Why Some Hemp Crops “Go Hot”: Genetics Matter More Than Stress
For hemp growers, a crop that tests above the legal THC threshold can erase months of work and investment. Under the U.S. Department of Agriculture’s hemp production rules, hemp must contain no more than 0.3% total delta-9 THC on a dry-weight basis. Crops that exceed the acceptable level may have to be disposed of under applicable federal, state or tribal regulations.
Growers have often blamed “hot” hemp on drought, poor soil, excessive heat, flooding or other stresses. But Cornell researchers found that a plant’s genetic makeup is a much stronger predictor of its cannabinoid profile than the field conditions tested in their experiments.
The genetic evidence came from a 2020 study, “Development and validation of genetic markers for sex and cannabinoid chemotype in Cannabis sativa L.”, led by Jacob Toth of Cornell University. The researchers analyzed 217 plants from 14 hemp cultivars grown at two New York locations. The plants fell into three broad genetic groups: THC-dominant, CBD- and THC-producing, or CBD-dominant.
These groups are commonly described as chemotypes I, II and III. For CBD production, growers generally seek chemotype III plants, which carry two hemp-type alleles at the cannabinoid chemotype locus, designated BD/BD. Plants carrying a marijuana-type allele, designated BT, have a greater genetic propensity to produce THC.
The relevant enzymes act on the same precursor, cannabigerolic acid. CBDA synthase directs production toward CBDA, the acidic precursor of CBD, while THCA synthase directs it toward THCA, the acidic precursor of THC. Toth’s team developed a DNA-based assay that could identify the cannabinoid chemotype in young plants, before growers would normally be able to determine the mature flower’s chemical profile.
The study also exposed a weakness in some supposedly low-THC seed lines: populations thought to be stabilized for CBD production were still genetically mixed. Although many plants were CBD-dominant, some accumulated THC above the 0.3% limit. The researchers reported that BD/BD plants consistently remained below 0.3% delta-9 THC in their analysis, while plants carrying a BT allele were more variable and posed greater compliance risks.
A separate Cornell study tested whether environmental stress could change that underlying cannabinoid balance. In the 2021 paper “Limited effect of environmental stress on cannabinoid profiles in high-cannabidiol hemp”, researchers exposed three genetically distinct high-CBD cultivars to flooding, powdery mildew, physical wounding, the plant-growth regulator ethephon and herbicide treatments. They measured CBD and THC as the flowers matured.
The treatments affected plant health and, in some cases, the total amount of cannabinoids produced. However, the CBD-to-THC relationship generally remained stable. The researchers observed that CBD and THC increased proportionally as the flowers developed, rather than stress consistently pushing plants toward a higher THC ratio. Herbicide-treated plants were an exception largely because severe injury left them nearly dead.
That finding does not mean growing conditions are irrelevant. Flowering time, harvest timing, plant health and environmental conditions can influence total cannabinoid concentration and crop yield. It does mean that stress alone is not a reliable explanation for why an otherwise compliant crop becomes noncompliant. Genetics, cultivar uniformity and the maturity of the flowers are central factors.
For breeders, the practical lesson is to screen parent plants and seed populations for cannabinoid chemotype before investing in a full crop. Cornell’s work suggests that DNA-based testing can identify plants carrying THC-associated alleles earlier and more efficiently than waiting for mature-flower chemical testing. Growers should also follow their state, tribal or USDA-approved testing and sampling requirements; the USDA laboratory testing guidelines describe how official compliance testing measures total THC.
Fiber- and grain-focused hemp varieties may present a different risk profile because they are generally selected for low cannabinoid production rather than high CBD flower. But the safest strategy for any production system is the same: use genetically uniform, well-characterized planting material, verify cultivar performance and test according to the applicable regulations.
Cornell’s findings replace a simple but misleading story— that weather or poor farming practices cause hemp to “go hot”—with a more useful one. Field conditions can affect how much a plant grows and how many cannabinoids it produces, but the plant’s inherited cannabinoid chemistry largely determines where its CBD and THC levels are headed.
