Best UK Onion Varieties: Sets vs Seed Tested

The best UK onion varieties depend on what you want from the crop. Storage, flavour, size, and bolt resistance all rank differently across the 30 widely sold cultivars. This guide ranks 18 varieties from a four-season trial on Staffordshire heavy clay, with side-by-side tests of sets and seed for each.

You will find a variety table with yields and storage figures, the planting calendar for spring and overwintered crops, and the specific cultivars that justify the extra effort of seed-raising. For the basic growing method, pair this with our how to grow onions UK guide and our shallot guide.

Eighteen UK onion varieties laid out at the end of the 2024 trial, ready for the storage test in Staffordshire

What separates a good UK onion variety from a poor one

Five traits decide an onion variety’s value to a UK home grower: yield per square metre, storage life, bolt resistance, flavour, and bulb size. The trial tracked all five for 18 varieties grown from both sets and seed across four seasons. No single variety topped every metric. The best all-round performers excelled in three of the five.

Yield ranged from 2.8kg to 5.2kg per square metre across the trial. Set-grown crops averaged 3.6kg, seed-grown averaged 4.1kg. The yield gap widens further if you measure by bulb weight at harvest rather than total weight.

Storage life is the single biggest discriminator. Sturon held its full sample without rot for 235 days in shed storage at 8-12C. Red Baron lost the first bulb at 168 days. Bedfordshire Champion lasted 195 days. Mammoth Improved went over within 90 days, despite being the largest variety in the trial.

Bolt resistance matters in cool wet UK springs. Centurion bolted on 2% of plants in 2023; Stuttgarter Giant bolted on 18% in the same year. Choose bolt-resistant varieties unless you are prepared to harvest early and pickle the rest.

Flavour is subjective but consistent across panels. Red Baron, Karmen, and Bedfordshire Champion topped UK taste tests. Bland varieties like Sturon work better cooked; punchy ones like Red Baron excel raw and roasted.

Bulb size depends on growing method as much as variety. Ailsa Craig from seed exceeded 750g; the same variety from sets averaged 580g. Sets compress the growing window and limit final size.

Variety comparison: sets and seed performance

The table below ranks the 18 trial varieties by overall reliability for UK home gardens. Yields and storage figures reflect the four-year average from Staffordshire trial beds. Bulb weight is the seed-grown average where seed and set differ.

The ranking shifts dramatically by use case. For a year-round kitchen onion, Sturon and Centurion dominate. For flavour and colour, Red Baron and Bedfordshire Champion win. For show or maximum size, Ailsa Craig and Mammoth Improved lead, with the trade-off of much shorter storage. For autumn planting, Senshyu Yellow and Electric are the only options worth considering.

Lifted Sturon and Red Baron from the 2024 trial, a few days into the outdoor cure before the storage test

Sets vs seed: which method gives the best bulbs?

Sets give faster bulbs and easier results; seed gives larger bulbs and a wider variety choice. The trial planted both methods for every variety where both were available. The differences are clear and consistent across four seasons.

Sets reach harvest 2-3 weeks earlier. A March-planted Sturon set was ready to lift on 2 August in 2024. The same variety from seed sown in late January under glass was ready to lift on 21 August. The earlier lift gives a longer cure window and slightly better storage.

Seed produces 18-22% larger bulbs. Set-grown Sturon averaged 280g per bulb across the trial; seed-grown Sturon averaged 340g. The difference comes from the longer growing season and the absence of the storage check that sets impose.

Sets bolt 30-50% less. Heat-treated sets are vernalised under controlled conditions before sale, removing most of the bolting trigger. Seed-grown plants face full UK spring temperature swings and bolt more readily.

Seed costs 70% less per metre of bed. A packet of 200 Sturon seeds costs around £3 and produces 150 viable plants. The equivalent 150 sets cost roughly £10. Over a 10-bed kitchen garden, the saving runs to £40-50 per season.

Seed gives access to varieties sets cannot match. Ailsa Craig, Bedfordshire Champion, Walla Walla, and most of the seed-only varieties produce bulb sizes and flavour profiles that no set replicates. These are the show-bench varieties and the kitchen specials.

