When to Plant Leeks

Leeks are a hardy, slow-growing crop that need a long season to reach full size. In the UK, sowing usually begins indoors or under cover in late winter to early spring, from January to March. Outdoor sowing typically starts around March or April once the soil begins to warm. Transplanting into final positions is usually done from late May through June when seedlings are pencil-thick and soil conditions are stable. Early sowings give you crops from late summer, while later sowings extend the harvest into winter.

Starting a Crop from Seed

Leeks are most often grown from seed rather than sets. Start seeds in trays or pots filled with seed compost. Sow thinly and keep the trays in a cool but bright place. Once the seedlings are about 15cm tall and thick as a pencil, they’re ready to transplant. Sowing under cover gives you more control over germination and a stronger start, particularly in colder regions or in wet early springs.

Why and When to Sow

Sowing leeks early gives them the time they need to develop thick, long shanks. In mild climates, you can sow as early as January, though most gardeners begin in February or March. If you miss the early window, you can sow into April, but this may result in smaller leeks or a shorter harvesting window. Choosing the right variety can also help extend the growing season — early, maincrop and overwintering types are all available.

Choosing the Right Leek Variety

Different leek varieties suit different growing times and end uses. Early leeks like ‘Oarsman’ or ‘King Richard’ are fast-growing and ready from late summer. Maincrop varieties such as ‘Autumn Giant’ or ‘Musselburgh’ are reliable and produce thick stems through autumn. For winter harvests, choose hardy types like ‘Bandit’ or ‘Below-Zero’ that can withstand frost and remain in the ground until needed. Mixing a few varieties ensures a steady supply for months.

Preparing the Planting Site

Leeks grow best in fertile, well-drained soil that’s been enriched with organic matter. Choose a sunny, open position. Avoid freshly manured ground, which can lead to soft growth and disease. Dig over the soil, remove weeds, and rake it to a fine tilth. If possible, prepare the ground in autumn and let it settle over winter. Leeks also do well in no-dig systems as long as soil structure is healthy.

How to Plant Leeks

The traditional method of planting leeks is to drop seedlings into dibbed holes around 15cm deep and 15cm apart, spacing rows 30cm apart. Don’t backfill the holes with soil — just water them in. As the plants grow, the soil will gradually settle around the stems, helping to blanch the lower portion and produce the long, white shanks leeks are known for. If you're planting in modules or plugs, ease them out gently to avoid root damage.

Looking After Leeks

Leeks are low-maintenance but need consistent moisture to grow well. Water regularly in dry weather, especially when young. Keep the beds weed-free to reduce competition for nutrients and water. Earthing up around the base can help blanch more of the stem, but this is optional depending on variety. Leeks rarely need feeding in fertile soil, though a liquid feed mid-season can support growth if they’re slow to bulk up.

Timing Based on Final Harvest Goals

One of the key things to understand about planting leeks is that timing affects not just size but also when you’ll harvest them. If you want early leeks for summer use, you’ll need to sow under cover in January or February. For maincrop leeks that you can pull from late autumn to early winter, March sowings are ideal. If your goal is to have leeks standing through winter and into spring, sow between March and April using hardy varieties and transplant in June or July. Each window suits a different use — earlier for fresh summer meals, later for standing overwinter and lifting as needed.

Indoor vs Outdoor Sowing

Sowing under cover in seed trays gives you more control in late winter or early spring. It allows you to start the growing process while the weather is still too cold or wet for outdoor sowing. This method also gives seedlings time to develop in a more protected space, producing stronger plants by the time they’re ready to transplant. Outdoor sowing is often simpler, but it depends entirely on soil temperature and condition. Cold, wet ground can delay germination or cause patchy results. If in doubt, start indoors and transplant when conditions improve.

Successive Sowing for Extended Harvest

One of the most effective techniques with leeks is staggered sowing. Instead of sowing all your seed in one go, sow in two or three batches from late January through April. This spreads your harvest across many months, so you’re not pulling all your leeks at once. You get a supply from summer right through to the following spring, especially when using a mix of early and hardy late varieties.

Soil Temperature and Sowing Conditions

Leek seeds germinate best at around 10–15°C. If the soil or compost is too cold, germination can be very slow or patchy. This is especially important when sowing directly outdoors. Use cloches to warm the soil if needed, or sow in modules indoors and move seedlings out later. If you're aiming for maximum germination, starting leeks in a propagator or heated greenhouse in late winter can give you a noticeable head start.