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.

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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
"I’ve planted onions from sets in different places on my allotment but they never get any bigger than when I put them in, what am I doing wrong? Lottie Digger "
Carol Carr on Saturday 16 January 2021
"They need good sunshine, and watering them is important as they have quite shallow roots. I often find the bulbs swell quite suddenly after a period of wet weather when it's been dry. So persist with watering - it's worth it. Finally, the soil does need to be fertile, so incorporate plenty of compost into the ground in preparation for planting in spring."
Ben Vanheems on Monday 18 January 2021
"Onions do indeed need sunshine. In 2019, they did very well in a raised bed with lots of sunshine. In 2020, it happened that I planted carrots adjacent, and the carrot fronds grew so big and thick, that I think they shaded the onions. I will take care to be more aware! 2019 onions were big and beautiful. The 2020 onions were very small (maybe it was the Covid, and they were just depressed)!"
Laurie on Saturday 30 January 2021
"Hi Laurie. I think it's because onion leaves (like leek leaves) are so thin and strappy that every bit of sunshine really makes a difference. I hope you have a better crop this year. :-)"
Ben Vanheems on Sunday 31 January 2021
"I planted onion sets for the first time in these wide plastic bulbs that some of my other plants came in. They’ve been growing quite well but I did notice flower bulbs coming up. I’ve been searching online and read that if my onion plants bolt they may be too tough to eat. I want to say I planted them about 2 or 3 months ago now so I didn’t think they were ready to harvest yet. The green parts on top look really full though. Should I just pull them up to prevent them from getting tough?"
Aaliyah on Friday 4 June 2021
"Hi Aaliyah. If the onions flower prematurely (called bolting) then the bulbs will stop forming and/or will become very tough and not good eating. You could try cutting the flower stalks out. Or if the bulbs are already a useable size, lift the plant up and use them straight away."
Ben Vanheems on Monday 7 June 2021
"Hello again, it's me Liz from August 2020. Well the onions which I sowed last June did survive the winter and we've just eaten the last of them. They didn't grow to any great size, but were perfectly edible, rather like a large spring onions and not too strong. Just about to start harvesting this years now and have a few more to follow later in the year. Thanks for your continued great website. "
Liz Prior on Tuesday 8 June 2021
"Hi Liz. Really delighted to hear they've survived the winter and given you a useable crop - what a bonus!"
Ben Vanheems on Wednesday 9 June 2021
"Hi, I put my onion sets in early this year - last week of February. They've done well and look good at this moment. But in the past my plot has suffered from downey mildew during late July/early August. Already the stems of some onions are showing lightish white streaks which to me don't look like the signs of maturing yellow. My question is this: should I lift the onions now, in mid July, or run the risk of them being infected by downey mildew during the next few weeks? Thanks in advance."
Bill on Wednesday 14 July 2021
"Hi Bill. If the onions are already a useable size, I'd be inclined to lift them now. They probably won't store as long as those that have been left to fully mature, but the risk is that if downy mildew does establish it could ruin the crop. Be sure to onion family plants in a new area of your garden next year - the infected area should not have onions planted again for at least four years, to avoid the disease passing from one crop to the next."
Ben Vanheems on Thursday 15 July 2021
"MY daughter told me that you are supposed to bend the stalks over on onions so the bulbs form properly. She said otherwise they just get thick stalks. Is this true for ordinary white onions?"
Margaret Sleigh on Tuesday 31 August 2021
"Hi Margaret. This was common advice some years ago, but the prevailing advice nowadays is to let the onions flop over naturally. Once most of the crop has leaves flopped over and the foliage is beginning to die back/yellow, then the whole crop can be dug up ready for curing then storing."
