Hemp and Cold Temperatures

Hemp flower harvest is well underway in the state of Vermont. Harvest can span over a several week period and proceed well into late fall. Although temperatures have been mostly mild throughout Vermont, some areas have experienced several light frosts in the past month, and as October continues on, temperatures will drop again. We have received many inquiries on the type of damage hemp plants might experience if they get frosted.

The good news is that hemp is quite frost tolerant! Ideally hemp should be harvested before the first hard frost, but if you must harvest after the first hard frost, yields are not likely to be impacted. Hardy and mature hemp plants can easily tolerate a frost of 29-32°F, while those temperatures would kill seedlings in the spring. A moderate freeze of 25-28°F can damage vegetation, and will impact semi-hardy plants. Colder temperatures will also cause green plants to turn purple/red, but the change of color of the vegetative tissues does not necessarily mean that the oils (i.e. CBD) in the plant will be affected. Freezes of 24°F and colder will cause heavy damage to most plants.

In our 2018 Hemp Cold Tolerance Trial, we tracked the temperature at the base of CBD hemp plants in plots with and without row cover, and tested the total potential CBD at 6 different dates in each treatment from October 18th to October 26th. Row covered plants had a higher average temperature and reached less temperature extremes than uncovered plants as expected, but the difference in temperature did not appear to correlate to total potential CBD concentrations. The lowest temperature was 27.8°F in the uncovered plants, indicating that light frosts will not affect the total potential CBD quality. For more information on the trial, see: https://www.uvm.edu/sites/default/files/media/2018_Hemp_Cold_Tolerance_Trial.pdf

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