Soil, Sun and Season
Do not plant leeks where you have grown other onion species in the previous three years. Plant in full sun in soil with pH ranging from 6.6 to 7.5. Before transplanting, work into the soil, about 6 to 8 inches deep, 1 to 2 pounds of 16-16-16 fertilizer per 100 square feet of garden area. Being perennials, leeks are genetically programmed to survive the winter. If you grow long, maturing varieties for winter harvest, cover them with a plastic tunnel or dry leaves before the first autumn freeze.
To sow seeds indoors, plant leeks 1/4 to 1/2 inch deep in flats eight to 10 weeks before the last anticipated frost in your area. Transplant them to paper cell containers when they’re about 2 inches tall or leave them in the flat. When the seedlings are 6 to 12 inches tall, plant them 4 to 6 inches apart and 4 to 8 inches deep in rows 8 to 16 inches apart. Tease their roots apart if you left them in the flat. Allow only a few inches of leaf to show above the soil. To seed them directly outdoors, plant seeds 1/2 inch deep and 1 inch apart in rows 8 to 16 inches apart about four weeks before the average date of last frost. They germinate best at temperatures between 45 and 77 Fahrenheit. They should emerge in five to seven days, when you can thin them to 4 to 6 inches apart. Leeks grow from 1 to 2 inches wide and 6 to 8 inches long. They’re ready to eat when they’re 1 inch wide. To grow white, edible stems, bank the soil 2 to 3 inches up the sides of the leeks two to three times during the growing season. Most cultivars mature in 100 to 120 days.
Water and Fertilizer
Moisten the soil thoroughly 18 inches a deep once a week. To conserve water and suppress weeds, lay a 2- to 4-inch layer of leaves, grass clippings or compost around the base of the leeks. In May or June, add 1/2 pound of water-soluble nitrogen (21-0-0) per 100 square feet in a furrow 2 to 3 inches deep, 3 to 6 inches to one side of your leeks. Cover with soil and water.
Insects and Disease
Onion maggots, the insect that will most likely infect leeks, look like houseflies. Their larvae can kill leek seedlings and eat their way into leek bulbs. Rototill the soil deeply the fall before planting. After you sow leek seeds, cover the seed beds with a floating row cover extending 6 inches on the sides of the row. Remove the cover when the plants are big enough to tolerate maggot damage. Leeks may be infected by the fungal diseases purple blotch and botrytis. Splotches of purple blotch appear in wet, rainy weather; the diseases are made worse by overcrowding plants and applying too much nitrogen fertilizer. Botrytis develops in warm, humid weather and can stunt or kill leeks. To treat purple blotch and botrytis, spray with a fungicide containing the active ingredient chlorothalonil. Although application rates vary brand, a dry product containing 80 percent chlorothalonil would require 1 to 1 1/2 teaspoons per gallon in a backpack sprayer. Apply every seven to 10 days in wet spring weather and on young leeks. Chlorothalonil is toxic if you inhale it or get it in your eyes. Wear protective eyewear, gloves, long-sleeved shirts and trousers, shoes and socks when applying. Do not apply on a windy day.