A sure sign of spring at AnswerLine are the calls from people concerned about the safety of their rhubarb plants. It seems like every year we have a week or so of really nice temperatures that allow the rhubarb plants to grow vigorously. Then the temperatures take a dive and we have a frost or freezing weather. There is an old wives tale that says rhubarb that has frozen is poisonous and that you should destroy or dig up your plants to stay safe.
That old wives tale is just that; a tale that is not correct. If your patch of rhubarb freezes, the fleshy part of the plant will freeze. After a day or two, the frozen leaves and stems will become soft and blackened. This is a result of the damage that freezing and thawing cause to the plant. Most people, when they pick rhubarb, are particular and choose the nicest, freshest looking stalks. They would not choose softened, black, or mushy stalks. Those stalks should be pulled and discarded; this is something most people would do without thinking.
Remember, only the stalks or petioles should be eaten because the leaves contain moderately poisonous oxalic acid. It is generally recommended that home gardeners stop harvesting rhubarb in early to mid-June. Continued harvest through the summer months would weaken the plants and reduce the yield and quality of next year’s crop. The rhubarb stalks may become somewhat woody by mid-summer, but they don’t become poisonous. Sometimes we have callers wanting to harvest enough for a crisp or a pie during mid-summer. We tell them to look for some smaller, tender stalks that could be pulled. If the rhubarb patch is an older, established patch pulling a few stalks should not cause permanent damage to the patch.
Enjoy your rhubarb.
My rhubarb crowns froze as it came out of the ground, what do I do?
Wait to see what happens when the weather gets warmer. It will be safe to eat.
This answer is in conflict with the University of Illinois Extension that says that the oxalic acid travels from the leaves to the stalks and then they become toxic. Now I’m wondering who to believe?
If a stalk of rhubarb has frozen, the University of Illinois says that those particular stalks–that will shrivel and turn black after freezing–would not be safe to eat. If you are out in the rhubarb patch, will you choose stalks that are shriveled and black? Or will you choose stalks that look crisp and nice? The stalks that look good are safe. Stalks that look shriveled or black may or may not be safe, but you would never choose to eat those anyway.
Think a clarification is needed here. If the rhubarb is hit with a frost the leaves will wilt and the oxalic acid present in the leaves will migrate to the stalk. Over a few days the entire stalk is likely to shrivel and turn black, however the morning after the frost the stalk may look fine. In a heavy frost or freeze the entire plant will wilt.
Just how toxic is oxalic acid? Hampshire College reports “Rhubarb leaves contain 0.5-1.0% oxalic acid, so that you would need to
eat quite a large serving of the sour leaves, perhaps 10 pounds to get a lethal dose. However, a fraction of that could nevertheless cause severe symptoms of oxalic acid poisoning.” http://helios.hampshire.edu/~nlNS/mompdfs/oxalicacid.pdf
That is the same oxalic acid that migrates to the stalks.
Every Spring the Help Line will get at least 1 call regarding frost and rhubarb. The recommendation is: if the leaves appear to be wilted due to frost DO NOT use the rhubarb as the oxalic acid will have already begun its migration into the stalks even though the stalk looks fine . Cut the stalks down to the ground and new growth will emerge. In a heavy frost or freeze the entire plant will appear wilted, the stalks will be limp. Again, cut the plant down to the ground.
Larry Stratton MG
University of Illinois Extension
Peoria County
This was a very helpful page! Thanks for having this up on the ISU Extension website.
With temperatures in nw iowa expected to drop we harvested some rhubarb and have attempted to cover as well. I was worried as how to know if my rhubarb has frost damage and is toxic. So if we are expected to have frost over the next few nights we need to cover and uncover then wait several days for the actual results ?
Hi, cover your rhubarb at night and uncover it during the day. Know that if the rhubarb gets for us damage or a couple days time so stems and leaves will start to wilt and turn a little black. Don’t pick those to eat, but then again you wouldn’t have chosen those stocks to eat. The whole rest of the plant will not become toxic. Any fresh rhubarb growing after that time will also not become toxic. If you have further questions please don’t hesitate to call us at AnswerLine. In Iowa the number is 1-800-262-3804
Should stalks be pulled or cut?
Pulling is often advised for harvesting rhubarb so it can be pulled if frozen. The frozen but now thawed stalks should pull easily.
What causes Rhubarb to bolt when it is first beginning to grow?
The rhubarb bolted early this year due to the really warm weather we had over the past few weeks.
If the rhubarb has frozen, will it loose it’s tartness?
Hi, it should not lose any tartness after freezing and thawing your rhubarb.
This is incredible article for rhubarb. I never felt its leaves would be toxic. I would for surely avoid them. thank you for help.
Thank you for this concise and informative article!
Hi Christena, thank you so much for your positive feedback. Here we are once again, faced with cold temperatures when our spring vegetables are coming forth.
What’s the lowest temperature that would cause any damage to rhubarb? We have 2 plants that were planted from crowns two years ago and 4 seedlings I started from seed over the winter.
I started to harvest some of our rhubarb already since it has so many stalks that are over 12 inches long (not including the leaves) and were at least an inch wide. We had a nice warm spell and the one rhubarb plant really filled out. I’m glad I found this website. I have a hard time finding information about growing rhubarb.
Thank you,
Hi Doreen, Rhubarb is a cool-season vegetable and hardy to cold temperatures. Light and moderate spring freezing temperatures in the low thirties and upper twenties are not likely to cause much damage to rhubarb, but once temperatures dip below 24 degrees Fahrenheit, serious problems could result. Rhubarb stalks that have been damaged by severe frost may be inedible, but new stalks that emerge after the frost will be perfectly safe to eat. Frost damaged leaves should be removed. Rhubarb leaves are filled with oxalic acid that makes them unfit for consumption. In freezing temperatures that acid moves from the leaves down into the stalk. Within two or three days, frost-damaged stalks will turn black and limp. When ingested, rhubarb stalks containing oxalic acid will create digestive issues or poisoning.