Gardener’s tip: Use sets for your main winter-storage crop and seed for your eating-fresh and show varieties. Plant Sturon sets in March for storage, sow Ailsa Craig and Bedfordshire Champion seed in January for size. The two methods complement each other across a single bed.

Best overwintering onion varieties for UK gardens

Autumn-planted onions deliver the May-to-July fresh harvest that fills the gap before spring-planted crops are ready. Three varieties dominate the autumn slot in UK gardens: Senshyu Yellow, Electric, and Radar. The trial ran all three for two seasons.

Senshyu Yellow is the standard for UK overwintering. The Japanese variety tolerates -10C in the ground, returns in March with strong leaf growth, and sets bulbs by late May. Average yield in the trial reached 3.5kg per square metre with under 5% winter loss.

Electric is the best red overwintering variety. The bulbs reach 200g with a sweet mild flavour suited to fresh use. Storage is short at 4-5 months, so plan to eat the crop between June and October rather than storing into winter.

Radar is the third reliable option. Slightly larger bulbs than Senshyu, slightly better cooking flavour, but more prone to neck rot in wet sites. Plant only on free-draining soil if you choose Radar.

Plant overwintering sets in late September or early October. The window is short. Plant too early and the leaves overgrow before winter; plant too late and the roots fail to establish before frost. The right size at planting is a 1cm-diameter set with one or two visible leaf tips.

Step-by-step planting calendar

Spring planting runs from mid-March to mid-April for most of the UK; autumn planting runs from late September to early October. The dates shift by 1-2 weeks for Scotland and the Highlands.

Step 1: Prepare the bed two months before planting. Apply 5cm well-rotted manure or compost to the surface in January for spring crops, or in mid-August for autumn crops. Onions need free-draining, weed-free, and well-fed ground.

Step 2: Mark planting rows at 30cm apart. Use a string line and cane. Keep rows straight to make hoeing between them easier through the season.

Step 3: Plant sets at 10cm spacing, just covering the tip. Push each set into the soil so only the tip protrudes. Birds will pull deeper-planted sets out; surface-set bulbs root faster but need bird netting for two weeks.

Step 4: Sow seed thinly at 1cm depth. Thin to 7cm spacing as soon as seedlings have one true leaf, then to 12cm at four leaves. Thin in the evening to avoid attracting onion fly to crushed leaves.

Step 5: Water well after planting. A thorough soak settles the sets and seedlings. Avoid further watering for 7-10 days unless the bed dries out completely.

Step 6: Keep weed-free for the first 12 weeks. Onions hate competition from weeds. Hoe weekly between rows. Hand-weed close to the plants to avoid root damage.

Step 7: Stop watering 6 weeks before harvest. Withholding water triggers the bulb to ripen and reduces neck rot. Foliage will yellow and flop, signalling the right time to lift.

Warning: Never plant onion sets before soil temperature reaches 7C, even if calendar dates suggest the window is open. Cold-soil planting triggers bolting in over 30% of sets, which destroys storage potential. Check soil temperature with a probe at 10cm depth. The Met Office’s climate data lists average soil temperatures by region.

Sturon sets at 10cm spacing in March 2024, prepared with a January compost mulch and ready for the new season

Common mistakes with UK onion varieties

Five mistakes account for most variety-specific failures in UK home gardens. Avoiding them adds 30-50% to typical onion yields.

Mistake 1: Choosing show varieties for storage. Mammoth Improved and Ailsa Craig produce huge bulbs but rot within 90 days of curing. Match the variety to the use. For winter eating, plant Sturon or Setton.

Mistake 2: Planting overwintering varieties in spring. Senshyu and Electric are bred for autumn planting and vernalisation through winter. Spring-planted overwintering sets bolt at 60-80% rates and yield almost nothing.

Mistake 3: Buying mixed-variety set bags. Most “garden centre value packs” mix old or low-grade stock from multiple varieties. Yields and storage are unpredictable. Always buy named, dated single-variety sets from a specialist supplier.

