Leeks are a nonbulbing member of the allium, or onion, family. Leeks were held in such high regard by ancient Egyptians that swearing by a leek was the same as swearing by the gods. The edible portion of the leek is the long, white shaft of layered leaves at the base of the plant. This portion of the plant ranges from 6 to 8 inches long, depending on how high the leek was blanched, or whitened, by hilling or trenching of the soil around it. Leeks have a long growing season and are often harvested in the fall.
Depending on the variety, leeks need an average of 80 to 120 days to produce. Leeks are a cool-season plant and can tolerate light frosts. Starting seeds indoors six to eight weeks before the soil dries out in the spring will give you a head start on your crop. You can also find seedlings in flats or bundles at your nursery. The two methods of planting your leeks are trenching and hilling. Trenching is digging a trench 8 to 12 inches deep, planting your seedlings at the bottom, and as they mature, fill in the trench with soil to produce a long, blanched shaft. You can also plant your leeks at ground level and hill the soil up around the shaft as they mature to produce the desired whiteness. Space plants four inches apart and rows eight to 16 inches apart.
Caring for Your Leeks
Leeks have very shallow root systems and are sensitive to water fluctuations. Make sure the soil around your leeks is moistened to a depth of 18 inches once a week during the growing season. Leeks grow best in a sandy loam soil with 4 to 6 inches of organic matter worked into the soil profile before planting. Leeks are heavy nitrogen feeders and benefit from a side dressing of 10-10-10 fertiliser in the summer. Planting your leeks alongside peas is a natural way to provide the precious nitrogen as peas are nitrogen-producing plants.
When to Harvest
Unlike other members of the allium family such as onions and garlic, the tops of leeks will not dry out to denote maturity. Leeks are ready to harvest when the stems have reached at least 1 inch in diameter. This varies by variety; some can be harvested at 1/2 to 3/4 inch for an increase in tenderness. Grab the entire plant and twist carefully to lift them out of the soil, or use a digging fork to prevent ripping and tearing. Cut off all green leaves, and cut off the ends if there is a slight bulbing by the roots. Leeks are notorious for having dirt between the leaves. Cut the leek in half lengthwise and wash out all dirt between leaves before using.
Overwintering Your Leeks
Leeks can tolerate temperatures down to -6.67 degrees Celsius without major damage. For this reason, in many parts of the country some leeks are planted later in the year to be overwintered for an early summer harvest. Hill 12 to 18 inches of mulch around the base of your leeks before a hard frost. In the spring, move the mulch back, water well and let your leeks continue to grow. This method gives you two harvests during the season.