Growing Onions from Sowing to Harvest

, written by Benedict Vanheems gb flag

Braided string of onions

Onions are a must-grow vegetable. Why? Well, where to begin! To start, onions are very easy to grow and, properly prepared, bulbs will store reliably for up to six months. As with potatoes, there’s something deeply satisfying about the weighty harvest you can get from even a small area, and as the starting point to so many recipes there’s every reason to grow your own. So let’s not hang about: here’s our Sowing to Harvest guide to onions.

Getting Started with Onions

Bulb onions come in traditional yellow and red, but look out for white varieties too, which are often bigger, milder and great thinly sliced into salads.

For an extensive list of varieties check out our Garden Planner where you can bring up a list of varieties for every crop (including onions of course!) and read through variety descriptions at your leisure. Drop some onions into your plan, then bring up the Plant List to check the best sowing, planting and harvesting dates for your specific location.

Onions love a sunny and open site in well-drained soil enriched with organic matter such as compost or well-rotted manure. If your soil is heavy and tends to remain overly wet then grow onions in raised beds or on mounds to improve drainage.

Starting onions in plug trays is fuss-free

Growing Onions from Seed Indoors

For the earliest start, sow onion seeds into plug trays or pots to transplant later as seedlings. This avoids the need for thinning out, encourages a more economical use of seeds and, given the protection of a greenhouse or cold frame, means sowing can start at least a month sooner in late winter.

Fill plug trays with seed-starting or general-purpose potting mix, pressing it down firmly into the cells. Sow a pinch of four to eight seeds per cell, then cover with more potting mix to a depth of a quarter to half an inch (1cm). Water with a fine spray.

Transplant the resulting seedlings while they’re still quite small to avoid disturbing the delicate roots. Make holes into prepared ground, planting each clump of seedlings about 4in (10cm) apart before firming in and watering.

Transplant clusters of onion seedlings to save time and space

Sowing Onions Outside

Direct sowings can commence in spring as soon as the soil is workable and has warmed up a little. Rake the soil level then mark out seed drills about half an inch (1cm) deep and a foot (30cm) apart. Sow the seeds very thinly, cover back over then water along the rows to settle them in. Thin the seedlings in stages until they’re about 2in (5cm) apart for lots of smaller onions or 4in (10cm) apart for fewer but bigger bulbs.

Covering early sowings or transplants with row cover or fleece helps to speed things along at the start of the season, and may help reduce the tendency to bolt.

Some especially hardy varieties of onion may also be sown in late summer to sit through winter and give an extra early crop in spring or early summer.

Planting onion sets offers a shortcut to success

Growing Onions From Sets

In many regions you may be able to buy onion transplants for immediate planting. An alternative is to plant sets. Sets are part-grown onions that are super-easy to grow and save time sowing. On the downside they don’t store as well as onions grown from seed or transplants, and they carry a higher risk of bolting (flowering) which makes the bulb too tough to eat. There are, however, heat-treated varieties available that are more resistant to bolting.

Nevertheless, sets are clear winners when it comes to convenience. Plant sets in mid spring into prepared, weed-free ground once the soil is workable and has warmed up a little. Leave just the tips poking up from the ground and space them 2-4in (5-10cm) apart, depending on the final size of bulb you’re after. Some sets may also be planted in early autumn, to give a harvest up to two months earlier next summer.

Caring for Onions

Onions transplanted from plug trays may be left as they are or thinned out once they’ve grown on a little to give bigger bulbs. You can enjoy the thinnings as green onions.

As shallow-rooted plants, onions must be kept watered in dry weather. Keep on top of weeds, hoeing carefully between rows then hand weeding within the rows so as not to damage the roots.

Curing onions on racks insures good airflow

Harvesting and Storing Onions

Harvest time is approaching once most of the leaves have bent down towards the ground. Bulbs will continue to swell over the next few weeks before coloring up nicely in time for harvest.

When they’re ready, lift them up with a fork or trowel then move those destined for storing under cover to dry. Any form of cover, from an airy shed to a greenhouse is ideal. In warm, dry climates simply leave the onions where they are on the soil surface. Space bulbs out so there’s good airflow between them. Racks can help with this. This drying process, called ‘curing’, takes about two weeks and toughens up the outer skin of the onion so it will keep for longer.

Store onions suspended in nets, tied into bundles or woven into beautiful onion strings. Onions should keep until at least midwinter, and as long as spring.

Bugs, Beneficial Insects and Plant Diseases

< All Guides

Garden Planning Apps

If you need help designing your vegetable garden, try our Vegetable Garden Planner.
Garden Planning Apps and Software

Vegetable Garden Pest Warnings

Want to Receive Alerts When Pests are Heading Your Way?

If you've seen any pests or beneficial insects in your garden in the past few days please report them to The Big Bug Hunt and help create a warning system to alert you when bugs are heading your way.