Mistake 4: Skipping the cure. Curing is the 2-3 week period after lifting where onions dry outdoors in the sun, sealing the neck and ripening the skin. Skip the cure and storage life drops by 60-80%. Lay lifted onions on wire mesh in a sunny spot until the necks are completely dry.

Mistake 5: Storing in damp sheds. Onions need 8-12C and 65-70% humidity for long storage. Damp sheds rot the necks within weeks. Store in net bags or trays in a cool, dry, frost-free space. Outdoor sheds work in winter only with insulation.

Why we recommend Marshalls and Suttons

Why we recommend Marshalls Garden for onion sets: After ordering from 5 UK seed suppliers across 4 seasons, Marshalls delivered the most consistent set quality with under 5% blanks across 20 deliveries. Their sets average 1.2-1.6cm diameter against industry-typical 0.8-1.4cm, which translates to faster establishment and earlier bulb formation. A 250-set Sturon pack costs £8.95 against £12-14 from garden centres, and the heat-treated variety range covers Sturon, Centurion, Setton, Red Baron, and Karmen.

Why we recommend Suttons for show seed varieties: Suttons stocks the widest UK range of seed-only onion varieties for kitchen and show. Their Bedfordshire Champion seed maintains the genetic line that produced the original Bedfordshire trial winners; their Ailsa Craig is the F1 form which germinates more reliably than the open-pollinated alternative. A 200-seed packet covers a 2 m² bed and costs £3.45.

Frequently asked questions

What is the best onion variety to grow in the UK?

Sturon is the best all-round onion variety for UK gardens. It yields 4kg per square metre, stores for 8 months at 8-12C, and resists bolting better than any other set in the trial. Red Baron is the best red variety for flavour. Ailsa Craig from seed gives the largest bulbs, often over 700g, but stores for only 12-14 weeks.

Should I grow onions from sets or seed?

Sets are easier and faster, seed produces larger bulbs and a wider variety choice. Sets reach harvest 2-3 weeks earlier with less bolting risk. Seed-grown onions average 18-22% larger by mid-September. Beginners and short-season gardeners should pick sets; experienced growers wanting size and variety should choose seed.

When do you plant onions in the UK?

Plant onion sets between mid-March and mid-April in most of the UK. Sow onion seed under cover from late January, transplant in April. Plant overwintering varieties like Senshyu Yellow from late September to early October. Avoid planting before soil reaches 7C, which is when bolting risk drops sharply.

Which red onion stores longest in the UK?

Red Baron stores 5-6 months in dry conditions, the longest of any UK red onion variety tested. Karmen and Electric store 4-5 months. Red Brunswick stores barely 12 weeks and is best used fresh from the garden. Always cure onions outdoors for 2-3 weeks before bringing them indoors for storage.

What is the largest UK onion variety?

Ailsa Craig grown from seed produces the largest UK onion bulbs, regularly exceeding 700g and reaching 1.2kg in show conditions. Bedfordshire Champion white onions reach 600-700g. Mammoth Improved is bred specifically for size and reaches 800g-1kg. None of these large varieties store as well as smaller bulbs like Sturon.

How do I stop onions from bolting in the UK?

Plant after soil reaches 7C, choose bolt-resistant varieties like Sturon and Centurion, water consistently, and avoid nitrogen-heavy feeds. Cold spells below 5C in May trigger bolting in susceptible varieties. Heat-treated sets resist bolting better than untreated sets. Overwintering varieties bolt less because they vernalise in winter rather than spring.

What onion variety is best for autumn planting?

Senshyu Yellow is the best autumn-planted onion variety for UK gardens. It tolerates -10C in the ground, fills the May-July gap before spring crops are ready, and yields 3-4kg per square metre. Electric is the best autumn red. Plant both in late September on well-drained soil for harvest the following June.

Now you have the variety rankings, read our shallot growing guide for the matching shallot varieties that complement an onion-growing plot.

Cured Sturon and Senshyu onions on wire mesh racks in late August, a 2-3 week outdoor dry phase before storage

Variety storage trays in November, six weeks after the cure phase, with Sturon expected to outlast Red Baron and Karmen by 60-90 days

Ailsa Craig and Bedfordshire Champion seedlings in late February, four weeks before transplanting to the bed