How to grow onions: from seed and even from another onion

Learn how to grow onions from seed and even from a spare onion – indoors or out. It's easy, fun to do with the kids and you'll get satisfying results

how to grow onions from seed
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Learn how to grow onions from seed and enjoy a good harvest of this essential cooking vegetables. Growing onions is also a fun thing to do if you've got (bored) children in the house, as it's relatively easy and doesn't require any special gardening skills. 

We do recommend getting young, first-year onion seeds as they give the best results.

Looking for more garden ideas? Check out our feature.

How to grow onions from seed

Growing onions will require a seedling tray, good-quality peat-free seedling compost, and something to replant them into – either larger pots and compost, or a patch in your garden if you have one. 

1. Begin by filling your seedling tray with the compost up to about two-thirds of each compartment. Then, carefully plant the seeds about one centimetre apart. 

2. Lightly cover the seeds with compost and soak the tray in water – you can do this in the sink. What you want is for the soil to absorb as much water as possible, but avoid submerging the tray in water.

3. Line the bottom with a plastic bag and place the tray in the brightest spot you can find, such as a bright window sill. Growing lamps help but aren't necessary. It also helps if the bottom of the tray is kept warm, for example by placing it on top of the fridge. Again, this will help the seeds germinate quicker but isn't essential. 

4. Water the tray regularly with a watering can and diffuser attachment. You should see germination after about a week to ten days. If the seedlings are growing in loops, gently snip them in half to allow them to grow.

5. When the seedling are taller and have three shoots each, you can replant them where you will be harvesting your onions. The should now be grown outside, provided there are no frosts. They should be ready to harvest in five months. 

How to grow onions, from an onion

Yes, it is possible. If you have an onion going spare, you can actually grow an onion from an onion, using the cuttings it produces. Start these indoors all year round, or out in the garden come spring/summer time. Here's how: 

  1. Take your onion, –  any variety will do – remove the peel and chop 2.5 cm in, off from the bottom. Retain this small section, you can cook or compost the rest.
  2. Leave you piece of onion out, bottom down to dry for about 12-24 hours until it starts to look calloused and is dry to the touch. 
  3. Take 4 toothpicks and slot them in to four sides, creating an 'x' like shape.
  4. Place your onion over a glass, filled to the brim with water. Do use a glass so you can see the root growth. 
  5. Leave it in a sunny spot and after 3-4 days you should see white roots beginning to sprout. 
  6. When there is a healthy amount there, your onion sprout is ready to be planted.
  7. Simply half fill a plant pot with draining holes, with well-draining soil, and add your onion, root side down and in the centre. Cover with soil and level the top, water it well and you can use a fertiliser also if you have some available.
  8. Whether you keep it inside in a container, or plant it out eventually, keep an eye on the soil to check that it doesn't dry out in the early stages of growth. We recommend about an inch of water a week but this depends on the weather, your garden, the type of compost and more. 
  9. You can expect a brand new onion bulb in about 90-120 days, in the meantime, keep it free from weeds and well watered as above.

Anna is a professional writer with many years of experience. She has a passion for contemporary home decor and gardening. She covers a range of topics, from practical advice to interior and garden design.