AMES, Iowa – Rhubarb, classed as a vegetable, is used as a fruit because its high acidity gives it a tart flavor. Only the leaf stalks – used in pies, tarts, sauces, jams, jellies, puddings and punch – should be eaten. Horticulturists with Iowa State University Extension and Outreach answer questions about rhubarb plant selection and harvesting. To have additional questions answered, contact Hortline at firstname.lastname@example.org or 515-294-3108.
After freezing temperatures in spring, some gardeners express concerns about the edibility of rhubarb. Rhubarb tolerates cold temperatures quite well. Temperatures in the upper 20s or low 30s usually cause no damage. Temperatures in the mid-20s or lower are usually necessary to damage rhubarb. 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.
Flower development is natural for rhubarb and most other plants. Drought, extreme heat and infertile soils may encourage flowering. Age is another factor. Old plants tend to flower more than younger ones.
Regardless of the reason, flower stalks should be promptly pulled and discarded. Plants will be less productive if allowed to flower and set seeds.
Flower formation can be discouraged with good cultural practices. Water rhubarb plants every seven to 10 days during dry weather. Sprinkle ½ cup of an all-purpose garden fertilizer, such as 10-10-10, around each plant in early spring. Control weeds by shallow hoeing, hand pulling or mulching.
Harvest rhubarb when the stalks are 10 to 15 inches long. Grasp the stalk near its base and pull up and slightly to one side. Immediately after harvesting the rhubarb, remove the leaf blades from the stalks with a sharp knife. Discard the foliage. The stalks can be placed in a plastic bag and stored in the refrigerator for two to four weeks.
When harvesting rhubarb, do not remove more than two-thirds of the fully developed stalks from any plant at any one time.
Gardeners should stop harvesting well established rhubarb plants in mid-June in Iowa. Continued harvest through the summer months weakens the rhubarb plants and reduces the yield and quality of next year’s crop.
The cultivars ‘Canada Red,’ ‘Crimson Red,’ ‘MacDonald’ and ‘Valentine’ have attractive red stalks and are good choices for Iowa gardens. ‘Victoria’ is a reliable, green-stalked cultivar. Rhubarb plants can be purchased from garden centers and mail-order companies.