Rheum rhabarbarum
Last revised February 15, 2010
Rhubarb. Photo credit: Bill Mansour, Oregon State University
Rhubarb variety names are often confusing. Different names are often assigned to the same variety in different regions or countries as people move planting material from one location to another. An excellent source of information on rhubarb names is: A Bibliography of Rhubarb and Rheum Species, Bibliographies of Literature of Agriculture, Number 62, 1988, U.S. Department of Agriculture. A good on-line source of information on rhubarb (Rheum) species as well as a wealth of other information and links on rhubarb is The Rhubarb Compendium.
Hothouse rhubarb is produced in Washington and Michigan with the total U.S. production of hothouse production being about 175 acres. Total field and hothouse production in the U.S. is about 1200 acres. This is produced mostly in the states of Oregon, Washington and Michigan.
VARIETIES
Rhubarb varieties are classified as red or green. Green types are again differentiated as green and speckled (pink).
Red stalk types:
Crimson (may also be called Crimson Cherry, Crimson Red, or Crimson Wine). This is reportedly the only variety of consequence in Oregon. It produces brightly colored red stalks with the unique characteristic of being red throughout under normal temperature and moisture conditions of the Pacific Northwest. Other vigorous red varieties are Valentine and Cherry Red (Cherry, Early Cherry?) which is reportedly grown in California, producing long, thick, deep-red stalks.
Speckled types (pink):
Victoria produces large stalks of excellent quality, long, round with smooth ribs. It develops pink speckling on a light green stalk with the pink color being more intense at the bottom of the stalk, fading to a solid green near the top. Victoria is commonly used for forcing.
Strawberry is very similar to Victoria, and may be the same variety. MacDonald is another "pink" type that produces well.
German Wine is similar to Victoria but slightly more vigorous and more intense in color, typically with a darker pink speckling on a green stem.
Green types:
Riverside Giant, a cold-hardy, vigorous producer with large diameter, long, green stalks.
For more information on varieties, see the Varieties chapter of The Rhubarb Compendium.
PROPAGATION
Rhubarb seed is not normally used to establish production fields. Healthy, vigorous 3 to 4-year old crowns are divided to obtain two or more buds per seed-piece. Five or six year old crowns will yield 8-10 good quality pieces.
Both red and green petiole varieties are available. Only the red varieties are important. As much as possible, obtain planting stocks only from reputable nurseries. When rhubarb planting stock is obtained from old fields, care should be taken to insure that crowns are free of diseases.
Crowns may be dug in late fall and stored in a cool place for planting in spring, or dug in late winter or as early in spring as possible. Plant crown divisions in March or April or as soon as soil conditions allow.
SOILS
Use a well-drained but moisture-holding soil with a pH of 5.5 to 7.0. The lighter soils will produce an earlier crop but require more irrigation and fertilization.
Fields to be planted should be plowed deeply and worked in the fall and/or spring. Choose fields free of problem perennial weeds.
FERTILIZERS
For the most current advice, see Nutrient Management for Sustainable Vegetable Cropping Systems in Western Oregon, available as a free download from the OSU Extension Catalog
A soil test is the most accurate guide to fertilizer requirements. The following recommendations are general guidelines:
Liming should be done if the pH is below 5.6. Manure at 25 to 35 tons/acre may be applied in the fall or as early as possible in the spring. (Do not apply manure or fertilizer within 2 weeks of the lime application.)
Fertilizer - In year of setting apply the following:
Nitrogen: 70-80 (N) 70-80 lb/acre
Phosphate: 70-80 (P2O5) lb/acre
Potash: 140-160 (K2O) lb/acre
In subsequent years apply the following:
Nitrogen: l40-160 (N) lb/acre
Phosphate: 70-80 (P2O5) lb/acre
Potash: 140-160 (K2O) lb/acre
Boron (B), Apply 1 to 2 lb/acre. A foliar application of 0.5 lb/A may also be made in early spring when plants are 6-8 inches tall.
Nitrogen fertilizer applications should be split into 3 sidedressings: before growth starts in the spring, after growth starts and after harvest. Nitrogen rates may be reduced in the first two years with manure applications.
PLANTING
The crown pieces are planted 3-6 inches deep, 2-3 feet apart, in rows about 4-6 feet apart or in a 4x4 foot grid to allow for cross cultivation. The most common spacing in Oregon is 2' x 6'. About 3600 plants would be needed per acre. Rhubarb intended for mechanical harvest is planted 18 inches apart in rows 4 feet apart. This spacing would require about 7200 plants per acre.
IRRIGATION
Irrigation is usually not necessary during the spring harvest of April and May in the Willamette Valley. Maintain adequate soil moisture after the harvest season, to insure good regrowth. A second crop may be harvested in early to mid July after which the field may be regrown or not harvested and allowed to go dormant through late July and August. If harvested in July, growth will be delayed the following spring.
Soil type does not affect the amount of total water needed, but does dictate frequency of water application. Lighter soils need more frequent water applications, but less water applied per application.
GROWTH ENHANCEMENT
Rhubarb requires a dormancy period of temperatures below 40 F to break dormancy and stimulate the production of leaf petioles. Winter conditions in the Pacific northwest easily meet this requirement. When temperatures begin to exceed 45-50 F, crown buds begin to develop. Early growth may be enhanced ten days to two weeks by the use of clear plastic row covers which may be applied in early February. Allow sufficient slack for stalk growth. Gibberellic acid may also be injected into the crowns at 0.0l grams of GA3 (Pro-Gibb) per crown. Injection is accomplished by using a custom-built, pneumatically activated, hand-operated injector wand that delivers the correct dosage when it is pushed into a rhubarb crown.
