By Esther E. McGinnis, Horticulturist
NDSU Extension
I have to admit that I get a little sentimental every time I think about rhubarb because it reminds me of my grandmother. She wasn’t necessarily the best of bakers but she did make a mean rhubarb cake.
Others in this region may have similar childhood memories and greatly value this spring crop as evidenced by the number of questions that NDSU Extension receives. The phones ring in Extension offices across the region particularly when a late-spring frost damages the rhubarb. The number one question is whether frozen rhubarb is toxic.
Before we can answer that question, we have to first talk about the leaves. Most people know that rhubarb leaves are toxic. The leaves contain oxalic acid which can damage kidney function. During World War I, rhubarb leaves were consumed as a vegetable substitute with dire consequences. The scientific literature documented poisonings and even deaths.
More recently, a debate has emerged as to how toxic the leaves are based on their oxalic acid levels. Some sources say that a large amount of leaves would have to be consumed to be fatal on account of oxalic acid. However, recent scientific studies show the presence of other compounds such as anthraquinone glycosides that may contribute to the toxicity of the leaves.
Rhubarb stems (petioles) which resemble celery stalks are the edible portion of the plant because they contain only trace amounts of toxic compounds. The stems are processed into delicious pies and cakes like my grandmother made.
The problem is during late-spring frosts. A hard freeze is widely believed to cause the movement of toxic compounds from the leaves into the stems. Consequently, people are concerned about harvesting rhubarb after a hard freeze.
To compound the complexity, universities across the region give conflicting advice on the risk of consuming frost-damaged stems.
When faced with conflicting advice, the first course of action is to look for peer-reviewed scientific research to settle the question. Unfortunately, I could not find any scientific studies to confirm or disprove this belief.
In the face of ambiguity, the better course of action is to use common sense. A hard freeze causes unmistakable damage to the plant and renders it inedible. Nobody wants to bake with mushy, frozen stems. Bakers want to use good quality ingredients.
To be on the safe side, wait 2 or 3 days after a hard frost and thoroughly inspect the plant. Dispose of mushy stems. If the leaves show browning, wilting or other damage, discard the entire stem. The freeze-damaged stems may be incorporated into the compost pile without any safety concerns.
Fortunately, rhubarb is a tough perennial and will recover from a late-spring freeze. The new growth will be just fine.
For more information about gardening, contact your local NDSU Extension agent. Find the Extension office for your county at https://www.ndsu.edu/agriculture/extension/county-extension-offices.
NDSU Agriculture Communication – May 24, 2022
Source: Esther McGinnis, 701-231-7971, esther.mcginnis@ndsu.edu
Editor: Kelli Anderson, 701-231-6136, kelli.c.anderson@ndsu.edu
FAQs
If stalks and leaves appear normal after a frost, the rhubarb is safe to eat. If stalks or leaves are damaged, and you've pulled the injured stalks, all future regrowth is safe to eat.
Is rhubarb poisonous after a frost? ›
Any stalks with wilted leaves after a cold spell should always be cut from the plant, and never consumed because frost can cause the substance that makes rhubarb leaves toxic to travel down the stalk.
Will rhubarb come back after freeze? ›
Fortunately, rhubarb is a tough perennial and will recover from a late-spring freeze. The new growth will be just fine.
Can rhubarb become toxic? ›
You would have to eat several pounds of rhubarb leaves to reach a toxic level—so don't feel like you need to toss the whole batch if tiny leaf scraps end up in your rhubarb muffins. But do avoid eating the leaves whole—even a few can make you feel sick.
Do I need to cover rhubarb for frost? ›
Rhubarb is hardy and needs no protection over winter. In fact, exposure to cold is necessary to trigger new growth in spring – seven to nine weeks of below 3°C (37°F), depending on the variety. In autumn, allow the leaves to die back naturally, then cut them all away to expose the growing points to winter cold.
What to do if rhubarb freezes? ›
Rhubarb in your garden that has frozen to the point where the stalks become damaged or mushy should be discarded. Check the leaves, too. If the stalks are firm and upright and the leaves have little damage, the rhubarb is OK. Discard the damaged ones and enjoy the stalks that grow later.
When should you not pick rhubarb anymore? ›
Harvest rhubarb from May to June in the northern hemisphere.
Start harvesting in May and plan to stop harvesting at the end of June. Rhubarb stalks get tough and woody as the summer progresses.
How do you revive rhubarb? ›
To renew your rhubarb, it will be necessary to divide the root. The root has become too old and tough to grow well. The time to divide the root will be as early in the spring as you can dig it up.
Is frozen rhubarb safe to eat? ›
All rhubarb stalks that have been exposed to freezing temperatures should be removed and discarded. The re-growth is safe to eat. After a hard frost, oxalic acid may move from the leaves into the leafstalk. When consumed, the oxalic acid can crystallize in the kidneys and cause permanent damage to the organs.
Why is my rhubarb dying back? ›
Die back is a common response to temperatures that are too high. Although rhubarb likes plenty of sun, providing some shade on hot summer afternoons may help. Also, be sure that the rhubarb has adequate water. During hot sunny weather, many large rhubarb leaves wilt and flop over to the ground.
Gastrointestinal (GI) conditions: Don't take rhubarb if you have a bowel obstruction, appendicitis, unexplained stomach pain, or inflammatory conditions such as Crohn disease, colitis, and irritable bowel syndrome (IBS).
Can rhubarb cause food poisoning? ›
Symptoms of toxicity include mild gastrointestinal symptoms, as well as more serious problems, such as kidney stones and kidney failure. Although reports of poisoning are rare, it's best to avoid eating rhubarb leaves, particularly if you have any condition that increases your risk of kidney stones.
What are the warnings of rhubarb? ›
Rhubarb can cause some side effects such as stomach and intestinal pain, watery diarrhea, and uterine contractions. Long-term use can result in muscular weakness, bone loss, potassium loss, and irregular heart rhythm. There is a report of kidney failure in someone who took a product containing rhubarb.
What is the cold weather for rhubarb? ›
Cool temps (28°F-75°F) ideal for rhubarb; below 28°F risks damage. Mulch and water protect rhubarb from extreme cold and heat. Watch for wilting or thin stalks, signs of temperature stress.
Should I let my rhubarb bolt? ›
So, if your rhubarb should happen to bolt, remove the flowering stalks just as soon as they are visible, to which the plant will likely respond by sending up another. If you keep at it, soon the plant will return to the desired priority for foliage production.
Can rhubarb survive in a raised bed? ›
Good garden drainage is essential in growing rhubarb. Planting on raised beds ensures against rotting of the crown. Working plenty of well-rotted manure or compost into the rhubarb bed before planting greatly increases production.
Is rhubarb bad after it flowers? ›
Another question that sometimes comes up is whether the flowering makes the leaf stalks poisonous. The answer is no, the leaf stalks remain edible, regardless of whether flower stalks are present. However, the leafy blade portion is always poisonous due to a high level of oxalic acid.
Can you plant rhubarb before last frost? ›
Start rhubarb seeds indoors 5 to 7 weeks before the last frost; transplant seedlings into the garden after the last frost. Rhubarb crowns can also be planted in autumn before the first freeze.
