Rhubarb is a favorite for many Iowa gardeners. The rhubarb leaf stalk (botanically called a petiole) is used in pies, tarts, sauces, jams, jellies, puddings, and punch. Although categorized as a vegetable, rhubarb is used as a fruit because its high acidity gives it a tart flavor. Below is more information about rhubarb including cultivars, growing conditions, fertilization, harvesting, care, and disease and insect management.
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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. These cultivars can be purchased from garden centers and mail-order companies.
Rhubarb performs best in well-drained, fertile soils high in organic matter. Heavy, clay soils can often be improved by incorporating organic matter, such as compost or well-rotted manure, into the upper 12 to 15 inches of soil. The planting site should also receive at least 6 to 8 hours of direct sun daily. Avoid shady sites near trees and shrubs.
Spring is the best time to plant rhubarb in Iowa. Plants can be purchased at garden centers or from mail-order catalogs. Digging and dividing large existing plants is another source of plants.
Plants growing in containers should be planted at the same depth as they are currently growing in their pots. Bare-root plants should be planted with the buds 1 to 2 inches below the soil surface. Rhubarb plants should be spaced 3 feet apart.
After planting, water thoroughly. Continue to water the plants throughout the first growing season. During dry weather, a deep soaking every seven to 10 days should be adequate.
It’s best to wait two years (growing seasons) before harvesting any stalks. The two-year establishment period allows the plants to become strong and productive.
Water rhubarb plants during dry weather to maintain healthy foliage throughout the growing season. Healthy plants are able to store large amounts of food in their roots, resulting in a good harvest the following year. During dry weather, a deep soaking every seven to ten days should be adequate.
Rhubarb requires annual fertilizer applications for good growth and large yields. Apply fertilizer in early spring before growth starts. Broadcast one-half cup of all-purpose garden fertilizer, such as 10-10-10, around each plant and lightly work it into the soil. Avoid getting fertilizer directly on the crown. In most garden soils, only nitrogen is needed.
Apply a 2- to 3-inch layer of mulch (dry grass clippings, shredded leaves, etc.) around rhubarb plants. A mulch helps conserve water and control weeds. When controlling weeds with a hoe, cultivate shallowly to avoid root injury.
Flower Stalk Removal
Rhubarb plants occasionally produce flower stalks that should be removed as soon as they appear because flower and seed formation reduces plant vigor and inhibits leaf stalk formation. Infertile soil, extreme heat or cold, or drought may cause the abundant production of flower stalks. Also, old plants tend to flower more than young ones. Rhubarb crowns often become overcrowded after eight to ten years. When this happens, the plant produces numerous small stalks, and yields decrease. This problem can be solved by dividing the plant.
Don’t cut back the rhubarb until the foliage and stalks have been destroyed by a hard freeze. To produce a good crop next spring, rhubarb plants must manufacture and store adequate levels of food in their roots. The foliage continues to manufacture food as long as it’s healthy. Once destroyed, the foliage and stalks can be removed.
When to Harvest
Begin harvesting rhubarb when stalks reach 10 to 15 inches long (usually sometime in April or early May in Iowa). Rhubarb can be harvested for eight to ten weeks, ending in mid-June. If harvested over a longer period, the rhubarb plants will be weakened and less productive the following year.
Do not remove more than one-half of the fully developed stalks from any plant at any one time.
When to Start Harvesting After Planting or Transplanting
Do not harvest rhubarb during the first two years after planting. This allows good crown and root development. During the third season, harvest for a four-week period. In the fourth and following years, rhubarb can be harvested for eight to ten weeks, ending in mid-June.
How to Harvest
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 leaves in the compost pile.
Fresh rhubarb stalks can be stored in a plastic bag in the refrigerator for two to four weeks.
Summer Harvest Should be Avoided
If the rhubarb plants are vigorous, pulling a few stalks and preparing a dish for a special occasion in summer shouldn’t be a problem. Harvesting rhubarb in summer should be a one-time event, not a frequent practice. Continued harvest through the summer months weakens the rhubarb plants and reduces the yield and quality of next year’s crop. In general, it’s best to stop harvesting rhubarb in mid-June in Iowa.
There is an erroneous belief that rhubarb stalks are poisonous in summer. The stalks will likely be a little tougher than those harvested in spring, but they are not poisonous. Plant vigor is the reason that rhubarb should not be harvested in summer.
Transplanting & Division
Early spring is an excellent time to transplant and divide rhubarb. As soon as the ground is workable, carefully dig up the plants just as the new growth begins. Dig deeply to ensure you get a large portion of each plant’s root system.
Once the rhubarb clump is lifted from the ground it can be divided, if needed. Divide the clump into sections by cutting down through the crown between the buds. Each division should contain at least two or three buds and a large piece of the root system.
Replant the rhubarb as soon as possible. The roots must not be allowed to dry out prior to planting. Set the transplants in the ground at the same depth as they grew previously.
