What if my rhubarb freezes?

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.

 

Liz Meimann

I received both my undergraduate and graduate degrees in Food Science at Iowa State University. I love to quilt, sew, cook, and bake. I spent many years gardening, canning, and preserving food for my family when my children were at home.

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26 thoughts on “What if my rhubarb freezes?

  1. 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?

  2. 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.

  3. 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

  4. This was a very helpful page! Thanks for having this up on the ISU Extension website.

  5. 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 ?

  6. 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

  7. Pulling is often advised for harvesting rhubarb so it can be pulled if frozen. The frozen but now thawed stalks should pull easily.

  8. 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.

  9. 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,

  10. 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.

  11. 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.

  12. What if it’s a lite freeze and just part of the leaf turns brown the rest is nice n green and the stalk is nice it doesn’t wilt or turn black. It safe to eat??.

  13. 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.

  14. 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.

  15. 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

  16. 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?

  17. 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 article can be found at https://aggie-horticulture.tamu.edu/travis/wp-content/uploads/2013/06/Rhubarb_Tx_Style2013.pdf. Thanks for contacting AnswerLine.

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