Show Comments


"Hi, I’ve started onions from seed indoors under artificial light. They are long day onions. Do you have a any recommendations on how many hours a day I should have the lights on. Thanks!"
Brian on Sunday 10 March 2019
"Long-day onions shouldn't start to produce a bulb until day length reaches around 14 hours. You don't want seedlings attempting to begin bulb formation early on, so keep your lights on for much longer than this. I would suggest, at this time of year, a length of around 11 to 12 hours would be perfect, thereby mimicking outside day length."
Ben Vanheems on Monday 11 March 2019
"Great simple stuff. I had forgotten about sets will try this Spring.zDQCX"
Dick Clapp on Wednesday 21 August 2019
"How do I do seed multiplication of onions? The same way with growing them?"
Salisu Harbau on Wednesday 6 November 2019
"Hi Salisu. If you are looking to grow onions from your own saved seeds, then simply leave onions to carry on growing so they can produce flowers and then the seeds. You can then collect the seeds when they are dry and shake them out into envelopes to keep until sowing the following spring."
Ben Vanheems on Monday 11 November 2019
"Hi, The video shows seedlings that were started in plug trays being planted. How long were they in the plug trays? I'll be starting mine under a light. Usually buy starts, first time trying from seed."
Bob Sloat on Thursday 23 January 2020
"It does depend somewhat on the weather and light levels. Generally a sowing in late winter/early spring will grow on in plug trays for around six to eight weeks before being planted out. "
Ben Vanheems on Thursday 23 January 2020
"I planted 120 red baron and Stuttgarter last year. Some went "tubey" but after harvesting in july. (I`m in mid Cornwall") I dried them outside when sunny then some weeks on window sills inside then hung them in string bags in the spare north facing bedroom from the curtain pole! I still have plenty of lovely firm onions to enjoy 7months later. I tend to plant in February but it`s been so wet this year they are going in on the next dry period here in Cornwall. Spuds have gone in today, three weeks later than usual!"
A Martin on Saturday 22 February 2020
"Hi ready for a laugh I put my onion sets in last week 7th April then scattered what I thought was grow more on top it was only today when putting more sets in I realised put weed feed and moss killer on those last week we have had 1 night of rain, shall I RIP them up."
malcolm tomkinsonC8CCN on Wednesday 15 April 2020
"Hi Malcolm. Mistakes happen, so don't be too hard on yourself! Weedkiller meant for lawns kills off dicotyledons - i.e. plants with branching veins. Onions are monocotyledons, so they may, in fact, be okay as they won't be targeted by that weedkiller. And by the time you harvest the onions traces of the weedkiller should be long gone (though I'd still wash them really thoroughly. So I'd say, keep them where they are as there's a good chance they'll grow."
Ben Vanheems on Thursday 16 April 2020
"I know this sounds silly but do the "ripe"onions sit on the soil surface when they are ready to harvest? never grown ant veg at all in my life but thought why not! so used onion set in long plastic box and carrot from seed in another ...both have got a bit greenery but I don't know what to do next can you please advise thank you M.Lilley (mrs)"
MARGARET LILLEY on Monday 25 May 2020
"No problem. The onions will just continue to grow and then from midsummer the bulbs start to swell up. You will know when they are ready when the leaves start to flop a bit and fall over or begin to go yellow. The onion do indeed sit on the soil surface - so they'll be very easy to see and judge how well they are coming along."
Ben Vanheems on Monday 25 May 2020
"Thank you Ben very helpful...."
maragretlilley on Tuesday 26 May 2020
"So, new to this gardening thing. I have planted some onions and they are growing beautifully. My question is do you have to bend the green part above the dirt so the growth goes into the actual onion? "
Dawn on Thursday 4 June 2020
"I have found this year less "tubey" onions. Been very warm and sunny and found one flower stalk pushing up so as I have well drained soil I`m soaking them every day.The answer does indeed seem to be feed occasionally but water water water! "
Anthony Martin on Friday 5 June 2020
"Hi Dawn and Anthony. No, there's no need to bend the green part to make it into an onion. The onion bulb will swell naturally, while the foliage is still very much upright and growing. The leaves tend to yellow and begin go to bend over naturally, which is when you can push them over to help with the maturing process. And yes, lots of water is important to help the bulbs swell - there's a direct relationship there!"
Ben Vanheems on Monday 8 June 2020
"Hello. During lockdown we do very strange things. I showed Ailsa Craig and Red Barron seeds about six weeks ago (June!) They're doing really well in my greenhouse and I've been snipping the tips of the seedlings off as some suggested. They're delicious in salad or stir fries. My question is am I being over optimistic in expecting them to survive the winter? If not, when should I plant them out? I have exceedingly hungry slugs, who nibble everything once my back is turned! Great website by the way "
Liz Prior on Thursday 6 August 2020
"Hi Liz, I think you may indeed be being a little optimistic in expecting them to overwinter for next year. They may well overwinter, but as onions are biennials they will be highly likely to bolt (flower) before they produce a useable bulb. I have heard of people growing their own sets though - the young bulbs are harvested while they are very young then dried out and stored to plant in the spring. I've never done this before and am unsure of the details, but it may be worth investigating."
Ben Vanheems on Monday 10 August 2020

Add a Comment

Add your own thoughts on the subject of this article:
(If you have difficulty using this form, please use our Contact Form to send us your comment, along with the title of this article.)

(We won't display this on the website or use it for marketing)


(Please enter the code above to help prevent spam on this article)

By clicking 'Add Comment' you agree to our Terms and Conditions