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Rhubarb (Rheum rhabarbarum) and sorrel (Rumex scutatus, R. acetosa) are two perennial vegetables hardy enough for Minnesota gardens. Both are in the buckwheat family, Polygonaceae, both have a sour flavor, and both are among the first crops that can be harvested in spring.
Sorrel is an early spring perennial vegetable, hardy into USDA Zone 3. Its bright lemony flavored leaves are welcome in mixed salads, on sandwiches, and in soups. The tartness is greatly reduced by cooking, so large amounts of sorrel can be added to soups.
Sorrel seed can be started indoors three weeks before last frost, or direct seeded in early spring. Choose a spot in full sun with good drainage. Mature plants should be spaced at least a foot apart; if the plant thrives and spreads outside its space, it can be divided in spring.
Once plants are established, harvest sorrel at any time from early spring until the growth is killed by frost. Pluck individual leaves, which can be used whole or chopped. Young leaves are much more tender than older leaves.
Remove seed stalks that emerge, just as you would rhubarb. Sorrel plants are not as long-lived as rhubarb, but new plants are easily started from seed and many gardeners treat sorrel as an annual.
Rhubarb is also called “pie-plant,” because the chopped stalks are often cooked, sweetened, and used as a pie filling, usually with strawberries. It can also be incorporated into breads and cakes, sweetened and used as a sauce, or cooked with sugar, strained, and drunk as a refreshing juice. Few people can eat much rhubarb as a raw vegetable; while juicy and crisp, it is very sour.
Different varieties have varying levels of sourness and fibrousness, and they also vary in color from almost pure green to almost pure red. Usually the skin is more or less red, while the flesh color varies from pale to darker green. Flavor is not correlated with color. Redder varieties are particularly desirable for pies, because the color of the filling is more attractive than the grayish color of cooked green varieties.
Plant stature and vigor also vary among varieties. In general, greener varieties are more vigorous and have longer stalks than red varieties.
Only the long, thick leaf petioles, the “stalks,” are edible. Rhubarb leaves are toxic and must never be eaten. The stalks do contain high levels of oxalic acid, which can tie up calcium and make it unavailable in the body. Eating an occasional dish containing rhubarb does not pose a serious nutritional threat; indeed, spinach, widely considered a health-promoting food, also contains high levels of oxalic acid. However, people with gout, kidney disorders, and rheumatoid arthritis may want to avoid foods high in oxalic acid and should consult with their physicians about consuming these foods.
Rhubarb plants are very large; fortunately, a single plant usually provides enough for any family. Each plant should be allowed a three-foot by three-foot area in the garden. For most gardeners, it’s most convenient to position rhubarb at the edge of the garden. It should be in a spot that receives all-day sun. Rhubarb is hardy in USDA Zone 4, and worth trying in Zone 3.
Rhubarb plants are often acquired from another gardener who is dividing a large plant. Although there is a possibility of receiving a plant infected with a virus, chances are good that the plant is healthy and vigorous, since it is large enough to need dividing. Plants can also be purchased at garden centers and from mail-order catalogs; nursery-grown plants will be virus-free.
Rhubarb seed is also sold through catalogs. The reddest varieties are not available as seed, only as plants. Start seed indoors six weeks before the last frost date for your area. Cover the seed lightly, keep moist, and supply bottom heat to improve germination and seedling health. It may be two or three weeks before seedlings emerge. Harden plants off for at least a week before planting out in the garden.
Prepare the soil by tilling or forking to a depth of at least two feet, to allow the new plants to root easily. Add plenty of garden and kitchen compost or composted manure to increase organic matter, improve drainage, and supply nutrients. It’s best to prepare the soil and let it settle for a few days before planting, so that newly-set plants don’t end up too deep.
Seedlings, divisions from a neighbor’s garden, and plants bought in pots at garden centers should be planted at the same depth they are growing in the pot. Bare-root plants bought through the mail should be planted with the crown of the plant just level with the soil. Water well after planting.
The most important feature of soil for rhubarb is that it be well-drained. Loamy soils are better-suited to rhubarb production than sandy soils, as they are more water-retentive and can also provide more nutrients to the plant. Soil pH is not important: any garden soil in Minnesota, whether acidic, neutral, or basic (“alkaline”), can support a good crop of rhubarb.
The rhubarb plant is considered a “heavy feeder.” Producing its large stalks and leaves, especially since many of them are harvested each year, requires that the plant take in large amounts of nutrients from the soil. Garden soil tests, because they are usually taken from parts of the garden that are planted annually, will not provide information about the rhubarb plant’s needs. Make it an annual practice to supplement the soil with either a balanced commercial fertilizer or rich compost, or both. Addition of manure or compost can add micronutrients and organic matter to soil (see the University of Minnesota Extension publication Composting and Mulching). Note whether growth is vigorous or weak, and adjust the next spring’s fertilizer application accordingly.
