Rhubarb usually can tolerate temperatures in the low 30s, even upper 20s (degrees F), but can suffer damage below that. As you know, one has to be careful about rhubarb, because if the leaves suffer cold damage, the oxalic acid from the leaves may filter into the stalks, making them toxic as well. Signs of cold damage are blackened, shriveled, limp leaves and limp, mushy stalks. If you see any damage to either leaf or stalk, discard both leaf and stalk. To be on the safest side, you can cut all stalks down regardless of whether they show signs of damage, and wait for the plant to recover and start new growth. However, some sources say that that might not be necessary. If you wish, you can wait a few days to see if any damage becomes evident, then discard only the affected leaves and stalks. Otherwise, if your plants do not show any sign of damage, and the stalks are firm and upright, some sources indicate that these healthy-looking stalks should be fine to harvest. For more information, you might visit:
https://hortnews.extension.iastate.edu/1997/5-2-1997/rhubarbsafe.html
https://www.ag.ndsu.edu/lamourecountyextension/frosted-rhubarb
http://web.extension.illinois.edu/state/newsdetail.cfm?NewsID=30622
Rhubarb usually can tolerate temperatures in the low 30s, even upper 20s (degrees F), but can suffer damage below that. As you know, one has to be careful about rhubarb, because if the leaves suffer cold damage, the oxalic acid from the leaves may filter into the stalks, making them toxic as well. Signs of cold damage are blackened, shriveled, limp leaves and limp, mushy stalks. If you see any damage to either leaf or stalk, discard both leaf and stalk. To be on the safest side, you can cut all stalks down regardless of whether they show signs of damage, and wait for the plant to recover and start new growth. However, some sources say that that might not be necessary. If you wish, you can wait a few days to see if any damage becomes evident, then discard only the affected leaves and stalks. Otherwise, if your plants do not show any sign of damage, and the stalks are firm and upright, some sources indicate that these healthy-looking stalks should be fine to harvest. For more information, you might visit:
https://hortnews.extension.iastate.edu/1997/5-2-1997/rhubarbsafe.html
https://www.ag.ndsu.edu/lamourecountyextension/frosted-rhubarb
http://web.extension.illinois.edu/state/newsdetail.cfm?NewsID=30622