How to Grow Onions from Sets

Learn how to grow and Harvest Onions from Sets. Follow my simple steps for a Successful Onion Harvest.

How to grow onions from sets

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Why grow from Onion Sets?

I mostly grow my onions from sets. I start my onion sets off early at the end of January- Early March. There’s a couple of good reasons why I do this.

  • Onions need a long growing season to mature and ripen, they are also sensitive to daylight & temperature. As the summer days grow shorter it can trigger the onion to go to seed before the onion is ready to be the earlier you start them off the better. It gives them opportunity to put on some extra growth early on.
  • Birds tend to pull up fresh onion sets and toss them around, mistaking them for snails, or they do it for a laugh…who knows! But, sets you have started off early are less desirable, due to their plumes of foliage…it puts the birds right off.

How to Start off Onion Sets

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Start onion sets off at the end of January- Early March

Plant onion sets in compost filled modules or trays , root down, around 1/3 depth. Keep the compost moist but not Water logged.

Compost / Coir mix

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A good Quality compost will work well, but to lessen the risk of onion sets developing rot in wet compost, add Coir peat to your compost, here’s a couple of reasons why I add Coir to my compost…

When Coir is mixed with compost it does a wonderful job of retaining the nutrients within the compost. Nutrients in soil are sometimes lost after watering..adding coir to your compost helps retain those precious nutrients, giving your plants the best possible start! Coir is also well known for holding moisture-without the growing medium becoming clogged and sodden, allowing a compost/coir mix to remain aerated even after a heavy watering. Perfect for Onion sets ( preventing Soggy bottoms! ) Also wonderful for hanging baskets and pots.

How to grow Onion sets

Plant Out

I plant out March/April once the sets have established a healthy plume of foliage. Plant 10-15cm apart, with roughly 30 cm between rows.

Onion seedlings


Onion sets will reach maturity around 80 days after planting. If growing from seed, 100-120 days. Onions require a free draining soil and a sunny position. I also like to dig in some well rotted manure before planting.

Onion bed

Onion Scapes

Onion Scapes, the free gift with every onion plant! As soon as these appear, remove them! They will steal energy from the plant and stunt the growth of the onion bulb.

Onion scapes

But don’t throw them away…eat them! Eat raw in salads or cooked in stir fry’s or pasta dishes. They have a mild garlicky onion flavour, delicious!

Onion Scapes


  • Harvest Onions when the leaves begin to flop over or you see the edges browning.
  • Dig them up using a fork, try to not damage the roots (they store better with intact roots)
  • Leave them to dry out in a sheltered sunny spot for about a week.

How to Cure Onions for long term storage

If you wish to store the onions long term, you then need to cure them. This can be done by spreading them out on a table, or hanging them up, or using racks, make sure the area is dry and airy, a polytunnel or greenhouse would be ideal. Let them dry out for 2 weeks until the skins are dry and papery and the roots are bone dry and wiry.

How to grow Onions from Sets
Happy Gardener!

How To Store Onions

For long term storage, keep onions in a cool, dark, dry place. *Moisture and light may lead to mould and sprouting of the onions. I either hang mine in the pantry or keep them in a basket in a dark cupboard.

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