Growing peas in Minnesota home gardens
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Peas (Pisum sativum) are a traditional home-garden crop all over the world, and have been widely cultivated for centuries. The most common type in American gardens is the shelling pea, also called the "garden pea" or "English pea." Tender, sweet peas are removed from thin, tough pods before eating. "Petit pois" is a French term for "small pea," and some varieties with very small peas are available. Although they may appear elegant on a plate, small peas are not necessarily sweeter, more tender, or better flavored than larger peas.
Edible pod peas include snow peas and sugar snap peas. Snow pea pods are thin and tender, eaten when there are still only tiny traces of peas inside. This type of pea is usually associated with Asian cuisine. Sugar snap peas, developed in the twentieth century, are eaten when the peas inside are nearly mature and the pods are still tender and juicy, similar to snap beans.
Pea vines are also edible, the tender shoot tips harvested and steamed or sautéed.
Finally, peas can be allowed to fully mature and dry, and the dried pea seed used for soups. Nearly all pea varieties on the market are open-pollinated, rather than hybrids, and all pea flowers are self-pollinating, so mature pea seed can also be saved for planting the following season. (See Saving Vegetable Seeds.)
Different plant types include tall vines, up to five feet, which need to be supported as they climb. These varieties bear over a longer period. Short-statured or "bush" types are only two to three feet tall, and will flower and then set their crops all at once, for a single harvest.
Older varieties of peas have compound leaves, each with many small, round leaflets, and the terminal leaflet modified to form a tendril. Newer varieties include many with more tendrils than leaflets, and a few with no leaflets at all, only tendrils. Leafless and semi-leafless vines may be less prone to disease, because of improved air circulation through the plants. They also may be better able to stand without a trellis, because the tendrils tangle themselves up into a mass.
Other qualities include number of peas per pod, length of pod, days to maturity, and stringed or stringless pods for edible-pod types. Garden centers and seed catalogs carry a wide selection of pea varieties. Some varieties are resistant to common pea diseases, including root rots and powdery mildew. Most seed is offered with a fungicide treatment to improve germination in cold soils.
Peas are in the legume family, Fabaceae, as are beans, alfalfa, and clover. Like these species, they "fix" atmospheric nitrogen in nodules on their roots, through a relationship with the soil-dwelling bacterium, Rhizobium. These bacteria, living in growths on the roots, can extract the nitrogen from the air and make it available to the pea plant. Many soils in Minnesota have adequate populations of Rhizobium to form the beneficial relationship with peas. However, to be sure, you can purchase powdered Rhizobium inoculum and treat the seeds before planting by adding a little of the powder to the seed packet and shaking. Be sure to use inoculum for pea seeds, and not specifically for beans.
The season for pea growing is short wherever a cool spring gives way quickly to a hot summer, as it does in much of Minnesota. Areas near Lake Superior and in the far northern part of the state may stay cooler into the summer and so have a longer time to grow peas. Most varieties of peas require about sixty days of growth before harvest, but will stop growing and not produce flowers or pods once temperatures get above 85°F, as often happens in June. Peas that are produced in hot weather may also have poor quality, so it's important to get an early start on planting. Preparing the planting area the previous fall may be a good idea.
Although hot weather is not desirable for pea production, the plants do need full sun, as do most garden vegetables.
As soon as the soil has thawed and can be worked at all, plant the seed. Tall, vining types to be grown on a trellis should be planted at the base of the trellis in a single row. Shorter bush types can be planted in a single row and encouraged to grow up a trellis, or they can be planted in a wide row, between 12 and 18 inches wide, where the plants will cling to and support each other. Trellises can be made of two vertical stakes with string or netting between them.
Place the seeds in a shallow furrow, six to seven inches apart, or, for a wide row, broadcast the seed over the prepared seedbed, with seeds about two inches apart in all directions. Evenly place the seeds into a furrow in the soil having a uniform depth and cover them with one inch of soil; then firm the soil over the seeds. A second planting may be made a week later, and another a week after that, for a longer harvest period.
Treat the pea seed carefully. Seeds that are cracked are unlikely to germinate, especially in the cool, moist soil of early spring.
Young pea plants can withstand light frosts, and will grow at any temperature above 40°F. Optimum temperatures for growth are between 55°F and 65°F. Once the plants have started to flower and set a crop, frost can be damaging, so watch for any forecast of late spring frost and cover the plants with sheets or row covers if necessary.
