NC State Studies ‘Seedless’ Hemp Varieties

When growing hemp for extraction of cannabinoids it is critical that only female plants are produced. Female flowers (“buds”) are densely covered in cannabinoid-rich trichomes as compared to male flowers, which produce very little in terms of cannabinoids. What males do produce is an abundance of pollen, which can threaten the yield and quality of a floral hemp crop. Female plants that are pollinated show a decrease in floral yield and cannabinoid concentration compared to non-pollinated plants. Meir and Mediavllia (1998) showed that unpollinated females produce up to 2-5x more extractable oils compared to pollinated plants.

When hemp was first legalized, growers utilized asexually propagated transplants (“clones”) to ensure that only female plants were produced. Since then, we have seen a shift towards the use of feminized seed. Compared to clones, feminized seeds are more economical and give the farmer more control over transplant production if they are unable to produce clones. But what happens if your neighbor is growing grain or fiber hemp, which has a mix of males and females? Hemp pollen is wind-dispersed and has been shown to travel at least 3 miles (Small and Antle, 2003). Unfortunately, this has already led to a number of lawsuits among neighboring farmers whose all-female floral hemp crop was pollinated by a neighboring grain/fiber crop (“Oregon industrial hemp litigation”, 2018).

To address this issue, breeders are developing triploid or “seedless” hemp varieties. Triploidy is both a naturally occurring genetic phenomenon and a well-established technique in the plant breeding world. Many crops we have come to love are triploids such as seedless watermelon, seedless citrus, and bananas.

What is a triploid?

Hemp, like humans, is a diploid organism. This means the plant has two sets of chromosomes: one set from the mother and one set from the father. Often you will see a diploid written as “2n” where “n” is the number of chromosomes from one parent (Figure 1). For example, each human parent gives their offspring a set of 23 chromosomes (n=23) so 2n=46. Hemp (Cannabis sativa) is 2n=20; 10 chromosomes from the male and 10 from the female.

A triploid organism contains three sets (3n) of chromosomes. In the case of hemp, a triploid is 3n=30. Triploidy can occur in nature due to random mutations. Plant breeders create triploids by crossing a tetraploid (4n) parent with a diploid (2n) parent (Figure 2).

So what does this mean for you, the hemp farmer? First, and most importantly, triploid plants are pollen insensitive. In other words, it is sterile. This means that you can bathe a triploid hemp plant in pollen and it will not produce seed. However, it is not unheard of to find a few seeds produced by a triploid/”seedless” plant. Cut open a few seedless watermelon and you may find one or two seeds, but it’s nowhere near the hundreds of seeds a regular (diploid) watermelon produces. There may also be added agronomic benefits to triploids versus diploids. In other agronomic crops, increasing the number of chromosomes can increase plant vigor, size, fruit yield, floral size, and other desirable traits. At this point we do not know if triploidy affords added agronomic benefits to floral hemp other than pollen insensitivity.

During the 2021 field season, we will conduct trials of newly developed triploid floral hemp varieties from Oregon CBD in environments with and without pollen pressure. We will compare the seed production of diploid vs triploid plants grown in the pollen pressure location as well as floral biomass and cannabinoid concentrations. We will also incorporate these triploids in our floral hemp variety trials (no pollen pressure) to determine if there are any added agronomic benefits compared to their diploid pair.

Key takeaway

The development and availability of triploid/”seedless” hemp is another tool for hemp growers to utilize. If proven to be pollen-insensitive, it will allow farmers to produce floral hemp without concern for neighboring pollen.

References: Meier, C. & V. Mediavilla. 1998. Factors influencing the yield and the quality of hemp (Cannabis sativa L.) essential oil. J. Intl. Hemp Assoc. 5:16-20 Oregon industrial hemp litigation: Won’t you be my neighbor? (2018, September 8). Small, E & T. Antle. 2003. A preliminary study of pollen dispersal in Cannabis sativa in relation to wind direction. J. Indust. Hemp. 8:37-50

Source: Dr. David Suchoff, NC State Extension, NC State University