You might enjoy reading more about rhubarb in a piece written by the University of Minnesota.
What is the lowest temperature that would not cause any damage to rhubarb? I previously asked this, but didn’t clarify I don’t want my rhubarb to have any damage at all in the spring before &/or during the harvest period. I prefer to cover them to avoid any rhubarb loss. I would like to know the lowest temperature that would cause damage, so I know I don’t need to cover them if for example it’s going to get down to 39 degrees, but I need to cover them at 38 degrees.
Thank you.
Hi Tony, the best advice is this: A light frost will not harm rhubarb. However, if temperatures were cold enough to cause the rhubarb leaves to wilt or become limp then damage has been done. The leaves should be removed and discarded. The leaf stalks can be used from any new, normal leaves. The leaf blade of rhubarb is poisonous regardless of whether it suffered cold damage as it naturally contains oxalic acid. The leaf stalk is the edible portion of this plant. However, when leaves become frozen, the oxalic content of the stalks increases, making them dangerous to consume. Per a U of WI article, it’s safe to harvest rhubarb if the plants show no signs of damage two or three days after the freeze event.
My rhubarb plant was quite large and desperately needed to be harvested. I wast aware of the freezing issue until after I harvested it. Over the last few days we have had temperatures hovering around freezing. When I harvested it I noticed a few shrivelled leaves. I honestly couldn’t tell you if it was on a stock I harvested it not. Do I need to be worried? I never gave them time to get black. I am new to growing rhubarb.
Brooke, According to ISUEO horticulture extension, “Rhubarb is a tough plant. Temperatures in the upper 20s or low 30s usually cause little or no damage. A hard freeze (temperatures in the mid-20s or lower) is usually required to cause serious damage. Rhubarb damaged by freezing temperatures will have black, shriveled leaves and soft, limp leaf stalks. It’s safe to harvest rhubarb if the plants show no signs of damage two or three days after the freeze event. Damaged rhubarb stalks (blackened foliage and limp stalks) should be pulled and discarded. New stalks that emerge after the freeze are safe to harvest.” Based on the info you provide, temperatures hovering around freezing and no mention of soft stalks, it is quite likely that the rhubarb you harvested is not a problem. Leaves can shrivel for reasons other than cold temperatures. For more info about rhubarb see: https://www.extension.iastate.edu/news/yard-and-garden-questions-answered-about-rhubarb
I have the reverse question about freezing temperatures and rhubarb. It is sometimes grown as an annual or a perennial in the south. I have read that if it is going to freeze (like December – February), then it’s ok to go ahead and harvest the whole plant without affecting its vigor for the next season. The logic is that it was going to die back to the crown anyway, so that you’re not weakening it anyway by harvesting. Is this true and if so will it be tough and stringy?
Hi Jan, I can’t confirm or deny the logic you suggest in regard to rhubarb in warmer climates. Perhaps your query could be answered by contacting your local or state extension horticulturalist. While I don’t know which state you hail from, I did find some information from the Texas AgriLife Extension Service, Travis County. The writer of this article suggests protecting the plant during a freeze to insure harvest in the spring.
The question Jan asked on June 16 is the question I have. It would not matter if the state was North Dakota or Texas, If the rhubarb is going to freeze hard that night, is it ok to harvest the entire crop, or does the root system gain any nutrients from the frozen stalks? I know we are told not to harvest after July or August. (or at least not much of the plant) Does this also apply to the night before a forecast hard freeze that is going to kill all the leaves and stalks anyway?
Hi Stan, Rhubarb is a cool-season vegetable and hardy to cold temperatures. Light and moderate freezing temperatures in the low thirties and upper twenties are not likely to cause much damage to rhubarb, but once temperatures dip near 20 degrees Fahrenheit, freezing is likely. The rule of thumb for harvesting rhubarb in the spring is to wait until the stalks are about 12 inches long and harvest only ¼ to ½ of the plant at a time. Harvesting all the stalks just before a frost is not recommended. Those leaves and stalks help support the root system of the plant. Even after they are killed by frost, the carbohydrates in the dying leaves and stems can be utilized by the roots and crown to build-up and store energy for new growth the next growing season. The same is true in the fall. Allowing the foliage to naturally die back after a frost is important to getting the plant to go fully dormant. Removing the leaves early can prevent the plant from going fully dormant in fall which can cause issues with overwintering and good growth next spring.
In warmer climates where rhubarb is grown as an annual, covering the young plants is advised as they may not be strong enough to withstand a complete harvest; further, it is unlikely that an annual plant would encounter a hard freeze with covering as the ground temperatures are warmer and will protect the plant with proper covering.
I needed to relocate my rhubarb plants. I know not to harvest, how to fertilize, etc. My question, since we are not suppose to harvest the first and second season, do I leave the leaves that have wilted and died on the plant or can I pull and dispose of them? I know not to feed the leaves to livestock or humans.
Many thanks, I have read may sites but this subject has not been addressed.
Hi Mary, I contacted our Ia State extension horticulralist regarding your question. His reply is as follows:
“Any dead leaves can be removed any time when they wilt and brown. It’s not essential that it happens (the plant would be just fine even if you didn’t cut them off) but it can help appearance and reduce the potential of harboring disease or insect pest issues (of which there are very few for rhubarb, which is why it’s not required).” Thank you for contacting AnswerLine.
If your rhubarb freezes, don’t worry! Rhubarb actually freezes quite well and can still be used in many recipes, even if frozen unintentionally.