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Poison rhubarb? - MessAge Media: Home Garden

Poison rhubarb?

Ask A Master Gardener June 5, 2019

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Posted: Thursday, June 6, 2019 5:00 am | Updated: 8:09 pm, Wed Jun 12, 2019.

DEAR Master Gardener: My rhubarb froze. I heard it is now poisonous. Is this true?

Dear Frozen Rhubarb: This time of the year in Aitkin County we can see a late frost this weekend. Official last frost is anytime between May 16-31 in northern Minnesota, but you know…and I know…that sometimes it is even in the first week(s)of June.

The old information about not eating rhubarb after it has suffered a freeze is, and is not, true.  

Once your plant suffers a freezing temperature, the fleshy part of plant will freeze and within a day or two those leaves will turn black and soft. This is the freezing/thawing damage to the plant. Pull and throwaway those stalks/leaves. Rhubarb leaves contain moderately poisonous oxalic acid. During the freeze/thaw the acid goes into the stalk. Don’t harvest these damaged stalks. New stalks/leaves will regrow, and you can harvest the new stalks. Always throw away the leaves, as they are not edible.

An old wife’s tale said that after a hard frost, oxalic acid, moved from the leaves into the leafstalk, and into the roots. Not true as the acid does not go into the roots. New growth is harvestable.

A word to the wise rhubarb grower: STOP harvesting rhubarb in early to mid-June. If you continue to harvest after this, you will weaken the plants and next years yield/quality will be reduced.  And, anyway they can become woody by midsummer. The plants need to store up sugar/energy to last through our tough winters!

Information from Iowa State Extension (https://blogs.extension.iastate.edu/answerline/2017/05/01/what-if-my-rhubarb-freezes/) and University of Illinois Extension (https://web.extension.illinois.edu/stat/newsdetail.cfm?NewsID=30622}

Ask A Master Gardener questions answered by Janice Hasselius, University of Minnesota, Master Gardener, Aitkin Co. Extension:

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