Harvest may start as early as mid-March. At the end of petiole harvest (May or June) new shoots will emerge. These will provide the reserves for the following year's crop. If growth and moisture reserves are adequate, a second harvest may be made in August or September, but stalks must be firm and not pithy.
INDOOR OR "HOTHOUSE" FORCING
Rhubarb crowns may be harvested for "forcing" indoors to obtain stalks for market earlier than would be possible from the field or during the off-season. This involves the digging of entire crowns and selecting the largest to place in a "forcing house" at a later date. Harvest of forcing rhubarb usually begins in early January.
Crowns used for forcing are usually two or three years old. Large crowns with a few large buds are preferred. Crowns from fields that have been harvested for outdoor production are not recommended for forcing, due to the poorer yields that result.
Structures used for forcing vary considerably in size and type. Consideration needs to be given if equipment will be used inside the house, and method and cost of heating.
Rhubarb crowns in the field must be allowed to go into dormancy and exposed to a certain rest period before any forcing is possible. This requires exposure to temperatures between 28 and 50 F for 7 to 9 weeks at the end of the growing season. Exposure to temperatures below 28 F reduce yields, while temperatures above 50 F contribute nothing to the required rest period. The amount of cold required before forcing can be started is referred to as "cold units." Cold Units are the accumulated number of degrees below 49 F (and above 28 F) as recorded at mid-morning. Varieties such as Victoria and German Wine will require about 470-500 cold units to force successfully.
Crowns may be subjected to cold treatment for accumulation of the required cold units either in the field, or in the forcing structure. In Oregon and western Washington where soil conditions allow, crowns are most commonly allowed to reach their rest period in the field, then plowed up for placement in forcing structures and forcing in mid-December. In other areas, where fall temperatures are more extreme, crowns may be dug in October or November and left in the field until excessively cold temperatures require their removal to the forcing structures. Gibberellic acid may be used to substitute for a possible lack of cold induction and to increase uniformity of growth.
At the appropriate time, crowns are plowed out of the ground and placed on the earth floor of the forcing structure as close to each other as possible. This requires about 1 square foot per crown. The spaces around each crown is filled with soil, leaving walkways where needed. At the time forcing is desired the soil is wetted and temperature raised to 56 F. This temperature has been found to produce the best yields. Temperatures between 50 F and 56 F produce more intense red color but slower growth. Temperatures below 50 F may reduce yields, and those above 60 F result in pale stalk color, a faster growth rate, and also may result in lower yields, especially if exceeding 65 F. Regardless of the temperature, stalk color becomes less intense as the crowns are exhausted.
Forcing rhubarb is usually picked about twice a week for about 4-6 weeks. Separate forcing structures may be sequenced, by starting forcing at different times, to provide forced rhubarb into May when field rhubarb becomes available.
It is important to maintain good, but not excessive, soil moisture around the roots in the forcing structure. Production drops off dramatically when the soil becomes dry.
HARVESTING, HANDLING, AND STORAGE
Yields of rhubarb depend on the number of pickings, and the age and condition of the field. In the Pacific Northwest yields may range from 6 to 12 tons per acre for red varieties. A yield of 8-10 tons/acre is most common at the first harvest. Fields may be harvested a second time with yields generally reduced by 50 percent from the first harvest. Green varieties tend to yield more. A well-maintained field may remain productive for 15 or more years.
Rhubarb in Oregon is all hand harvested. For processing, both ends of the petiole are trimmed so that no leaf tissue remains. For fresh market a small amount (1/4 inch) of leaf tissue is usually left attached to the petiole and the basal end is not trimmed. Splitting of the petiole will be more serious if the entire leaf is removed.
A machine has been developed by USDA Agricultural Engineers and may be available through the Wilde Manufacturing Co. of Bailey, Michigan. Fields intended for machine harvest are planted at much higher in-row populations than fields for hand harvest (as noted above).
Stalks should not be pulled during the first year of growth. Stalk color is best after the field is 2 to 3 years old. In subsequent years, harvesting can be expected to start in March and to end in June. This will vary with management practices, with the variety being grown, and is somewhat driven by market demand.
Plants should not be over-pulled at any time, as a certain amount of foliage is required for the development of the present crop as well as next year's crop. A well-cared-for field will last for l0-15 years or longer.
In the Willamette Valley, early rhubarb is harvested for processing from April 25 to May 25. The prime harvest period is from April 25 to May 15. Late rhubarb is harvested from June 25 to July 25. The prime harvest period for late rhubarb is June 25 to July 7.
STORAGE (Quoted or modified from USDA Ag. Handbook 66 and other sources)
Store at 32 F and 95 to 100% relative humidity. Fresh rhubarb stalks in good condition can be stored 2 to 4 weeks at 32 F and high relative humidity. Rhubarb can be hydro-cooled or air-cooled, and the temperature of the stalks should reach 32 or 33 F within 1 day of harvest. The topped bunches or loose stalks should be packed in crates, and the crates should be stacked to allow ample air circulation; otherwise, there is danger of heating and mold growth.
Moisture loss in storage will be much less if the bunched or loose stalks are packed in crates lined with perforated polyethylene film.
Fresh rhubarb cut into 1-inch pieces and packaged in 1-lb perforated polyethylene bags can be held 2 to 3 weeks at 32 F with high relative humidity.
PACKAGING
Rhubarb is commonly packaged in 20-lb cartons, place pack; or, 1-lb film bags, in cartons containing 10 bags each.