A staple vegetable in New England gardens and farms from Colonial times, the demand for the crop is as strong as ever. Rhubarb acreage has declined, plantings aren’t being replaced. The fabled tough and resilient plant has difficulty getting established and is susceptible to Phytophthora root rot. We used to plant it on heavier soils that weren’t well drained and it thrived. Heavier soils with high organic matter produced abundant crops for years. Now, it struggles on our best soils. The one part of rhubarb production that hasn’t struggled is its price, $3-$5 per pound is common.
Rhubarb likes cooler, well drained soils with a pH of 6-6.8, and afternoon shaded locations. It prefers summer temperatures below 75 degrees. It needs to be exposed to temperatures below 40 degrees to go dormant. The redder the variety gets, the harder it is to grow. Green Rhubarb yields range from 10-18 tons per acre, red varieties are about half the yield. Consumers will say they prefer the red color, but a blind taste test might reveal no significant difference. In our field trials, the greener the strain, the easier it is to grow. Rhubarb leaves contain oxalic acid and should not be eaten.
In row plant spacing is generally 3 feet between plants and 5-6 feet between rows. While plants can be grown from seed, a stand with more uniformity is obtained from divided crowns. Commercially available crown size can vary from one shoot to three shoots per crown. Nourse Farms #1 graded divisions are 3 buds, #2 divisions are one bud.
Plan to plant Rhubarb on heavily composted soils. California recommendations are 15 tons of compost prior to planting. Rotate soils for 1-2 years into green manure crops without using any herbicides prior to planting. I’ve been recommending heavy mulch after planting for weed control and lowering soil temperatures. 3-4 inches applied every spring will keep out summer weeds, conserve moisture, and keep the soil temperature down. Drip irrigation is an easy, low-cost way to manage soil moisture. Heavier yields and longer crop cycles can be realized with proper care.
Harvesting too soon after planting is one of the biggest mistakes. Allow rhubarb to establish for 3 years prior to harvest, developing 2 foot diameter crowns. Do not harvest the year after planting to allow for the plant to develop. Two years after planting, harvest lightly depending on the amount of growth in your field. Harvesting too much is the second biggest mistake. I recommend leaving a minimum of 25-30% for healthy regrowth. Other recommendations vary from leaving 2-3 stalks to 30-50%. Another rule of thumb is to end harvest when new stalks emerge thin or spindly. Flower stalks should be removed with a knife as they appear. Young plants can be uprooted if flower stalks or young stalks are pulled.
PESTS AND DISEASES
Rhubarb is not immune to pests and disease. Slugs, Leafhoppers and Rhubarb Curculio need to be controlled early. Leaf spots, Ramularia and Ascochyta are also preventable with early intervention. Tank mixing insecticides with fungicides should begin to be applied in early May through the growing season. The prevalence of Phytophthora Root Rot justifies the preventative control in spring and fall. It will manifest itself in water logged and poorly drained situations and spread down the row.
Plan to replant every 6-8 years after reaching maturity. Plantings that are too old will have dense growth that results in small sized petioles, reducing yield and increasing harvest costs. Regular division every 6-10 years and re-locating the field will add to crop yields. Growers should consider dividing 10-15% every year as an annual maintenance.
An annual schedule should include early spring herbicide application, prior to growing, to kill overwintering perennials and prevent seed germination. In late April, a straw application after plants begin to grow. Applying Gramoxone or other burn down material with a backpack sprayer to selectively kill any emergent weeds should occur every other week through most of the season. This crop will benefit when long residual herbicides aren’t used. Irrigate 2-3 times per week. Two inches per week will sustain the crop, three to four inches will help it flourish before and during harvest. Maintain good soil moisture for your soil type.
In spring apply 200-250 pounds of 20-10-20 three times, adjust N-P-K levels according to your soil test. Application intervals are before growth begins, before harvest, and after harvest. Remember to consider adding micro nutrients and slow release products. The total pounds nitrogen applied should be reduced according to compost quantity. Apply 15-30 tons of compost in early Novem- ber, 3⁄4-1 1⁄2 pounds per sq/ft. Account for any nitrogen in the compost and decrease the spring application accordingly.Varieties have special requirements and their own personalities. MacDonald’s downside of greenish stalks is offset by its vigor. It is easy to grow and is a consistent producer. Cawood Delight is my favorite, it fills the red color requirement. The plant is compact with very thick stems. Flavor isn’t as much of an issue as tender- ness. Harvesting too late in the season will result in tougher and mealy stalks.
Forcing can be accomplished in two ways. Potting divisions in large containers, rhubarb can be forced in a greenhouse. After harvest, the pots would move outside until the following spring. Remember they won’t like hot spots or black ground covers. Regular division would occur after 3-4 years of harvest.
In the field, row covers can be applied to a few rows. As tem- peratures exceed 70 degrees, remove the covers. The goal is to begin harvest 1-2 weeks early. Pay attention to weeds and killing frosts! Remove the covers and weed every two weeks. Cover during frost nights, 2 covers may be necessary some nights.Information used to write this article can be found in The Rhubarb Compendium online at www.rhubarbinfo.com.