You can start leeks off indoors from late winter onwards on a windowsill or in a greenhouse. Fill small pots or modular trays with peat-free seed compost, water gently, then sow the seeds thinly, about 1.5cm (¾in) deep.
Keep the seedlings warm (above 10°C/50°F) and in good light, and keep the compost slightly moist. Move into larger pots/modules if the roots appear from the drainage holes. Young leeks are usually ready to be planted outdoors after about eight weeks, when 20cm (8in) tall.