When the U.S. legalized hemp production in 2018, growers didn’t have a definitive or reliable cultivation guide to turn to. Primary among the many questions facing new hemp growers includes what cultivars to grow, when (and how often) to sample for cannabinoids and when to harvest.
In 2020, researchers, grower-cooperators and private laboratories across the Midwest collaborated in an effort to better understand production strategies and relative performance of hemp cultivars.
In short, this program provided hemp growers an opportunity to receive discounted cannabinoid analysis in exchange for structured data sharing. The data from these efforts, along with university station trials conducted in 2020, were fed into an interactive data visualization tool known as the Midwestern Hemp Database (MHD). That year, more than 130 growers participated, providing 779 sample submissions across 152 distinct variety entries. Here’s what we learned.
Growing Season Overview
Producers participating in the project established seedlings/clones in greenhouses and nurseries for several weeks prior to transplanting them into the field, a process that peaks in mid-June and extends into early July across the Midwest.
Across all entries into the MHD, average flowering initiation took place in early- to mid-August. We observed extreme heterogeneity within and between the photoperiod-sensitive cultivars, particularly when it came to flowering date, flowering uniformity and maturity. Some cultivars, for example, began flowering consistently within a short number of days, while others initiated flowering unevenly over a long period of time. Due to the non-uniformity of the flowering process, plants of the same cultivar could reach maturity at different points in the growing season, which could have adverse impacts on testing and harvesting strategies at the field level. On average, growers harvested varieties in late September and early October, about 45 days after flowering.
Data from the MHD shows that many cannabidiol (CBD)-dominant cultivars exhibit a near-linear relationship between total percentages of CBD and tetrahydrocannabinol (THC). Given this presumed relationship, total CBD is often not able to surpass about 8% without exceeding the federal regulatory threshold of 0.3% total THC. This suggests cultivars with a stable CBD:THC (about 25:1) ratio throughout flowering will help maximize profitability while maintaining compliance. The reality is many hemp cultivars currently on the market will “go hot,” or have illegally high levels of THC, if not monitored appropriately—a quarter of the samples submitted into the MHD were above the 0.3% total THC regulatory limit.
One of the biggest mysteries among hemp growers was what causes a hemp plant to go hot. A 2020 study from Cornell University found that fields tend to go hot because of genetics, not because of environmental conditions or production practices. As such, heterogeneity across and within varieties can make agronomic performance and cannabinoid development unpredictable.
Testing and Sampling
As cannabinoids do not begin to develop rapidly until flowering starts, growers prefer to delay sampling until after terminal flowering to eliminate unnecessary testing costs. Sampling for cannabinoids begins upon flower initiation in mid-August, peaking in mid-September and slowly decreasing until harvest.
Across all entries in the MHD, the average harvest date is 45 days after flowering; a cross section of the cannabinoid data shows that by this period, 117 samples (15%) had exceeded the regulatory threshold for total THC. Perhaps even more concerning is that 61 samples (7.8%) exceeded the regulatory limit within 30 days after flowering. Growers must submit pre-harvest reports no fewer than 30 days prior to harvest—this means in some cases, growers would have to submit pre-harvest reports before flowering even begins in order to comply with federal, state or tribal regulations. Without previous production history for a given cultivar, this is a tremendous challenge. Cannabinoids accumulating more rapidly as flowering progresses makes it even more difficult to make harvest-timing decisions. For these reasons, researchers from the MHD encourage growers to test early and often following flower initiation to ensure compliance, especially when experimenting with unfamiliar cultivars.
Perhaps as important as when to sample is where to sample. Preliminary research conducted at the Institute for Advanced Learning and Research and referenced in the U.S. Department of Agriculture’s final rule on hemp indicates that THC concentration is greatest in the top third of the plant when compared to the middle and bottom portions. Often, the most visually pleasing and largest buds form on the apical shoot of the plant, making growers reluctant to sample from that portion of the plant during in-season testing. Coupled with expected variation in flowering progression within cultivars, growers should be very conscious of where and when they take pre-harvest samples. Be sure to stick to the same sampling and testing protocols regulatory agencies use to assure accurate results. (See exact sampling protocol here.)
There is also currently a great deal of variation regarding sample collection, preparation and analysis across the U.S. This variation is leading to a great deal of disparity among laboratory results, which has problematic implications for growers wishing to track cannabinoids across the season using laboratories that are not approved under specific state or tribal rules.
Part of an ongoing effort by the National Institute of Standards and Technology (NIST) is to encourage standardized analysis of a wide range of cannabis compounds. Overall, 116 laboratories participated in a NIST project designed to evaluate how cannabis products are being tested and labeled for various cannabinoids. The study showed some surprising results. While labs’ measurements of CBD, THC and precursors of various cannabinoids (like THCA or delta-9 THC) generally hovered around the known, accurate concentration levels, they still showed a wide range of results. This goes to show that growers should utilize approved laboratories when submitting samples for analysis prior to harvest to minimize variability and potential issues with compliance.
The MHD is intended as a resource for growers to use until more information about cultivar performance can be gathered. We encourage growers to think about how this information may help them in their production endeavors. A report summarizing the 2020 growing season findings and associated project details are available at the project website.