If the rhubarb can’t be planted immediately, place the clumps in a plastic bag and store them in a cool, dark location. This temporary storage should be fine for a few days.
Rhubarb also can be successfully transplanted in early fall. Fall-planted rhubarb should be mulched with several inches of straw. The mulch provides additional time for the rhubarb plants to reestablish before the ground freezes.
After planting, water thoroughly. Continue to water the plants throughout the first growing season. During dry weather, a deep soaking every seven to 10 days should be adequate. Let plants establish for two growing seasons before you begin harvesting.
Are the Leaves Poisonous?
Rhubarb leaves and stalks contain small amounts of oxalic acid, which is known to be toxic at high doses. Oxalic acid is a naturally occurring compound in rhubarb and many other plants (some of which are at much higher concentrations). The concentration of oxalic acid is higher in rhubarb leaves than in the stalks. For this reason, it is advised not to consume rhubarb leaves.
Can I Put the Leaves in the Compost Pile?
While the rhubarb leaves contain a poisonous compound called oxalic acid, they can be used in the compost pile. Oxalic acid and soluble oxalates are not readily absorbed by the roots of plants and these compounds quickly break down when composted. Compost containing decomposed rhubarb leaves can be safely worked into the soil of vegetable gardens.
Can I Eat After Rhubarb is Exposed to Freezing Temperatures?
There is a fairly common perception that rhubarb is not safe to eat after the plants have been exposed to freezing temperatures. Rhubarb can tolerate springtime temperatures in the upper twenties and low thirties. After a freeze event, examine foliage and stalks. 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 in spring, should be safe to eat. Rhubarb plants showing no sign of damage are fine and can be harvested.
Are Plants Poisonous During the Summer?
There is an erroneous belief that rhubarb stalks are poisonous in summer. The stalks will likely be a little tougher than those harvested in spring, but they are not poisonous.
Small Spindly Stalks
Newly planted rhubarb will initially produce small, spindly stalks during its 2-year establishment period. After planting rhubarb, wait two years (growing seasons) before harvesting any stalks. The two-year establishment period allows plants to become strong and productive.
The stalks of large, old plants may be small and spindly because of overcrowding. Large, old plants can be dug and divided to reduce overcrowding.
Poor plant vigor due to poor cultural practices is another possibility. Follow good cultural practices, such as removing flower stalks, fertilizing, and watering, to promote plant vigor.
Overall, rhubarb has very few disease issues. There are a few diseases that may affect rhubarb and they are uncommon in healthy plants growing in ideal growing conditions.
When it occurs, Phytophthora crown or root rot is a serious disease of rhubarb. Slight, sunken lesions at the base of the stalks enlarge rapidly, resulting in wilted leaves and collapse of the entire stalk. The crown and roots turn brown or black and begin to disintegrate.
Ascochyta leaf spot starts as numerous small yellowish-green areas on the upper surface of the leaves. Within a week of initial symptoms, the leaf tissue turns brown and dies, resulting in angular spots. These spots have white centers surrounded by a red zone and then a grayish-green zone. Often the dead tissue will drop out, giving the leaves a shot-hole appearance which may be confused with insect feeding.
Anthracnose stalk rot is first noticed when leaves begin to wilt and large, water-soaked lesions form on the stems. The lesions quickly enlarge and turn black. The stems may have a twisted appearance and the whole stem may collapse.
These disease problems can be avoided or minimized by planting in well-drained soil in a sunny location. Planting disease-free plants where rhubarb has not been grown for four to five years also helps avoid disease problems. Additionally, good cultural practices, such as leaf clean-up in the fall and regular fertilization to promote vigorous growth, help prevent diseases.
Few insect pests cause problems on rhubarb and, even when present, rarely cause enough damage to warrant control.
The rhubarb curculio is a large, rusty snout beetle about three-fourths inch long. It causes minor damage by puncturing rhubarb stalks. The rhubarb curculio lays its eggs in the stems of wild dock and other weed hosts. Eliminating weeds in and near the rhubarb planting in July, after the eggs are laid, will aid in controlling this insect.
The stalk borer also punctures rhubarb stalks. Eliminating grassy and large-stemmed weeds around rhubarb plants also helps control the stalk borer.
- When is the best time to plant rhubarb?
- What are the best rhubarb varieties for home gardens?
- What would be a good planting site for rhubarb?
- When can I start harvesting newly planted rhubarb?
- When should I stop harvesting rhubarb?
- Can I harvest rhubarb in summer?
- What is the correct way to harvest rhubarb?
- When can I cut back rhubarb plants?
- When can I transplant rhubarb?
- When can I divide rhubarb plants?
- My rhubarb plants are blooming. What should I do?
- Why is my rhubarb flowering?
- Why are my rhubarb stalks small and spindly?
- Are rhubarb leaves safe to put in the compost pile?
- Is it safe to eat rhubarb after the plants have been exposed to freezing temperatures?
- Rhubarb in the Home Garden (publication)