What effect does harvesting the marijuana plant after the first frost have on THC levels?
THC will not be affected by the frost. But, plant tissue can incur some damage. Temperatures that dip below 48.2F (9C) usually affect the marijuana plant in some way. Leaves will suffer damage and need to repair themselves prior to resuming growth or ripening—a process that could take a few days of good weather to complete. If cold weather persists for several days, the damage to the plant is multiplied because it is weakened and increasingly less capable of repairing itself.
To return the marijuana plant to normalcy, the temperature needs to rise into the 60’s, and the plants need to get adequate sunlight to power photosynthesis. When temperatures dip even further down to 40F (4C), leaves, flowers, and other plant parts suffer even more. It takes a couple of days for a plant to fully recover from the damage, and it’s not likely because of the generally cloudy conditions associated with fall. The energy (sunlight) available is limited and the maturation of the bud and THC production is curtailed. Stopping the production of THC helps the plant save energy, particularly when it’s not needed to protect from insects and UV light. Under these conditions, the THC that the marijuana plant already produced is not eradicated by the frost, but you won’t find any new production. Unless there is a genuine possibility of warm, sunny weather in the forecast, you’ll probably want to harvest the marijuana plants.
While I don’t doubt what you’re saying is true. In southern states where you might only get a light cold snap, even down to freezing, I’ve often let outdoor plants go through one or two cold snaps. The cold seemed to promote last-days heavy bud growth that turned the buds an awesome shade of purple.
The seeds were bag-seeds from some Tex-Mex mids that everyone started calling grape-jelly bud as it tasted just like grape jelly and would leave resin on your lips half-way through a normal joint. It was truly some of the best regular-seed smoke I’ve ever grown and in still remembered by many in the area.
Because of this good experience, I have no qualms about letting plants that aren’t quite ready to go through a night or two of light cold and long for some more grape jelly bud.