Continuous use of high phosphorus fertilizer such as 10-10-10 or 15-30-15, or high rates of manure or manure compost results in phosphorus buildup in the soil. Although phosphate fertilizer applied to soil is bound tightly and resistant to movement in the soil, some runoff may occur. It can then become a major pollution concern in our lakes, rivers and streams. High levels of phosphorus support over-production of algae, which causes significant reduction in water quality (see Preventing Pollution Problems from Lawn and Garden Fertilizers). If your soil tests high in phosphorus, use a low phosphorus (such as 32-3-10, 27-3-3, or 25-3-12) or no phosphorus (such as 30-0-10 or 24-0-15) fertilizer.
Proper watering will enhance good production. Soak the soil thoroughly when watering, to a depth of at least one inch each week during the growing season.
A giant rhubarb plant, growing vigorous in its patch of garden, can be a strong competitor against weeds, and weeds are therefore unlikely to be a problem. During the first year or two, while the plant is establishing itself, keep weeds controlled by hoeing or hand-pulling. Frequent, shallow cultivation will kill weeds before they become a problem. Be careful not to damage the plants when cultivating: keep your tool away from the plant itself.
Rhubarb plants are often neglected once the harvest season is over, but it’s a good practice to continue watering and weeding all summer long. The plants can live fifteen or more years with good care.
The rhubarb plant will produce the next year’s buds at the outer edges of its crown. With each passing year, the plant will become slightly wider, and the center may not produce any new stalks. Like many perennial plants, rhubarb can benefit from division every few years.
Very early in the season, just as new growth is starting, use a clean, sharp shovel to cut the plant in half or in thirds. Remove the divisions to newly prepared planting sites, or give them away. Alternatively, try to dig up the entire plant, and divide it using a sharp knife.
Because rhubarb makes such nutritional demands on the soil, it may be worth moving the plant to a new site every so often, and rotating another crop into the former rhubarb patch.
Wait until the second season, or the third season if the plants were started from seed, before harvesting.
One of the first crops of the year, rhubarb is an eagerly awaited spring vegetable. Begin picking stalks as soon as they have reached their full length: depending on the variety, they may be only 12 inches long, or as long as two feet.
To pick rhubarb, hold the stalk firmly, pull, and twist. Using a knife to cut the stalks from the plant is not recommended. The knife can carry diseases from plant to plant, and the remnant of the stalk serves as a point of entry for other pests. But immediately upon harvesting, use a knife to trim the leaves from the stalks and discard. They are toxic, and leaving them on can speed wilting of the stalks.
The harvest season for rhubarb lasts until the end of June. Until then, pick as many stalks as you wish. Then allow the plant to keep all of its leaves, to build its reserves of energy for the next year. A common myth is that the entire plant becomes toxic later in the summer. This is not true; indeed, if a few stalks are pulled on one occasion later in summer to prepare a special dish, plant health and vigor will probably be unaffected. The stalks may be tougher than spring-harvested rhubarb, however.
When a seed stalk emerges from the plant, cut it off as soon as you notice it. If the plant is allowed to flower and set seed, it will have used up energy unnecessarily.
Rhubarb is commonly used in dishes with strawberries, but the season for strawberries and the season for rhubarb only overlap in June. Some gardeners freeze rhubarb for a few weeks until the main harvest of strawberries starts.
For assistance in diagnosing unknown problems visit the University of Minnesota Extension diagnostic site 'What’s wrong with my plant?'
Insect pests are generally not a problem on rhubarb. Rhubarb curculio, a dark-colored, snouted beetle about ½ - ¾ inch long, can bore holes into the crown and stalks. Stalk borers may cause similar problems. Control them by removing curly dock, a weed that is the normal host for rhubarb curculio, and nearby grassy weeds.
Planting rhubarb in a sunny, well-drained site is important to avoid infection by soil-dwelling pathogens. In a wet year or a poorly drained site, root-rotting fungi can invade the plant. Start over in a new spot if this happens.
There are two leaf spot diseases that commonly occur on rhubarb in Minnesota. These can easily be managed through selective harvesting of infected stalks combined with removing and destroying all leaves after the first hard frost in the fall.
Viruses occasionally infect rhubarb, causing abnormal growth, loss of vigor, or unusual leaf coloration. If a rhubarb plant appears to be infected with a virus, it’s best to remove it from the garden and start over.
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