Growing leafy greens in Minnesota home gardens
Collards, endive, kale, lettuce, spinach, and Swiss chard are popular salad vegetables. Other specialty greens, such as arugula, cress, mache, mustard greens, and various Asian greens are becoming more common in home gardens. All of these leafy greens have similar cultural requirements.
Collards and kale (Brassica oleracea L.) are related to broccoli, cauliflower, and kohlrabi. They are tolerant of cold temperatures, and are usually cooked before eating. Endive and escarole (Cichorium endivia L.) are variants of chicory, and related to daisies. Escarole generally has larger leaves than endive. Both are grown like lettuce, and can be blanched (covered so that they are not exposed to sunlight). There are three types of lettuce (Lactuca sativa L.) - looseleaf, head or romaine, and Boston or bibb. Leaf lettuce grows quickly and is the easiest type of lettuce to grow. A cool weather vegetable, they grow best at 60-65° F. They can go dormant in hot weather. Spinach (Spinacia oleracea L.) is related to amaranth, and its leaves can be smooth or ruffled in texture. Swiss chard (Beta vulgaris L.) is a close relative of beets, and is now available with stems in different, bright colors - gold, pink, orange, purple, red, and white.
All leafy greens are generally direct seeded, although kale, lettuce, and Swiss chard can be transplanted. To produce transplants, plant seed indoors (see Starting Seeds Indoors) in early April or about 4 weeks before transplanting. For fall crops, plant seed in June. Harden seedlings by reducing water and temperature for 2-3 days before transplanting. Transplant kale 8 inches apart, looseleaf and bibb lettuce 8-12 inches apart, head lettuce 10-12 inches apart, and Swiss chard 4-6 inches apart in rows 18 inches apart. Closer spacing will result in smaller, "baby leaved" plants, and farther spacing will result in larger heads or plants.
To direct seed collards, endive, kale, lettuce, and spinach sow seeds as soon as soil can be worked in the spring and about 3 months before the average fall frost date for a fall crop. Plant seeds 1/4-1/2 inch deep, in rows 18-30 inches apart. Thin collards, endive, kale, loose leaf and bibb lettuce to 8-12 inches apart. Thin Swiss chard to 4-6 inches apart. Thin spinach to 2-4 inches apart. For a continuous spring supply, plant spinach seeds every 1-2 weeks until outdoor temperatures reach 80° F.
Soil pH and fertility
Leafy greens grow best in well-drained, moisture-retentive soil with pH of 6 to 6.5. Have your soil tested (see Understanding Your Soil Test Report) to determine your soil's pH and whether it should be amended.
Incorporate well-rotted manure or compost at planting. Addition of manure or compost can add micronutrients and organic matter to soil (see Composting and Mulching). A garden fertilizer can be used if compost is not available, applied at the rate of 2 pounds per 100 sq. ft. Sidedress fertilizer once during the growing season at a rate of 1 pound per 25 feet of row. Do not use a fertilizer containing a weed killer ("Weed and Feed"), or it will kill your vegetable plants.
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 at the rate of 1/2-1 pound (1-2 cups) per 100 sq. ft.
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. Sandy soils may require more frequent watering. Mulching 3-4 inches deep with herbicide-free grass clippings, weed-free straw, or other organic material will help retain soil moisture and help suppress weeds, decreasing the need for frequent cultivation.
Frequent, shallow cultivation with a garden hoe or trowel will kill weeds before they become a problem. Leafy green roots are very close to the surface of the soil, so it is important not to cultivate too deeply. Cultivate just deeply enough to cut the weeds off below the surface of the soil. Be careful not to damage the leafy green plants when cultivating.
Harvest single leaves as soon as they reach a usable size. Spinach and some lettuce varieties can produce new leaves, especially if individual leaves are harvested at the "baby" stage, so multiple harvests are possible before the plants exhaust their resources. Alternatively, the whole plant can be removed by cutting it off at or just before the soil surface. Endive and escarole can be blanched before harvesting by placing a 4-8 inch cardboard disk or plate over the head 3 days before harvest. Flavor is best in leafy greens if harvested before the weather becomes hot and dry.
Leafy greens have few significant problems. When outdoor temperatures are high or day lengths long, they can "bolt", or form flowering stalks and seeds. When they start to bolt, leaves will generally become bitter and inedible, as energy produced in leaves are transferred to the flowers and seeds. Some varieties are more tolerant of hot weather than others.
High summer temperatures and uneven watering can also exacerbate tip burn in lettuce, in which leaf edges deteriorate. Remove leaves with tip burn before eating. Choose varieties with resistance to tip burn, and maintain even soil moisture during the growing season.
Flea beetles, little black "jumping" insects the size of a head of a pin that chew holes in leaves, can be a problem with leafy greens.
Cercospora leaf spot causes circular leaf spots with a reddish purple border and a white to tan center on Chard and beet leaves. Remove infected leaves from the garden and avoid sprinkler irrigation to reduce the spread of the disease.
Reviewed by Vincent Fritz and David Zlesak 2009