Leeks are an extremely winter-hardy cool-season crop, milder than either onions or garlic. Members of the onion family, leeks do not form bulbs or produce cloves, but develop an edible 6 to 10-inch long round stem as much as 2 inches in diameter. The leek has flat leaves very similar to garlic. Leeks are a biennial crop grown annually and will grow in any garden that will produce good onions. Different varieties of leeks can be harvested practically year round with early varieties ready to harvest from early to mid-fall. Mid season varieties are ready from early to mid winter and late season varieties can be harvested in early to mid spring. 

For a winter crop, plant 10 to 15 seeds per foot of row directly in the garden in the fall. Thin the seedlings to 4 inches apart. Leeks may also be started by seed in the house during January.  Hardened-off seedlings can be set out in the garden as soon as the soil can be worked. There are a couple of ways to set out the seedlings. One way is to dig a 6 inch deep trench and set the seedlings 6 inches apart in a row in the bottom of the trench. You can also poke holes 6 inches apart with the handle of a rake or hoe about 6 inches deep, then dropping the seedlings into the holes. Watering after setting will wash enough soil in around them to cover the roots. The holes will fill in gradually on their own as the plants grow, covering the developing stalks and causing blanching. Do not bank the soil around the plants for blanching until they are at least the size of a pencil. Early banking while they are small will cause the plant to rot and die. Hilling encourages taller growth thus producing a larger blanched edible stem. They are slow growers, requiring 120 days or more to reach 1 to 1 ½ inches in diameter.

Leeks prefer full sun and fertile, well-drained soils. Growing leeks in moisture-retentive soil helps their hardiness to frosts and light freezes. Incorporate plenty of organic matter and a complete fertilizer into the area before planting. An additional side dressing of nitrogen in May and June will ensure good growth and high yields. To keep a balance in your soil, try planting leeks along with peas, as leeks are heavy feeders and will benefit from nitrogen-producing peas. 

Leeks require regular watering; water stress will reduce yields and plant size. Organic mulches help conserve water, supply extra nutrients and reduce weeding. Moisten the soil thoroughly to a depth of 18 inches every 7 days. Water needs are critical since rooting depth in leeks is shallow and drought stress during growth will decrease yield. Leeks may be harvested and used when larger than one inch in diameter. Leeks mature best during temperatures averaging below 75 degrees F. Both the leaves and stems of leeks may be eaten. After they reach sufficient size, harvesting of leaves can begin but beware that harvesting too many may affect the growth of the stalk. One leek seed or transplant will yield one sheath 6-8 inches long and 1-2 inches wide. Plant a 3-6 foot of row length per person to have a sufficient supply for fresh and storage purposes.

Control weeds through regular cultivation being careful to avoid damaging the shallow roots of the leeks. Weed control is particularly important during the first 2 months of growth. Onion maggots destroy seedlings and will continue to feed on the leek's bulb as it expands. Read about this pest from UC Integrated Pest Management at http://www.ipm.ucdavis.edu/PMG/r584300211.html. Other common pests and diseases that may affect your leeks in California see UC IPM Pest Management Guidelines: Onion and Garlic UC ANR Publication 3453. Also go to http://www.ipm.ucdavis.edu/PMG/crops-agriculture.html the University of California Integrated Pest Management website.

To prepare leeks for eating, remove the thick top greens just before use. Leeks can be a source of frustration for cleaning as soil is often lodged between its multiple layers. When preparing larger leeks, remove outermost layers of the vegetable and trim away the roots. Dice the leeks and add them to a bowl of water, swishing briefly with your hands to dislodge any dirt. Allow the pieces to sit undisturbed for a few minutes and then carefully remove the leeks with a slotted spoon or tongs to avoid stirring up the particles. Drain and mop dry using a paper towel. Cut them into rings, slice lengthwise or in squares depending on the recipe. When the base can be easily pierced with a knife, the leeks are ready. They are familiar in soups and stews, but can be served in a number of ways. Their delicate stems have recently found favor among Far-East Asian and Mediterranean cuisine.

Leeks will keep in storage for several weeks if maintained at temperatures near 35 degrees F and a relative humidity near 90 percent. Leeks are low in sodium and calories; 3 ½  ounces contain 61 calories, 20 percent of the RDA for iron and 20 percent of the RDA for vitamin C. Leeks also contain kaempferol, a type of phytochemical which helps ward off many types of cancer.

Some interesting tidbits about leeks: Legend has it that in 640 Welsh warriors led by King Cadwallader (the last Briton king), placed leeks in their hats during battle to distinguish themselves from Saxon enemies. Also, references to leeks growing in Egypt are found in the Bible, and ancient Egyptians held the leek in such high regard that swearing by this member of the onion family was the same as swearing by one of the gods. 

UC Master Gardeners of Napa County provide free home gardening advice. Visit, call or complete the Plant Problem Diagnosis Sheet  for assistance.

Monday, Wednesday and Friday, 9 a.m. to 12 p.m.
UC Cooperative Extension 1710 Soscol Avenue, Suite 4, Napa
(707) 253-4143 http://ucanr.edu/sites/ucmgnapa/

References:

United States Department of Agriculture, University of Arkansas, and County Governments Cooperating

Utah State University

http://www.goodlifegarden.ucdavis.edu/crops/id/32