Botanically speaking, the leek is Allium porrum, an onion relative. Unlike onions, though, leeks don't form distinct bulbs. Plants grow 23 feet tall; edible, mild-flavored stem resembles long, fat, green onion. Leeks need very rich soil. Plant them to grow and mature in cool weather. In most areas, late summer to early fall planting is best. Sow seed or set out transplants. 'Arkansas' (108 days) and the heirloom 'American Flag' (120 days) are proven selections. Leeks are quite cold hardy and can overwinter in ground; in fact, cold temperatures improve flavor. When plants have considerable top growth, mound up soil around fat, round stems to blanch them white. Harvest when stem bases are 12 inches in diameter. Offsets may be detached and replanted. If leeks bloom, small bulbils may appear in flower clusters; plant these for later harvest. Leeks are free of many of the pests and diseases that attack onions.