Rhubarb

Introduction

Rhubarb (Rheum spp.) has a long history of medicinal use in Asia. Its use as a food crop is much more recent, with widespread table use beginning in the 19^th century. The leaves of the plant are toxic, but the fattened petioles are edible and consumed most commonly in pies and other sweetened desserts, in beverages, and occasionally as a vegetable. Several species of Rheum exist, and modern culinary varieties were likely derived from crosses between some combination of these species. The genus Rheum is not closely related to other vegetable crops; it belongs to the family Polygonaceae, along with buckwheat and many weeds including sorrels and knotweeds.

Rhubarb is a perennial crop that requires a dormant winter period below 40ºF to stimulate vigorous spring growth. It grows most vigorously in cool conditions, and growth is suppressed at high temperatures (>90ºF). For this reason, rhubarb is well-adapted to Northern latitudes in the U.S. and Canada. Once established, a rhubarb planting can remain productive for 8–15 years. 

Types and Varieties

Rhubarb varieties are classified as red, green, or speckled (pink). Currently, the market generally prefers red over green or speckled.

In New England, the most common variety grown is Macdonald, also known as Macdonald's Canadian Red or Macdonald Crimson. This cultivar has large stalks and a vigorous and upright-growing habit, and is resistant to wilt and root rot. It is excellent for pies, canning, and freezing. It can have medium to heavy seed stalk production. At the beginning of the harvest season (late May), the petioles may be a deep red but will lose some or all of their color as the weather warms and as harvest extends through June into early July. 

• Red Types: Crimson (may also be called Crimson Cherry, Crimson Red, or Crimson Wine) does well in New England. It produces brightly colored red stalks with the unique characteristic of being red throughout. Other vigorous red varieties include Valentine and Cherry Red.
• Speckled/Pink Types: Victoria produces stalks that are large, of excellent quality, long, and round with smooth ribs. Stalks are light green with pink speckling that is more intense towards the bottom of the stalk. Stalks are solid green near the top. Victoria is commonly used for forcing. Strawberry is very similar to Victoria, and may be the same variety. 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 is a cold-hardy, vigorous producer with large diameter, long, green stalks.

Soil Fertility

Plant Nutrient Recommendations According to Soil Test Results for Rhubarb

Planting Year

Established Plantings

Planting

Dormant crown pieces should be planted in the early spring, 3"–6" deep, 2'–3' apart, in rows 4'–6' apart or in a 4'x4' grid to allow for cross cultivation. The most common spacing in New England is 3'x6' (2,420 plants/acre).

Field Culture

Maintain adequate soil moisture throughout the harvest season, to ensure good stalk quality and regrowth. 

Flower stalks should be removed when they emerge, by cutting them off close to the crown. Removing these stalks as they appear will extend the harvest season of good quality leaf stalks. 

Rhubarb requires a dormancy period of temperatures below 40ºF to break dormancy and stimulate the production of leaf petioles. Winter conditions in New England easily meet this requirement. When temperatures begin to exceed 45ºF–50ºF, crown buds begin to develop. Clear plastic row covers, applied in early February, may help achieve leaf production 10–14 days earlier. Allow sufficient slack for stalk growth. 

Harvest and Storage

Harvest may start as early as mid-May. When petioles are of sufficient size for the market, they are pulled, not cut, from the plant. For fresh market, a small amount (1/4") of leaf tissue is usually left attached to the petiole and the basal end is not trimmed. For processing, all leaf tissue is trimmed from the petiole.

Stalks should not be pulled during the first year of growth. Stalk color is best after the plants are 2–3 years old. 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. At the end of petiole harvest (late June/early July) new shoots will emerge. These will provide the reserves for the following year's crop. Yields of rhubarb depend on the number of pickings, and the age and condition of the field but should yield an average of 6 tons (600 20-lb cartons) per acre. A well-maintained field may remain productive for 15 or more years. 

Fresh rhubarb stalks in good condition can be stored 2–4 weeks at 32ºF and 95%–100% relative humidity. Rhubarb can be hydrocooled or air-cooled, and the temperature of the stalks should reach 32ºF–33ºF within one day of harvest. If not cooled properly, there is danger of 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.

Disease Control

A handful of fungi can cause leaf spots on rhubarb. Ramularia rhei causes round, tan lesions with red borders on leaves and petioles. The centers may fall out of leaf lesions, causing a “shot-hole” appearance. Ascochyta (Didymella) rhei causes slightly larger spots that may have yellow haloes when immature, eventually darkening to purple or brown. Spots caused by both species may coalesce, involving significant leaf area. Phytophthora cactorum is the most common cause of crown rot. Early symptoms of crown rot include yellowing, wilting, and stunting. Leaf spots and crown rot diseases can be problematic following cool, wet springs. Since both types of diseases overwinter in infected plant tissue, good sanitation practices should control most disease problems. Remove and dispose of infected plant tissue during the summer and after the first frost. Remove all dead foliage in the fall. Improved growing conditions may help minimize disease. Full sun and well-drained soil are essential for the prevention of crown rot. Fertilize plants when growth begins in the spring. 

• azoxystrobin (Quadris F): 6 to 15.5 lb/A; PHI 0d, REI 4h, Group 11.
• Bacillus subtilis strain QST 713 (Minuet^OG): 12 to 24 oz/A; PHI 0d, REI 4h, Group BM02.
• chlorothalonil (Echo 720): 3 pt/A; PHI 30d, REI 12h, Group M05. Do not apply more than 1.35 lb a.i./A per year. 
• copper oxychloride plus copper hydroxide (Badge X2^OG): 1 to 2.25 lb/A; PHI 0d, REI 48h, Group M01. Use preventively.
• cyprodinil plus fludioxonil (Switch 62.5 WG): 11 to 14 oz/A; PHI 0d, REI 12h, Groups 9 & 12.
• fluopyram (Velum Prime; aka Luna Privilege): 6.84 fl oz/A; PHI 0d, 3d (greenhouse), REI 12h, Group 7.
• laminarin (Vacciplant): 14.0 to 22.0 fl oz/A; PHI 0d, REI 4h, Group P04.
• Pseudomonas chlororaphis strain AFS009 (Howler EVO^OG): 2.5 to 7.5 lb/A; PHI 0d, REI 4h. Use preventatively. 
• pydiflumetofen plus fludioxonil (Miravis Prime): 9.2 to 13.4 fl oz/A; PHI 0d, REI 12h, Group 7 & 12. Not for greenhouse use.
• pyraclostrobin (Cabrio EG): 12 to 16 oz/A; PHI 0d, REI 12h, Group 11. Do not make more than 2 sequential applications before alternating to non-Group 11 fungicide.
• thyme oil (Guarda): 1 gal/29–159 gal water (0.625%–3.3% v/v); PHI 0d, REI 0h.