Weeds pose a significant challenge for hemp growers. In addition to competing for water, sunlight, and nutrients, weeds also contaminant yield. The stems of weeds are considered a contaminant in hemp grown for the stalks, impacting processing. Weed seeds in the grain harvest require additional cleaning and sorting. Managing weeds in hemp is a challenge because there are fewer tools available to growers.
There is currently only one registered conventional herbicide for use in this crop. However, there may be another herbicide registered for use in the near future. Until then, Sonalan® HFP, is the only conventional option for hemp growers. Non-chemical management strategies should be considered as part of integrated weed management in hemp. These strategies could include avoiding fields with known high weed densities, stale seedbed technique, tillage and seedbed preparation, choosing vigorous hemp varieties, and equipment sanitation to minimize weed seed spread (Sosnoskie et al., 2024). Additional strategies could also include use of cover crops, though this has not been well studies in hemp.
Pre-emergent Herbicide Option
As a reminder, ethalfluralin, trade name Sonalan® HFP Herbicide (EPA Reg. No. 10163-356), was registered for use in hemp back in 2023. It is used for the control of annual grasses and broadleaf weeds. This product can be applied before planting or after planting but before crop emergence. The product needs to be incorporated mechanically or with irrigation or rain. In a recent study on application method in hemp, there was greater control of grasses when Sonalan® HFP was pre-plant incorporated compared to post-plant preemergent application where both methods received 0.5 inches of irrigation water (Sosnoskie et al., 2024). However, June and July were particularly dry months at the study location, which could have been the reason the incorporated application provided better control of the grasses. Soils that are wet and subject to prolonged flooding can also impact the efficacy of the product. Because Sonalan® HFP has not been tested on all hemp varieties or on all soil types, it is recommended to test the product on a small area first. Injury to the crop could include, stubby roots, swollen stem bases, and stunted plants (Sosnoskie et al., 2024).
The herbicide is applied at a rate of 1.5-3 pints per acre, but will depend on soil type. Lower rates should be used in coarse soils. Applicants need to have the supplemental label when applying this product. The supplemental label can be found here, which expires May 1st, 2026. When using a supplemental label, all applicable directions, restrictions, and precautions on the container label must be followed. The label is the law!
Reference
Sosnoskie, L. M., Bohrer, M., Thomas, A., & Petzoldt, J. (2024). 2024 Weed Science Hemp Field Day handout. Cornell Hemp. https://bpb-us-e1.wpmucdn.com/blogs.cornell.edu/dist/a/7491/files/2024/09/2024-Weed-Science-Hemp-Field-Day-Handout.pdf
