If your regular day temperature is above 50ºF, start your leek seeds outdoors immediately. If there are still cold temperatures, start your leeks indoors and transplant when temperatures are 60-65ºF.
When starting indoors, plant seeds 1/2” deep & thin to 1” apart when the seedlings are about 3-4” tall. Harden them off by moving them outdoors as soon as your soil is workable. If starting outdoors, plant them 6” apart, then thin as they grow, about 6 weeks after sowing. Germination can take 2-3 weeks to occur.
Leeks can be grown in a variety of soil types, but do best in moist, firm soil in a space that used to have lettuce, cabbage, or peas. Pick a spot that leeks can take time to mature in, since they can take 25-40 weeks, and are often left to overwinter and pick as needed during the winter.
Transplant leek seedlings 8-10” apart. Water well and make sure all of the roots are covered. Add additional soil around the plant as it grows to produce the long white stalks. Trim back the long dark leaves a few times over the summer.
Fertilize as needed every 4-6 weeks from planting time, but do not feed leeks that will be overwintering after late summer.
Blanching should be started in late summer, and the more you blanch, the more of the plant will be usable at harvest. Pile up dry, fine soil around the stem, or use a mulch to cover up to the lowest leaves. If you use wet soil, the plant can rot. You can also use piping/tubing around the plant as a collar.
Harvest your leeks when they reach a large, ideal size for cooking, preferably in mid fall through the next spring. Harvest the largest ones first, so that smaller leeks can grow more. If your leeks get too large, they will not taste as good.
Loosen the soil around the plant before pulling them, and use a fork to help pull their roots up, so you don't break them off. Allow your leeks to stay in the ground just before the ground freezes or use mulch or hay to over-winter leeks if you live in milder climates.
Pull any leeks before they flower in spring. Or leave them, but cut the flower, and you’ll get bulbs forming at the base of the leek, which you can harvest in summer and use like onions.
Cut green tops off of leeks and store in a plastic bag in the refrigerator for up to 2 weeks.