How to grow onions and leeks from seed

Growing onions and leeks from seed is fun and rewarding
Image: Onion 'Performer' from Thompson & Morgan

Popular members of the allium family, onion and leek seeds require little more than a sunny windowsill to start them off. Rewarding and cost effective, onions raised from seed are often bigger than those grown from sets - ideal if you’re hoping to win a prize at the village show! Here’s everything you need to know about growing onions, spring onions, shallots and leeks from seeds.

How to grow onions and shallots from seed

To get large uniform bulbs like these, thin your rows of seedlings as they grow
Image: Onion 'Hylander' from Thompson & Morgan

For the best results, start your onion and shallot seeds off indoors, around February or March. An early sowing extends the growing season, allowing more time for your onions to grow large. 

• Sprinkle your seeds thinly over a tray of good quality seed compost.
• Cover with a light layer of soil. 
• Keep your seed trays moist to help with germination.
• Once the seedlings get a few inches tall, prick them out and transplant into small pots of new compost. 
• Once they’ve established and grown a little more, move them out into their final positions.
• They prefer light, well dug and fertile soil.
• Plant your onions about 5 inches (13cm) apart, with 12 inches (30cm) between each row.
• Keep the soil moist, but not waterlogged, as the onions develop. 

If you don’t have space to start them off indoors, you can sow your onion seeds directly outside in March and April. They need to be growing strongly by the time the weather warms up at the end of spring to give the bulbs a chance to swell. Simply sow your onion and shallot seeds in shallow drills outdoors, about 1.3cm (½inch) deep. Mark out rows, spaced about 20cm (8 inches) apart, with string tied between canes. This will remind you where they are when you come to plant out other crops! To give them more room as they grow, thin your seedlings to about 10cm (4 inches) apart, using the thinnings as salad onions. 

Growing onions and shallots from seed gives you the chance to try varieties that can’t be bought in the supermarket. For onions, the classic award-winning ‘Red Baron’ produces a deep red-rimmed shiny bulb, while ’Globo’ can produce bulbs up to 1kg in weight! And if you’re after variety, try a premade seed mix like our ‘Four Colour Bulb Mix’.

If you prefer the more delicate taste of shallots, ‘Figaro’ produces lovely banana shaped bulbs with distinct flavour and clean white flesh. For a variety with good bolting and mildew resistance, try ‘Lorient’ which produces teardrop shaped bulbs with blush-coloured flesh and a lovely sweet flavour. 

How to grow spring onions from seed

Spring onions are a fast maturing crop that do very well in containers
Image: Spring onion 'Pompeii' from Thompson & Morgan

Spring onion seeds can be sown directly outdoors from March, and you can continue to sow them every three weeks until August for regular crops throughout the summer. Taking up very little room and ideal for containers, you don’t have to worry too much about spacing as spring onions develop much smaller bulbs, if any at all. Some of our favourite varieties include: 

• 'Bulgaarse Reuzen - Lincoln' - early crop with great disease resistance and resilience to bolting. 
• 'Lilia' - red Italian variety with small pungent bulbs and stems.
• 'Purplette' - eye-catching spring onion with deep purple, glossy bulbs, which looks great on display in a container. 

If you want a very early spring harvest, try sowing a batch of the quick growing and hardy variety ‘White Lisbon’ in September. With an RHS Award of Garden Merit, it’s one of the best varieties for overwintering. 

How to grow leeks from seed

Leeks are amazingly frost tolerant and well worth growing from seeds
Image: Leek 'Northern Light' from Thompson & Morgan

Sow your leek seeds outdoors in March and April for harvests from late September through to January. Because they take up quite a lot of space, many gardeners start them off in seed beds for a few months until they’re ready to move to their final positions. 

• Prepare your bed the autumn before, digging in plenty of compost or manure.
• Thinly sow your leek seeds into shallow drills about 1.25cm (½ inch) deep, and cover.
• In early summer, when the leeks are much bigger, transplant them to their final growing positions in the vegetable patch. 
• Your rows should be 15cm (6 inches) apart, and each leek should be planted at 15cm intervals. Use a dibber to create planting holes that are 15-20cm deep.
• Keep the leeks well watered during dry weather and weed regularly. 
• About three weeks after planting out, apply a dressing of a balanced organic fertiliser or Growmore, and a second dressing about three weeks after the first.
• ‘Earthing up’ as your leeks grow increases the length of the white stem. 

Leeks are hardy and can be left in the ground until required. ‘Musselburgh’ and ‘Below Zero’ F1 hybrid are especially cold tolerant varieties, making them a great choice for areas with snow during the winter. For an exceptionally long cropping season, try ‘Northern Lights’. The leaves of this attractive leek change from blue-green to a stunning shade of purple as the weather gets colder. 

Sow onion and leek seeds for a cost effective crop to liven up your favourite meals. If you don’t have the space to sow your own seed, then take a look at our guide to growing onions and shallots from sets. Find other popular alliums to grow, growers guides, and our full range of products at our onions, garlic and shallots hub page. Keep us up to date with your allium harvest via social media! We love to hear from you.