Weighing the risk of cannabis cross-pollination

Industrial hemp and marijuana growers must work together to ensure a bright future for all sectors of the budding cannabis industry.

A male industrial hemp plant.
A male industrial hemp plant with flowers ready to open. Photo by James DeDecker, MSU Extension.

When passage of the 2018 Farm Bill legalized industrial hemp, many people in agriculture celebrated the new opportunity that this crop symbolizes for our industry. The spring day when we planted our first hemp plots at Michigan State University Extension’s Upper Peninsula Research and Extension Center also had an aura of historic significance. Hemp had not been legal to grow for over 60 years in Michigan, and today we are initiating research and outreach to support the potential (re)development of an entire value chain surrounding this multipurpose plant. Now that our first hemp crop is up and growing, a new concern is emerging with it that could threaten the future of Michigan’s cannabis industry.

For those less familiar, industrial hemp is cannabis cultivated to produce fiber, grain or non-intoxicating medicinal compounds such as cannabidiol (CBD). As defined by law, industrial hemp has less than 0.3% THC (tetrahydrocannabinol), the psychoactive component in marijuana. In fact, the primary difference between hemp and marijuana is this legal THC threshold, which results from selective breeding for different uses. Yet as members of the same species, the two crops have more in common than not, including the vexing ability to crossbreed.

Cannabis is what’s known as a dioecious species, meaning that male and female flowers are borne on separate plants. There are some monoecious varieties of cannabis with male and female flowers on the same plant, and stress can also induce the production of male flowers on female plants, but these are exceptions to the plant’s normally dioecious nature. Flowering is induced when day and night lengths become equal. Male cannabis plants flower for a period of two to four weeks, and a single male flower can produce 350,000 pollen grains. Pollen is carried to female plants on the wind and can travel great distances when conditions are favorable. Bees will collect cannabis pollen but are generally not attracted to the female flowers to contribute to pollination.

In the 1970s, marijuana growers found that preventing pollination by rogueing out male plants or producing only females (through clonal propagation or sowing of feminized seed) could greatly increase the yield and potency of their crop. This works because cannabis is one of the few plant species that can actively increase the number and size of its female sex organs in response to prolonged virginity, according to Small and Naraine, 2016. The longer female plants go unpollinated, the more flowers are produced and the larger they get.

Cannabinoids, including the valuable end products THC and CBD, are concentrated in the female flower tissue. A study by Meier and Mediavilla, 1998, found that pollination decreased the yield of essential oils in cannabis flowers by 56%. Today, most marijuana is sinsemilla (Spanish for “without seeds”) and seeded crops are considered inferior, commanding a lower price in the marketplace. The same strategy is now also being applied by industrial hemp growers producing CBD.

Industrial hemp grown for grain or fiber is a different story. Male plants and pollen are required to create hemp grain used for food, feed and oil. Fiber hemp does not require pollination, but the prohibitive cost of planting feminized seed or female clones means that fiber fields will usually include male plants. As a result, the recent introduction of hemp grown for grain and fiber in Michigan increases the risk of pollination for marijuana and CBD hemp growers. I say that industrial hemp increases rather than creates this risk because cannabis pollen has been blowing across the Midwest long before 2019.

A study by Stokes et al., conducted in 2000, years before hemp and marijuana were legalized, found that cannabis pollen comprised up to 36% of total airborne pollen counts in Midwest states during the month of August. This pollen likely came from wild hemp or illicit marijuana fields where male plants were not controlled, minor sources that could be greatly compounded by legal hemp production.

Mitigating the risk of cross-pollination in cannabis presents a unique challenge. The most straight forward strategy involves geographic or physical isolation. Industry experts recommend a minimum distance of 10 miles between outdoor cannabis fields. Research has shown that pollen can travel much further than 10 miles, but the amount of pollen transported decreases logarithmically with increasing distance from the source. Therefore, the risk of pollination should be negligible beyond ten miles from a pollen source.

Important variables related to pollen transport and viability include wind speed, direction, precipitation and humidity, topography, physical barriers, time since release, etc. For example, a study by Small and Antle, 2003, on pollen dispersal in cannabis found that a 3-mile isolation distance downwind was equivalent to a 0.6-mile distance upwind in terms of the amount of pollen deposited.