Soil pH and fertility
Many soils, from sandy to heavy clay are suitable for peas; but the soil should be well-drained. Peas grow best in slightly acidic to slightly basic soil, pH between 6 and 7.5, but soils with somewhat higher pH (more basic or "alkaline") should also produce a good crop of peas. Have your soil tested to determine your soil’s pH and whether it should be amended. Incorporate well-rotted manure or compost at planting. Although the pea plants will have a supply of nitrogen once the association with the Rhizobium bacteria is formed, they do need some nitrogen in the soil early in their growth.
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.
Because peas grow during a naturally wetter and cooler time of year, it may not be necessary to water them at all. But during dry springs, proper watering will enhance good production. Try to water the soil, not the vines, to prevent disease. Soak the soil thoroughly when watering, to a depth of at least one inch each week during the growing season. Sandy soils may require more frequent watering. Mulching three to four inches deep with grass clippings, weed-free straw, or other organic material will help retain soil moisture and help suppress weeds.
Before planting, cultivate and hand-pull weeds. If peas can get a head start, they can out-compete weeds. But if the weeds get the head start, the pea plants will not perform well. Frequent, shallow cultivation will kill weeds before they become a problem. Although peas will form a taproot, fibrous lateral roots can be fairly close to the surface of the soil, so it is important not to cultivate too deeply. Be careful not to damage the plants when cultivating. A wide row of peas, with bushy plants forming a continuous mass, usually stays weed-free within the row, and only needs weeds controlled between rows.
To harvest pea shoots for stir-fries or steaming, cut six-inch lengths of vine. Edible-pod varieties are preferred for shoot harvest. Some cooks prefer leafless and semi-leafless types, finding the tendrils more tender and sweeter than leafy shoots.
To harvest shelling peas, observe the plants carefully, sampling the crop each day, once the pods have begun to fill with peas. Optimum pea harvest occurs as soon as the peas have achieved their full size, slightly larger than the dry seed you planted. They will also be sweet, tender, thin-skinned, and non-starchy. Once peas have reached maturity, they will quickly decline in quality, and will be inedible as fresh peas within one to three days.
Over-mature peas have a starchy flavor, less sweetness, thick, tough skins, and a firm or hard interior. There can be a temptation to pick the peas before they are mature, but yield is reduced because the peas are picked too small, and both flavor and sweetness may be lacking.
Pick the peas and either shell immediately, then cool, or cool immediately for shelling later. The best way to remove heat from a large harvest of peas on a warm day may be to dunk them in very cold water until chilled, then dry and refrigerate. Once cooled, they will hold their quality for more than a week in the refrigerator.
To harvest snow peas, wait until the pods have reached their mature length as described on the seed packet. The peas within the pod should be visible only as small traces; if they are allowed to mature further, the pods may become tough. Some varieties have fibrous strings along the edges of the pods; these should be removed before cooking. As with shelling peas, it's important to cool snow peas quickly after picking them.
To harvest sugar snap peas, wait until the pods appear almost filled with peas. Sample frequently as the crop matures, and harvest when peas and pods are still sweet, juicy, and tender. Remember that quality declines quickly once maturity is reached. Cool quickly as soon as they are picked. Some varieties will need to have the strings removed before cooking.
After harvest, typically in June, remove any trellis, and turn the pea plants into the soil. Wait at least two weeks for the soft tissues to break down, then seed or transplant a second crop for fall harvest (see Planting Vegetables in Midsummer for Fall Harvest) or plant a cover crop (see Green Manure Cover Crops for Minnesota), or put in some annual flowers.
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Cutworms can sever plants off near the soil due to their feeding.
Aphids are small, soft-bodied insects less than 1/8 inch long that occur in a variety of colors. They use piercing-sucking mouthparts to remove plant sap, and often spread disease in the process. Pods are rarely damaged. Treatment is usually not required as aphids have many natural predators, such as lady beetles. However, in widespread infestations (more than 50% of all leaves in field infested), insecticidal soaps and horticultural oils can be used to decrease aphid numbers.
Powdery mildew, a foliar disease common in hot weather, can be a problem for pea varieties that are not mildew-resistant. Leaves and pods can become covered in a whitish mold. Choose resistant varieties, and start peas early to avoid hot weather.
A number of root rotting pathogens, especially Aphanomyces, can infect peas and other legumes, including beans. The plants begin to brown and die from the soil up, and eventually collapse. Root rot is exacerbated by cold, wet weather, followed by hot dry conditions, particularly at the flowering stage. It's very important to rotate peas and beans around the garden, not planting in the same spot more than one year in four.
Damping off and a few other pea diseases may occur and can typically be managed through cultural control practices.