Leeks (Allium ampeloprasum var. porrum) are a gourmet vegetable that may be grown easily in Minnesota. They have a mild onion flavor and are usually used in soup, but they may also be eaten raw, braised, or in casseroles or quiche.
Leeks resemble overgrown green onions, with a long, cylindrical white shaft, but the leaves are thick, flat and folded. Plants grow to 2-3 feet in height. The edible portion is the shaft, usually 6-10 inches long and up to 2 inches in diameter.
Most leeks require a long growing season of about 120-150 days, and a minimum of eight hours of bright sunlight daily. Some newer cultivars require as few as 90 days to maturity, and these may be most suitable for Minnesota conditions. Leeks do best in a slightly acid soil, with a pH of 6.0-6.8, but will grow well in even a slightly alkaline soil.
In northern climates, start leek seeds inside in late February or March. For best results, transplants should be no more than 10-15 weeks old when set out in the garden. Harden off the plants for 5-7 days before transplanting by putting them in a cold frame. If you don’t have a cold frame, set them outside for longer periods each day while returning them to the shelter of your home or garage at night.
Transplant leeks as soon as early spring weather has stabilized and daytime temperatures are at least 45 degrees. Trim the roots of the transplants to 1” to facilitate transplanting, if necessary. Plant them 2-6 inches apart with 12-36 inches between rows.
Leek leaves consistently emerge opposite each other, directly above the previous leaf. Close spacing works well if plants are set out so the leaves will grow into the between-row space, rather than towards the plants on either side. This will make the best use of space, light and air circulation.
To produce long white shafts, some gardeners plant leeks in furrows. Set transplants at the bottom of a six inch deep furrow. As the plants develop, raise the soil level along the stems up to the leaves, gradually filling the furrow. Another method is to hill leeks by planting them at normal soil level, then mounding compost or similar mulch material around the plants several times during the growing season.
A transplant solution of half strength 20-20-20 or 10-10-10 fertilizer will get them off to a good start. In mid-summer, side-dress with a balanced fertilizer such as 10-10-10 at a rate of one cup per 10 feet of row.
Leek roots are fibrous and shallow, so take care not to damage them by hoeing close to the plant.
Water the soil around the leeks thoroughly every week. Avoid excess watering since it promotes fungal disease.
Leeks are fully developed when the stem diameter exceeds one inch. Some smaller varieties mature at ½”-¾” diameter. A quality leek should have a firm, white shaft more than 3 inches long. Swelling at the base, called “bulbing,” is undesirable. Unlike their onion and shallot cousins, leek tops do not die back as the crop matures. The top growth, called the flag, should be dark blue green.
Harvest leeks by either gently twisting and pulling them from the earth or digging and lifting them. Trim the leaves to a more manageable length at harvest, if desired. Thoroughly clean leeks before cooking. There is often a small amount of soil held tightly between the leaves, so slice the whole leek lengthwise, separate the layers, and rinse to remove any soil.
Leeks are fairly frost tolerant, so you can delay harvest until after the first few frosts. Temperatures as low as 20F may not harm some varieties! Mound mulch around your leeks to protect them, and you can enjoy digging fresh vegetables out of your garden into late fall.
Although in milder climates leeks can be overwintered and will continue to grow again in the spring, this is not recommended in Minnesota.
Edited by Jill MacKenzie, Former Extension Specialist, Horticulture, University of Minnesota Extension 9-07.
Reviewed by Cindy Tong, Extension Post-Harvest Horticulturist, Univ. of Minn. Extension, 1-08 and Vince Fritz, Extension Horticulturist – Vegetables, Univ. of Minn. Extension 1-08. Picture credit Sabrina Hart.
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