Ben Vanheems on Tuesday 31 August 2021
"Hi, I know only the method of growing bulb onion through nursery sowing of bulb onion seeds and transplanting seedlings which I believe is okay when growing onion on smaller plots. I am very interested to grow bulb onion on a larger plots for marketing and as such, my biggest challenge will be to reduce as much as possible my farm input costs. One area in cutting cost is by-passing the nursery process and doing direct seeding into gardens. Please advise if this is practicable. john Pono Papua New Guinea"
John Pono on Wednesday 15 September 2021
"Hi John. This is certainly possible - it's how onions are grown on commercially, on a field scale. In the garden onions can be direct-sown into well-drained soil and then thinned in stages to their final spacing, though I'm not sure that would be practical on a large scale. You would need to sow at just the right rate to ensure no thinning is required. It may be asking your local agriculture advisory service for the specifics on what might be best in your climate and soil etc."
Ben Vanheems on Thursday 16 September 2021
"Hi Ben, I thank you for the highly valuable feedback on my query of 15th September, 2021. The information is valuable for my purpose. On the same subject I have 2 additional queries if you could kindly provide your input on these. 1. Is there a specific BO variety recommended or bred specifically for direct seeding or sowing in the field or any available variety is ok for direct seeding? We are using GLADALAN BROWN variety which is quite dominant in Papua New Guinea. 2. I need up to 50kg of BO seeds per month for my project. How and where can I source these quantity on a consistent basis? "
John Pono on Thursday 30 September 2021
"Hi John. I'm afraid I cannot advise on where to source seed either within PNG or for export to PNG. Again, I would suggest contacting your local agriculture advisory service, or you could try a Queensland, Australia-based service, as your climate will be very similar? Certainly, the priority will be to use a variety that can cope with your climate - that is essential. If the Gladalan Brown variety is dominant, then that may be a good starting point. You'll also want to consider pest and disease resistance of ease of growth, particularly as you are growing commercially and don't want to have the risk of extensive crop loss."
Ben Vanheems on Thursday 30 September 2021
"Hi Ben, thank you for your feedback. Again, the info you provide is highly relevant and important. Thanks again. john pono papua new guinea"
John Pono on Thursday 30 September 2021
"Hi Ben I have King onions that are now 4months old The bulbs have grown well and now the leaves are starting to bend.How do I prepare them for harvest,Shall I dig the soil around them so that the bulbs are fully exposed.When is the right time to uproot them. I am in Harare,Zimbabwe. "
Clem Mhonda on Tuesday 19 July 2022
"Hi Clem. You can just leave the foliage to completely flop over and turn yellow/die back. At this point gently dig the bulbs up. Let them dry off on the soil surface for a day, then move them to somewhere dry, warm and airy to continue drying out, ready for storing."
Ben Vanheems on Tuesday 19 July 2022
"Can I sow onion seeds as early as October indoors, for planting outdoors? I've been given some seeds, but would like to harvest them as early as I can, to avoid delays in my crop rotation. I am in East Anglia."
Bruce Ronaldson on Friday 23 September 2022
"Hi Bruce. I wouldn't sow onions that early. They will be too big and leggy and would then get hit hard by frost as they would need to be planted. Best to wait until late winter before sowing them (late January or February)."
Ben Vanheems on Monday 17 October 2022
"Hi Ben it's me again, by my reckoning you must have more queries about onion growing than another crop! Anyway silly me, planted far too many red and white onion sets last autumn.which are doing well and beginning to enlarge with the longer days. However, I am now worried that I won't have sufficient room for all my other veg once they are ready to be transplanted out. I think I know what you've going to say, but is it too late tomove some of them closer together? Maybe I could use them as small said onions or plant a few slow growing veg inbetween? Thanks again for your wonderful website which I really enjoy and couldn't manage without. "
Liz Prior on Friday 22 March 2024
"Hi Liz. You could try transplanting them if you very carefully more all the soil with the roots, using a good-sized hand trowel, for example. But it's a gamble. Another option may be to harvest half (or some) of the crop at salad onion size to free up space to sow/plant something else. This would be what I do I reckon. They're probably close to salad onion size in another few weeks only."
Ben Vanheems on Friday 22 March 2024

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