How to grow onions from seed

A table displaying which months are best to sow, plant and harvest.
Jan Feb Mar Apr May Jun Jul Aug Sep Oct Nov Dec
To do
To do

Do To do in January

Do To do in February

Do not To do in March

Do not To do in April

Do not To do in May

Do not To do in June

Do not To do in July

Do not To do in August

Do not To do in September

Do not To do in October

Do not To do in November

Do not To do in December

Onions are easy and cheap to grow, whether from seed or from heat-treated sets. Follow our step-by-step guide, below, on growing onions from seed (skip to Step 3 for instructions on planting out sets).

Onion sets are miniature or immature onions, which can be planted out in March or April for a quick-growing crop. Sets are available from garden centres or by mail order, and although the range of varieties is not quite as wide as from seed, it’s is an easy, reliable method of growing.

Whether you grow from seeds or sets, you can start to harvest bulbs in early summer and have plenty to store through winter.

As with the majority of vegetables, onions grow best in a well-lit position, with soil that has been enriched with generous amounts of plenty of organic matter.

You will need

  • Onion seeds
  • Multi-purpose compost
  • Pots or seed tray
  • Trowel
  • String
  • Watering can, with rose
  • Onion hoe or circle hoe

Total time:

Step 1

Sow seed in a pot or tray of seed compost. The seeds are small, but try and space them about 1cm apart. Lightly cover with a thin layer of compost then stand pot or tray in water to moisten.


Step 2

When the seedlings are a few inches tall, prick them out and transplant into fresh compost. Once established, transplant seedlings into the garden, 10-15cm apart.


Step 3

Alternatively, in spring, plant heat-treated onion sets, into soil that has had large stones and weeds removed, and been enriched with organic matter. Plant each set 10-15cm apart, with tip protruding. Water newly planted sets and cover immediately with horticultural fleece, to prevent birds from lifting them. Once firmly rooted, the developing crops can be uncovered.


Step 4

Using a watering can with a rose, thoroughly water seedlings and sets directly after planting to settle soil around roots or bulbs. Repeat at regular intervals if the weather is dry.


Step 5

Hoe regularly to remove weeds, which will compete for water and nutrients with developing bulbs. A hand-held ‘onion hoe’ or ‘circle hoe’ are both perfect for weeding in small spaces.


Step 6

In late summer, draw away earth from the bulbs to expose them to the sun. Harvest after the leaves turn yellow and the stem bends over. Allow leaves to dry before carefully lifting bulbs.


Once dried, a convenient way to store onions is to string them up in onions ropes and hang them in a cool, dry, frost-free place. Select only unblemished bulbs with narrow, well-closed necks. Wide-necked onions do not store well so use them first