Leeks are a long-season plant so start seed indoors in February and early March. Sow the seed 7 mm (1/4″) deep in a soil-less medium and keep it at 20 C (68 F) during germination and growth. Transplant seedlings outside later in the spring when they are around 20 cm (8″) tall. Space the transplants 10-15 cm (4-6″) apart in rows 30-45 cm (12-18″) apart. Seed can also be sown directly into the garden from April through June.