Plant in the fall or spring, depending on your climate. If you live in a mild climate (growing zone 7 or warmer), you can plant leeks in the fall for a spring harvest, then plant again in the late spring for a fall harvest. Leeks planted later in the year will overwinter and grow in the spring. If you live in a climate with harsh winters, leeks should be planted in very early spring, as soon as the ground is workable. Sow the seeds in a warm, sunny location. Fill seed trays with a seed starting mix (not potting soil) and sow the seeds about 6 weeks before the last frost of the winter. Leek seeds germinate best at a temperature around 77 °F, so keep them in a warm, sunny room. If you're starting seeds in the fall, you can keep them outdoors in a sunny spot. Keep the starting mixture moist. If you wish, you can direct sow the seeds instead of starting them in trays. Prepare a planting bed with plenty of compost. Plant the seeds 1⁄2-inch-deep and several inches apart. The seedlings are ready to transplant when they grow to be at least 6 inches.
Prepare a permanent bed for the seedlings. Choose a spot in the sun that has well-draining soil. Work compost into the soil to a depth of at least 8 inches. Dig a trench 6 inches deep. Leeks must be planted deep in the soil so that the base of the stem stays out of the sun and becomes "blanched." The blanched part is the white, tender, edible part of the leek. Plant the leek seedlings at least 6 inches deep and 6 inches apart. Pat the soil around the base of the leak so that it covers the roots and reaches the cleft where the leaves spread apart. The process of slightly mounding the dirt in this way is called "hilling." Instead of hilling, you can help the leek stems blanch by putting a cardboard tube over each plant, so that it rests around the base. This serves the dual purpose of keeping the sun away and keeping dirt out of the leek's leaves.
The leeks' roots are shallow, and must be protected so that they stay moist. Water the leek bed thoroughly after planting, then mulch the leek bed with straw to protect it throughout the growing season.
About halfway through the growing season, hill the soil around the base of the leeks again, so that it reaches the place where the leaves cleave apart. The higher the hill, the larger the edible white part of the leek will be. However, hilling too high can cause the leeks to rot.
Leeks are generally ready to eat when their stems are about 1 inch thick. However, if you have a taste for young leeks, they can be harvested at any time. Younger leeks are less flavorful but they are more tender, and may be eaten like scallions.