Sowing: Start leeks indoors up to 12 weeks before the last spring frost. Sow seeds indoors in a warm, well-lighted location—in a bright window or under grow lights. Sow seeds ⅛” (3 mm) deep in seed starting mix. Firm lightly and keep mix just moist. Seedlings emerge in 8-16 days at 70°F (21°C). When seedlings are 2” (5 cm) tall transfer them to individual pots. Transplant leeks to the garden a week after the last frost or later when seedlings are 6-12” (15-30 cm) tall. Move seedlings to a sheltered place outdoors to “harden off” 1 week before transplanting.
Growing: In the garden, space plants 2-4″ (5-10 cm) apart in trenches 5” (13 cm) deep–trenches spaced 6-10” (15-25 cm) apart. Or drop seedlings in holes dibbled about 6” (15 cm) deep leaving just one or two leaves above the soil surface. As leeks grow, fill in the trench gradually or, if planted on level soil, “hill” them by drawing soil up around the stems. Large transplants will grow the best leeks. Grow leeks in full sun to partial shade. Keep the soil moist during the growing season. Side-dress the plants with fish emulsion every two weeks.
Harvesting: Leeks are ready for harvest 120-170 days after seeding, early autumn to late spring. Short-season leeks (with thinner stems) can be harvested during the summer. Long-season leeks (with thick, cylindrical stems) are harvested from late summer through winter. Harvest when stems reach 1-2” (2.5-5 cm) in diameter and the leaves are 6-8” (15-20 cm) tall—after the trench has been fully filled in. Leeks do not store well; harvest as needed. In mild-winter regions, leeks can be left in the ground and lifted as needed.
Read more detailed leek growing articles at Leeks.