Carentan
Leek in the US

Leeks are best started indoors, to be honest. Get your seeds going about 1/2 inch (1 cm) deep in trays or modules, usually February through April for many zones, but check your specific USDA zone for the best timing – it can be as early as January for warmer areas or April/May for colder ones. Keep them somewhere warm, around 60-68°F (15-20°C), on a windowsill or in a greenhouse. They don't need much light to start, but once they pop up, give 'em plenty. Sow a few more than you think you'll need, just in case.

Once your seedlings are a few inches tall, maybe pencil-thick, you can prick them out into individual pots or deeper trays. Get 'em hardened off gradually for a couple of weeks before planting out. This means putting them outside during the day and bringing them in at night. Plant them out when they're about 6-8 inches (15-20 cm) tall, once all danger of last frost has passed. Space them about 6 inches (15 cm) apart in rows, leaving 12-18 inches (30-45 cm) between rows. You can plant them quite deep too, to get a longer white stem.