seed breeder can’t give you a detailed history of how a packet of seeds was bred or what they were crossed with, you never really know what you’re getting.

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Plant breeding is a fundamental process of growing cannabis. Breeding is highly technical and typically done on a commercial scale, but with legalization increasing, breeding is becoming more popular. You can even do it yourself.

The Basics of Breeding

Cannabis plants can be either male or female. Cannabis consumers are mainly concerned with female plants, because only females produce the sticky buds that we all know and love. But male cannabis plants are important for the breeding process, as they are needed to pollinate the bud-producing females.

Take the strain Super Lemon Haze as an example. It’s a hybrid (or a “cross”) of Super Silver Haze and Lemon Skunk—these are the parent strains. At some point, the breeder decided that they liked some attributes of Super Silver Haze and some of Lemon Skunk and decided to combine the two.

To do this, you need a male of one strain to pollinate a female of the other. Once pollinated, the female will then produce seeds that express the genes of both the male and female plant. Those seeds will be harvested and grown separately, and voilà: You have created a hybrid.

So how do you know whether to pick a male or a female of each strain that you’re crossing?

“Often in cannabis, the traits of the female carry over to progeny (seeds) more than the male. That said, the traits of the male are often obvious to the discerning grower so one should definitely choose a male that will complement the traits of the female,” says Nat Pennington, founder and CEO of Humboldt Seed Company who’s been breeding cannabis for 20 years. “So much is possible with truly intentional breeding strategies.”

How to Breed Cannabis Plants

After two parent strains are selected for breeding, a male and several females are put into a breeding chamber to contain the pollen. A breeding chamber can be as simple as an enclosed environment with plastic sheeting on the sides, or a specially designed sterile environment for large-scale breeding.

“A healthy male can pollinate up to 20 females, and by pollinate, I mean absolutely cover the plant with seeds.”

A single male plant can pollinate tens of females. “It’s always a good idea to have only one male, genetically speaking, per pollination effort,” says Pennington. “A healthy male can pollinate up to 20 females, and by pollinate, I mean absolutely cover the plant with seeds.”

This is intentional breeding—any grower who’s accidentally grown a male and pollinated a crop will know that one male can easy pollinate hundreds of females, filling your whole crop with seeds.

Once in the breeding chamber, you can grow the plants vegetatively for a few weeks to let them get bigger, but it’s not necessary. Put them on a flowering light cycle: 12 hours of light, 12 hours of dark.

The mature male will grow pollen sacs within the first couple weeks of its flowering phase. Pollen will release from the sacs, move through the air, and land on the female plants, pollinating them. Having an enclosed breeding chamber is important to contain the pollen and also to prevent outside pollen from getting in.

You can also help along the pollination effort by shaking pollen from the male onto the females, or by collecting pollen from the male and directly applying it to the females. These female plants will continue to grow and flower, during which they’ll grow seeds (as well as buds). These seeds will express the genetics of both the male and female plant.

When the seeds are mature, they are harvested and stratified (or dried). “The secondary process of maturation happens after the plant is dead, and the seed needs to be stratified before it will germinate,” says Pennington. “In general, harvest for flower takes place three to four weeks before harvest for seed.”

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These seeds—now a hybrid of the two parent strains—will be grown on their own, outside of the breeding environment.


But the process doesn’t end there. The hybrid strain that you buy at the dispensary has likely gone through many rounds—or generations—of breeding to strengthen its genes and to ensure that its descendants are healthy and consistent.

Just as you and your sibling might have different physical attributes from your parents, each seed created from a round of cross-pollination will have different attributes from its parent strains. Maybe you have your father’s eyes and your mother’s hair, but your sister has your mother’s eyes and hair. Each cannabis seed is unique and will express different traits, and different combinations of traits, from one or both of the parent strains. These seeds with various expressions are called phenotypes.

Homozygosity ensures that a plant will consistently produce the same seeds with the same genetic makeup over and over again.

A plant that produces a set of phenotypes that have a lot of variety are said to be heterozygous. With cannabis, you typically want seeds that are homozygous—ones that have the same set of genes. Homozygosity ensures that a plant will consistently produce the same seeds with the same genetic makeup over and over again, ensuring that buyers and consumers will get the same plant or seed time and again.

After a strain is crossed, a breeder will then have to select which phenotype of the new strain they like best. For large-scale growers, they want to choose the best phenotype for mass production.

Back to the Super Lemon Haze example: This strain takes a lot of its bud structure, trichome and resin production, and overall appearance from Super Silver Haze. But it takes its flavors and aromas from Lemon Skunk.

Lemon Skunk also tends to grow extremely tall and has loose buds, whereas Super Silver Haze grows smaller and has denser buds. Through selecting specific phenotypes, a breeder can pick one that has the attributes they want to keep. In this case, a phenotype that has the structure and bud density of Super Silver Haze and the flavors and aromas of Lemon Skunk.

Most likely, there were early phenotypes of Super Lemon Haze that grew tall and loose like Lemon Skunk, or tasted more like Super Silver Haze. But the breeder discarded those phenotypes and keep growing the ones that have the attributes of what we now know is Super Lemon Haze.


High-quality breeding still doesn’t stop there. Once a breeder has crossed a strain and narrowed down a phenotype and finally has the one, they will usually backcross that strain to strengthen its genetics.

Backcrossing is a practice where a breeder will cross-pollinate the new strain with itself or a parent—essentially, inbreeding the strain. This makes the strain more homozygous, and strengthens its genetics and desirable characteristics, and also ensures that those genes continue to pass down from generation to generation.

The hybrid that you bought from the dispensary has gone through months and even years of growing, crossing, and backcrossing, as well as a selection process to pick the best phenotype of that strain.

Breeding is about time and patience. Says Pennington: “To be a breeder, you have to be willing to accept the fact that you won’t have uniformity in the offspring, [you’ll get] lots of ugly ducklings in the hunt for your golden goose. To make seeds that will actually reflect the golden goose takes time, and it takes more than just a one-off cross. Even after you found your golden goose, expect to have to do a whole number of stabilizing backcrosses to reproduce your golden goose in seed form.”