Many areas in Iowa experienced record or near record temperatures in early April 1997. Those cold temperatures have prompted questions on the edibility and use of rhubarb. Answers to those and several other rhubarb related health questions are presented below.
Is it safe to use rhubarb after the plants have been exposed to freezing temperatures?
The leaves of rhubarb do contain oxalic acid and soluble oxalates. Consumption of rhubarb leaves can cause nausea, vomiting, abdominal pains, and even death. The concern expressed by some individuals is that the oxalic acid and soluble oxalates would move from the leaves to the stalks upon exposure to freezing temperatures. In fact, however, the movement of these compounds into the stalks is not a problem. Gardeners should examine their rhubarb and base their harvest decision on plant appearance. Cold damaged rhubarb leaves will shrivel and turn black. Damaged stalks become soft and mushy. Damaged rhubarb stalks should be pulled and discarded. Any new growth which emerges later this spring would be safe to eat. Rhubarb plants showing no sign of damage are fine and can be harvested.
Do the rhubarb stalks become poisonous by summer?
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.
Is it safe to harvest rhubarb if the plant is flowering?
While the flower or seed stalks should not be used, the leaf stalks are edible. However, the flower stalks should be promptly pulled and discarded. If allowed to develop, the flower stalks reduce plant vigor and next year's production. Flower stalk formation may be caused by drought, infertile soils, and extreme heat. Age may be another factor. Old plants tend to flower more than young ones. Flower formation can be discouraged with good cultural practices. Water rhubarb plants once a week during dry weather. Sprinkle 1/2 cup of an all purpose garden fertilizer, such as 10-10-10, around each plant in early spring. Manure is an alternative to a commercial fertilizer. Apply a 2 to 3 inch layer of well-rotted manure around rhubarb plants in spring. Dig and divide large, old rhubarb plants in early spring or late summer.
Are rhubarb leaves safe to put into the compost pile?
While the rhubarb leaves do contain poisonous materials, they can be used in the compost pile. Oxalic acid and soluble oxalates are not readily absorbed by the roots of plants. Compost containing decomposed rhubarb leaves can be safely worked into the soil of vegetable gardens.
This article originally appeared in the May 2, 1997 issue, p. 57.