How to Grow Onions From Seed? (Complete Guide)

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If you’re wondering how to grow onions from seed, this article is for you.

How long do onions take to grow from seed? You can grow onions from seed in approximately 46-48 weeks. Some types mature much faster like green onions (also known as spring onions).

Keep on reading to find out how to sow them, when to transplant them, and how to store them. We will also share common problems when grown outside and how to prevent them.

Step-by-step guide: Germination of onion seeds

  1. In late winter, prepare your seed trays with light compost with excellent drainage, which is not too rich in nitrogen, because this can stunt growth.
  2. Next, you need to choose what type of onion you want to grow. See the types of onions below to make an informed choice.
  3. Many seed providers recommend soaking seed prior to planting. See FAQs below for why this is good practice.
  4. Onion seeds are tiny, black, and fiddly to handle! Make a drill line with a pencil tip, water the line and then you can drop the small seeds into this line easily and they stick.
  5. Now turn over the soil to cover them but no deeper than half an inch. Do not plant too deeply, because this is a frequent reason for non-germination.
  6. Lightly water from time to time – check the soil for dryness if in any doubt.
  7. Transplant seedlings started indoors outside to the garden or your chosen pot, about 5-8 weeks after sowing, depending on local weather conditions.

growing onions from seed

Types of onions you can grow

You can grow onions from seed or from pre-grown small bulbs, which are usually called onion sets.

  • Large bulb types range from very large, yellow Spanish onions to red onions to the reliable “Ailsa Craig”. It is an old favorite of onion growers, with its mild taste and honey-colored flesh. This variety is best eaten quickly because it does not store well. For onions to store for the winter, choose from Red Baron or Reliance, or check with your local seed providers for advice on good bulbs which last.
  • Salad or spring onions (or green onions) have really white skins, which are used chopped in salads. These grow in bunches, like the White Lisbon variety. A Japanese variety called Isikhura, allows you to pull out bulbs as you need them. The rest of the bunch continues to grow all summer. These onions can be picked for 6 months from spring to autumn if transplanted outside.
  • Pickling onions are grown for their petite size and their small bulbs. These enjoy sandy soil to thrive. Sow these in April and pick them in August for preservation in vinegar.

Can you start onions in seed trays?

Yes, you can certainly start onion seeds in seed trays.

However, you will need to transplant them into either a row outside or a deeper pot. Seeds need at least 5 days to germinate but it could take up to 10 days so be patient!

Keep your seed trays at a temperature of around 70°Fahrenheit. The seeds will take a lot longer if your soil is cool. When you see tiny shoots growing, allow these to grow several sets of leaves and keep watering. Transplant them into a new home on a dry day. Read on to learn how to prepare the bed/pot.

Can I direct sow onion seeds?

  • Yes. To prepare the soil for onions try to apply manure in autumn, if possible. If that is not possible, just add some now and dig it in well and leave for a few days.
  • Choose an open site with good drainage. Rake the soil well to remove large stones. Onions do not like acid soil; if yours is, then buy some lime and apply it in autumn.
  • If planting in a pot, fill it with suitable soil and add some drainage holes.

Now follow the planting instructions above and thin the row when needed.

Spacing for planting on

  • Bulb onions need about an inch between sets but green salad onions can be planted closer together. You will be picking these frequently and this will thin the crop naturally.
  • When you have very crowded rows outside, remove a few – this is known as thinning. You can try to plant these in a new row but it is important not to discard thinnings on the soil outside. Their very distinctive smell will attract onion root fly.

Indoors, you can try to place thinnings into the soil in a different pot and see if they take.

How do I know when my onions are ready?

Usually, the plant tells you visually; the leaf tips start to turn crispy and sandy brown and then die off. The plant is not dead! It’s ready.

  • Check the bulb; does it feel solid? If so, wait for another 2 weeks to allow the bulb to form fully. You can also try lifting the plant by the leaves and if it is ready, the onion normally lifts out of the ground easily with a small tug.
  • To harvest a full row, wait for a dry day. Then, use a garden form to remove each bulb being careful not to spear them with the fork! Traditionally, farmers left onions to dry on the field for a few days. You can do this or take them home and lay them in a sunny, warm position to dry out.

Read on to see how to store them best for winter use.

Storing your onions

  • After lifting them from the soil or ground, lay them back on the soil to dry out. This helps to conserve them.
  • Next, carefully check any bulbs for softness and use these in cooking because they will not store well. If any of the bulbs have small white worms, please see the diseases below.
  • Use the long dried leaves to tie your onions together. They make a pleasing plaited shape and you can then hang this decorative collection of onions to use when you need them.
  • You can also place them in a cool shed or dark area in a cupboard and they will store for months.

Final tips

Onions produce beautiful purple flowers but you have to remove any flower heads which form; these will rapidly go to seed and your onion bulb is unusable.

Common problems and diseases

Why are my onion leaves wilting?

Early on in the growing season, this may be because the planting soil is too rich in manure.

Try a potassium feed and see if that helps. Later in the season, it may be that your onions are ready to pick. The leaves tend to die back before the bulb is ready.

  • Mildew on leaves

It looks like mold and the tips of the leaves go brown. This is usually due to bad drainage.

Make sure to grow your onions in a different site next year with better drainage. If in a pot, it may be worth re-planting the onions in new soil, if it is early in their growing cycle.

  • Onion fly

Little white creatures at the base of the onion are the dreaded onion fly.

Onions grown from seed are reported to be more susceptible to this insect, so if it happens, the following year, try planting from sets.


Should I soak onion seeds before planting?

Many experienced onion growers swear by soaking seeds and the main reason is that they sprout quicker.

It feels like the spring rain is giving them a little nudge before they start growing.

Some growers prepare compost teas to give the seeds some protection from disease but as a first-time onion grower, plain water will work as well. Leave them in a jar of water for at least 1 hour or longer, then drain the water and plant them immediately.

How many onions do you get from one plant?

You will only get 1 bulb per plant unless they are a bunching variety like spring onions.

Fun activities with onions for younger growers

If you slice the roots of the onion off and leave a tiny piece of onion too, this will re-grow. Sit this on top of the soil and it will turn into a full onion.

Turn your window sill into a science lab for younger gardeners!

If an onion you bought starts to sprout, this can be planted in the water. The white roots grow quickly so the children don’t get bored waiting. Plant it in soil later if you like.

Mike Smith

I love Gardening and this is my site. Here you will find some really useful plant-related tips and tricks.