This report from Marijuana Moment published 30 December 2024.

A new study on the spread of cannabis pollen sheds light on the various seasonal and conditional dynamics that contribute to the risk of cross-pollination between crops, finding that “pollen dispersal rates escalate from summer to autumn” and that pollen tends to travel further from its source during daytime.

The research, published in the journal Scientific Reports, explores a little-studied issue that can have major consequences for hemp growers. Cross-pollination with nearby marijuana, for example, could cause a crop of planted hemp to exceed the federal cut-off for THC or risk hybridizing hemp strains that were bred specifically for certain traits.

Cross-pollination becoming a significant challenge

Cross-pollination between crops in nearby fields “has become a significant challenge,” the report says, “leading to contaminated seeds, reduced oil yields, and in some cases, mandated crop destruction.”

“This investigation represents a pioneering effort to assess the potential risks associated with windborne hemp cross-pollination, emphasizing the variability in risk risk across different seasons and geographic regions,” wrote authors from Virginia Tech, adding that they believe it to be “unprecedented in its comprehensive simulation of pollen dispersal’s regional and seasonal inhomogeneities, specifically focusing on hemp.”

The team used meteorological data and modeling “to simulate wind-borne hemp dispersion” across the conterminous United States—meaning the lower 48 contiguous states and Washington, D.C.—on a county-by-county basis. For each county, they took weather data from a point nearest the center of the jurisdiction “and averaged across local noon and midnight hours for each month from July to November, to represent average ‘day’ and ‘night’ conditions respectively.”

Using models, they simulated pollen dispersal “traveling in the downwind and vertical directions,” counting the number of particles deposited “within 250 meter-wide bins up to 50 km downwind of the source.” Overall, the study consisted of 31,070 simulations.

Authors said their goal in conducting the research was to aid hemp farmers and policymakers in better understanding the spread of hemp pollen and its varying risks across regions, seasons and times of day.

Difficult to put a geographical distance on pollen spread

“Cross pollination has been a challenging issue for hemp growers, and I would love for the industry to continue to grow in the U.S.,” Manu Nimmala, the lead author of the study and a doctoral student in engineering mechanics at Virginia Tech, said in a university post about the study. “We hope this information can better inform policymakers as well as the farmers involved.”

Ultimately, the study concludes that variations in pollen dispersal patterns over time and region make it difficult to definitively prevent cross-pollination. Instead, it suggests adopting more adaptive strategies to manage risks and prevent losses.

“Such variability complicates the establishment of uniform isolation distances, suggesting the superiority of adaptive risk management strategies,” authors wrote. “These strategies could incorporate weather pattern considerations to mitigate cross-pollination risks more effectively and could include measures like intertemporal zoning, farm quotas, cross-pollination damage insurance, and regulatory policies.”

“It might not be possible to establish a very strict isolation distance the way you would with other crops, as cross-pollination risk also depends on the size of the farm or pollen source, time of year and weather conditions,” explained lead author Nimmala. “With this data, we would advocate for community planning, and encourage farmers to talk to each other about the crops they plan to plant each season. These patterns could inform both spatial and temporal isolation strategies, and maybe even insurance coverage related to cross-pollination damage.”

As for the dynamics of how pollen spreads, the study says its findings “reveal that pollen deposition rates generally escalate from summer to autumn, attributed to the reduction in convective activity during daytime and the increase in wind shear at night as the season progresses.”

“Notably, we detected pronounced diurnal variations in pollen dispersion: nighttime conditions favor deposition in proximity to the source, while daytime conditions facilitate broader dispersal albeit with reduced deposition rates,” it continues, asserting that the study “lays the groundwork for developing sophisticated approaches to managing agricultural cross-pollination risks, potentially influencing both policy and practice.”

Hemp is particularly susceptible to unintentional cross-pollination

Co-author Shane Ross, a Virginia Tech professor, said in the university post that hemp is especially prone to unintentional cross-pollination because of the pollen’s small particle size compared to that of other crops.

“Due to its small size, hemp pollen travels farther and deposits in greater quantities than other wind-pollinated crops,” he said. “Our map simulates just how far pollen can spread, highlighting states and areas with the highest vulnerability to cross pollination.”

The report notes that while its models focused on cannabis pollen, and specifically hemp, “the methodologies are broadly applicable to the dispersion of any lightweight particles.”

“This is only a first step in preventing unintended cross-contamination,” Ross said. “Our map reveals hard physics-based data about vulnerable areas, but it is up to the states and policymakers to decide the best approaches to mitigate the problem.”

Hemp shifting from being an illegal crop to a meaningful agricultural commodity

The new report comes amid increased attention by researchers and government officials on hemp production as cultivation surged following the nationwide legalization of hemp—cannabis that contains less than 0.3 percent THC—through the 2018 Farm Bill.

While sometimes tedious, many of the changes signal policymakers shifting their treatment of hemp from illegal crop to meaningful agricultural commodity. For example, the U.S. Department of Agriculture (USDA) recently announced changes to a federal hemp crop insurance program. The adjustments, which take effect next year, ease certain crop-rotation requirements and remove smoke damage as a cause of covered loss.

Nationwide, hemp operators insured $750,000 in “covered liabilities” on 2,600 acres of production in crop year 2024, USDA officials said as part of that announcement.

USDA has been working to bolster the nation’s hemp industry, including by appointing a number of industry stakeholders this summer to a federal trade advisory committee meant to support efforts to promote U.S.-grown cannabis around the world.