While geographic isolation may be a technically feasible strategy, accomplishing it in the field is more complicated. Maintaining isolation distances requires identifying where cannabis is being grown. Marijuana growers in Michigan are currently regulated by the Michigan Department of Licensing and Regulatory Affairs (LARA). Industrial hemp producers are regulated by the Michigan Department of Agriculture and Rural Development (MDARD). Although these agencies maintain records on where cannabis is produced, there is currently no coordination between the agencies regarding this issue and the location of cannabis fields is not public information. Maintaining geographic isolation would therefore require voluntary sharing of location information by growers. Even if growers could be encouraged to share this sensitive information, enforcement of isolation distances would be difficult.

Physical isolation accomplished by growing marijuana or CBD hemp indoors with air filtration systems can achieve the same result as geographic isolation, but also dramatically increases the cost of cannabis production. Growing grain or fiber hemp indoors is not practical given the scale required to achieve profitability with these lower value commodities. However, it may be possible to physically isolate grain/fiber hemp and the pollen it produces using strategic windbreaks or irrigation. Borders of thick crops or trees planted downwind may be able to intercept a great deal of pollen. A study by Ushiyama et al., 2009, found that windbreaks reduced dispersal of maize pollen by 30-60% depending on their design. Precipitation or irrigation water can weigh down pollen and prevent it from floating away on the wind. However, research on the use of these techniques in cannabis is lacking.

As a result, other states and local units of government are responding to the risk of cross-pollination in cannabis by simply banning marijuana, industrial hemp, or male cannabis plants specifically. In Michigan, Ballot Proposal 1 of 2018 legalized both recreational marijuana and industrial hemp, so it is unlikely that either would be banned at the state level to address cross-pollination. Municipalities in the state can legally restrict where marijuana is grown, so that may offer some flexibility for hemp production in communities that opt out of marijuana. Local governments do not currently have the power to control where industrial hemp is grown, but MDARD could potentially implement such a policy in the future as part of their hemp regulatory plan.

That said, cooperation and a little creativity should hopefully make it possible for all sectors of the cannabis industry to coexist. One potential solution to cross-pollination that captures this spirit of cooperation is temporal isolation. As noted above, flowering in cannabis is controlled by day length. Artificial shading can therefore be used to induce flowering at almost any time of the year. This technique is feasible for marijuana and CBD hemp growers working on a relatively small scale. It requires a shading structure and extra labor to cover and uncover plants daily, but is not prohibitively expensive.

Forcing flowering via controlled light regimes is likely not a realistic option for grain and fiber hemp grown at a larger scale. However, auto-flowering cannabis cultivars that flower based on age rather than photoperiod do exist. If the auto-flowering trait could be bred into elite cannabis cultivars, it could be used to off-set the release of pollen in hemp from flowering in female marijuana and CBD hemp plants.

Until more research can be conducted to assess the risk of cross-pollination in cannabis and policy created to mitigate that risk, the best advice I can offer is for cannabis growers to start an open dialogue. After so many years of prohibition, it would be a shame to see factions develop within the industry that limit potential growth by favoring either marijuana/CBD hemp or grain and fiber hemp. Together, with cooperation from forward thinking regulators, we can identify equitable solutions to the problem of cross-pollination.


  • Meier, C. and Mediavilla, V. 1998. Factors influencing the yield and the quality of hemp (Cannabis sativa L.) essential oil. Journal of the International Hemp Association 5(1):16-20.
  • Small, E. and Antle, T. 2003. A Preliminary Study of Pollen Dispersal
  • in Cannabis sativa in Relation to Wind Direction. Journal of Industrial Hemp, Vol. 8(2).
  • Small, E. and Naraine, S.G.U. 2016. Expansion of female sex organs in response to prolonged virginity in Cannabis sativa (marijuana). Genetic Resources and Crop Evolution 63:339–348.
  • Stokes, J.R., Hartel, R., Ford, L.B., and Casale, T.B., 2000. Cannabis (hemp) positive skin tests and respiratory symptoms. Annals of Allergy Asthma and Immunology 85:238-240.
  • Ushiyama, T., Du, M., Inoue, S., Shibaike, H., Yonemura, S., Kawashima, S., & Amano, K. (2009). Three-dimensional prediction of maize pollen dispersal and cross-pollination, and the effects of windbreaks. Environmental biosafety research, 8(4), 183-202.

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