Leeks are as intrepid a crop as they come. Hardy enough for cool climates, flexible enough to grow in the heat with sufficient mulching, and no seeds necessarily required.
If you do start from seed. Sow them in pots or pallets about 12 weeks before you intend to plant them in the garden. When the shoots are about 6 inches tall, harden them off for a few days before transplanting. Prepare firm, moist soil and mulch in the growing beds. If you live in warmer climates, simply transfer the seedlings directly to the growing site.
As the leeks grow, mound more dirt and mulch around the base to develop the blanched or white part of the leeks. Continue to add mulch until you have a 6 inch blanch. This is where most of the mild onion flavor will develop.
If you buy leeks from the market, there is no need to start from seed. Instead, you can use a process called vegetative propagation. The same process by which potatoes are reproduced results in growing a genetically identical plant. Just as in selecting varieties for seed, it is important to buy the healthiest looking plants from the market. Then, trim and plant 2 to 3 inches of the stem and root and care for them by the same methods described above. If the new plant bolts, or develops a seed head, trim the buds off. Baby leeks will begin to develop and look like a head of tiny grass. When the leek is harvested, these can be rooted and planted. Leeks are mature and ready to harvest when the stem exceeds one inch in diameter.
From your favorite market-fresh leek you can grow and propagate hundreds more.