Onions masterclass: best expert content

Last Updated on December 10, 2025 by Thompson & Morgan Horticultural Team

Homegrown onions have great storage potential
Image: Onion ‘Crusado’ F1 from Thompson & Morgan

For the best independent advice on growing onions, we’ve found a selection of expert articles, videos and Instagram posts to help you raise a bumper crop. Whether you prefer to plant onion sets in autumn or spring, or you want to grow huge, exhibition-sized bulbs that you’ve carefully nurtured from onion seeds, here are some top tips…

Decide whether to grow onion seeds or sets

‘Rumba’ onion sets growing on in module trays
Image: MandyCanUDigIt

What’s the best way to grow onions – seeds or sets? For the biggest onions, traditional wisdom says you should sow them from seed on Boxing Day, says Mandy Watson over at MandyCanUDigIt. “Especially if you want onions for the exhibition bench in September – they need all that time to grow so big.” But for beginners, Mandy recommends using onion sets that don’t need to go in the ground until the beginning of April.

Start your sets in module trays to outsmart the birds

Over at The Grey Gardener YouTube channel, Adam doesn’t plant his onions out until April when the chance of frost is passed. But by starting the sets off in module trays in the greenhouse first, he gets an inch or two head-start of green growth which stops the birds pecking them up. After a few weeks under cover, he transplants them into shallow, well-spaced holes in the ground and waters them in.

Choose red onions for mild flavour

Autumn-planting ‘Electric’ onion sets ready to go in the ground
Image: The Unconventional Gardener

“Onions have always been one of my most reliable crops – easy to grow, and easy to store and use, so they do all get eaten,” says The Unconventional Gardener, Emma Doughty. She’s a fan of autumn-planting red onion sets, chosen for their mild but excellent flavour. Explaining the extreme wartime shortages of this everyday vegetable, her Onion Club article is a fascinating read about a time when onions were worth their weight in gold and “given as birthday presents, or donated as raffle prizes!”

Try award-winning onions for tried-and-tested success

If you’re not sure about the best onion varieties to grow, visit Carol Bartlett over at The Sunday Gardener for advice. She explains how the RHS decide which varieties should get the Award of Garden Merit. “Trialled and assessed for bolting, size, yield and…storage quality,”  Carol says you won’t go far wrong if you try “Sturon, Centurion, Hercules, Autumn Gold Improved, Griffon, Setton, Vulcan, and Turbo.”

Grow onions in containers if you’re short of space

Container-grown onions
Image: @lisaskitchengarden

According to Lisa from @lisaskitchengarden, “companion planting is such a great way to maximise your growing space.” She grew her onions in containers alongside other crops like aubergine, chilli, basil and parsley. With her entire 18 square metre veggie patch crammed with plants, she acknowledged that the resulting onions were not as big as they might have been, “but they did grow!”

Start onion seeds early for the biggest bulbs

Over at @gardeningwithjase, Jason explains that serious exhibition growers often start their onion seeds under artificial lighting in heated greenhouses. But, he still manages to win prizes without any special equipment by starting his onion seeds in November to give them a longer growing season. Follow him on Instagram to see if he can take another ‘Best in Show’ prize.

Enrich your soil well in advance of planting onions

Nigel always grows his onions from seed
Image: MuddyBootz

Nigel Jewkes prefers to grow onions from seed using a heated propagator – mainly because he finds that they’re less likely to bolt. Watch his excellent video over at MuddyBootz for a step-by-step guide. But the real trick to successfully growing from seed is the soil – it needs to be enriched with plenty of manure the autumn before spring planting. According to Nigel, “if you prepare the soil you shouldn’t need to feed – in fact too much feed can give your onions thick necks.”

Direct sow onion seeds from mid-February until end-March

Sow ‘Ailsa Craig’ seeds for giant, show vegetables
Image: Onion seeds ‘Ailsa Craig’ from Thompson & Morgan

Another fan of growing from seeds, Simon from Garden of Eaden says you can “direct sow onions outdoors anytime from mid-February to the end of March, provided the soil surface is dry and friable.” Then, when large enough to handle, simply “thin onion seedlings to 10cm (4″) apart for medium sized bulbs.” Although he recommends watering freely during dry spells, Simon says you should stop once the bulbs start to ripen.

Use sulphate of potash for bigger bulbs

Do you want to know Steph’s top tip for BIG onions? Sulphate of Potash!! She sprinkles some around her autumn-planted onions and garlic in March to give them a boost. But if you plant your onions out in spring, she recommends waiting for a month or so before giving them the special treatment. Follow her at @stephs_allotment for lots more helpful tips.

Weed onions frequently for a healthy harvest

Autumn planting onion sets like ‘Shakespeare’ form good sized bulbs
Image: Onion ‘Shakespeare’ (Autumn Planting) from Thompson & Morgan

“Onions don’t like too much competition from other plants, so keeping the patches relatively weed free should help the bulbs access water, space and nutrients a lot easier,” says Adam Leone at Carrot Tops Allotment. But Adam freely admits that weeding is like marmite: “it’s a job that you either love or hate.” He hasn’t made up his mind about whether it’s something he enjoys, but at least, “you really do get a great sense of gratification when it’s been done.” And a good crop of healthy onions!

Use netting to keep pigeons off your onion sets

Attacks from allium leaf miner can be devastating
Image: Tomasz Klejdysz/Shutterstock

For a great summary of the most common onion pests, diseases and problems, head over to Allotment Garden to read John’s thorough troubleshooting article. Netting is essential when growing onions, but there are far worse problems than pesky pigeons. “White rot is a serious fungal disease,” explains John. “The spores can survive in the soil for fifteen years so normal crop rotations are not effective even as a deterrent.” Read his top tips on how to avoid spreading it round your plot.

Mesh protects onions from leaf miner

A mixed colour collections is a great way for gardeners to grow onions to suit any culinary preferences
Image: Onion Mixed Red, White & Brown (Autumn Planting) from Thompson & Morgan

Having grown onions for the very first time, Naam Kaur shares her successful red and white bulbs over at @naams_garden. They suffered a little from leaf miner, so she’s definitely going to grow them under mesh next time.

Wait until the stems flop to harvest your onions

Charley shows off her onion haul
Image: @charleyspatch

Over at @charleyspatch, Charley uses the stem flopping system to figure out when her onions are ready for harvest. Those that haven’t ‘flopped’ get to stay in the ground for a bit longer. Not discouraged by the smaller bulbs, Charley simply views these as an opportunity to get pickling. Grown in raised beds from autumn-planting sets, she leaves hers to ‘cure’ in the sun for a few hours before moving them to a portable mini greenhouse to finish the job.

Dry and plait your onions for storing

After drying her onions, Lucy plaits the foliage
Image: @shegrowsveg

Drying onions and shallots properly is vital if you want to store them for future use. Over at @shegrowsveg, Lucy shows off her DIY onion drying rack that was cleverly upcycled from an old plastic grow tent. Lucy explains, “once the foliage is semi-dried I will start making plaits, my go-to space saving storage method.” Watch her quick onion plaiting video if you’re not sure how it’s done. 

Onions are relatively easy to grow and taste so much better than shop-bought varieties. In fact, if you grow your own, simply roast the bulbs and enjoy them as a vegetable in their own right. For more advice on growing alliums, visit our onions, shallots and garlic hub page.