Planting Onions UK | How to Grow Your Own Onions from Sets or Seed
Introduction: A Staple Crop for Every Garden
Onions are one of the easiest and most useful vegetables to grow in the UK. Whether you’re a beginner starting your first allotment or a seasoned gardener filling raised beds, planting onions is a satisfying way to produce a long-lasting crop that stores well and adds flavour to every meal.
This guide will walk you through everything you need to know about planting onions in the UK — including when to plant, how to grow from sets or seed, soil preparation, spacing, aftercare, and harvesting for best results.
1. When to Plant Onions in the UK
Timing depends on how you’re planting them.
From onion sets:
- Spring planting: March–April (most common)
- Autumn planting: September–November (for overwintering varieties)
From seed:
- Indoors: January–February (for transplanting later)
- Direct sowing: March–April (in mild regions)
Spring sets are ideal for beginners, while autumn sets offer an earlier harvest the following summer.
2. Choosing Between Onion Sets and Onion Seed
Onion sets:
- Small, immature bulbs grown the previous year
- Quick, easy, and reliable
- Less prone to pests during early growth
- Perfect for beginner gardeners
Onion seeds:
- Cheaper and more variety
- Take longer to mature
- Require more care at the seedling stage
- Best started indoors and transplanted later
If you’re new to onion growing, start with sets. Seeds are ideal if you want specific varieties or are growing large quantities.
3. Best Onion Varieties for UK Gardens
Here are some popular, reliable types for UK growing conditions:
| Variety | Type | Notes |
|---|---|---|
| Sturon | Spring set | Hardy, reliable, classic onion flavour |
| Centurion | Spring set | Early harvest, bolt-resistant |
| Red Baron | Spring set | Popular red onion, stores well |
| Radar | Autumn set | Hardy overwintering variety |
| Electric | Autumn set | Deep red colour, good winter performance |
| Ailsa Craig | Seed | Large, sweet onions, good for shows |
| Bedfordshire Champion | Seed | Traditional British favourite |
Choose a mix of red and white onions for variety in the kitchen.
4. Preparing the Soil for Planting Onions
Onions grow best in:
- Full sun
- Well-drained, light soil
- Neutral to slightly alkaline pH
Soil prep tips:
- Remove weeds and large stones
- Dig over soil to a depth of 20–25cm
- Mix in garden compost or well-rotted manure (a few weeks before planting)
- Rake to a fine tilth before planting
Avoid freshly manured beds — this can cause onions to rot. If your soil is heavy clay, consider using raised beds.
5. Spacing for Onion Sets and Seedlings
Give onions enough space for bulbs to swell properly.
| Method | Spacing (in row) | Between rows |
|---|---|---|
| From sets | 10–15cm | 25–30cm |
| From seed/seedlings | 10cm | 25cm |
Planting too close can lead to small or misshapen bulbs, while too wide wastes space.
6. How to Plant Onion Sets Outdoors
- Push each set into the soil with the tip just showing above the surface
- Space as per the guide above
- Firm gently — don’t bury them too deep
- Water lightly to settle the soil
Tip:
Birds sometimes pull up sets mistaking them for worms. Cover with fleece or netting until they establish.
7. How to Sow Onion Seeds
Indoors (January–February):
- Sow in trays or modules using seed compost
- Place in a cool, bright location (unheated greenhouse or windowsill)
- Harden off and transplant outdoors in March–April
Direct sow (March–April):
- Sow seeds thinly in drills 1cm deep
- Space rows 25cm apart
- Thin seedlings to 10cm once established
Sowing too thickly will lead to crowding — thinning is essential.
8. Watering, Feeding, and Weeding
Watering:
- Water in dry spells — especially in spring and early summer
- Avoid waterlogging or overwatering
Feeding:
- Add a general fertiliser (e.g. growmore) before planting
- Feed every 3–4 weeks with a high-potassium fertiliser (e.g. tomato feed) for stronger bulbs
Weeding:
- Keep beds weed-free — onions don’t compete well
- Weed by hand to avoid damaging shallow roots
Mulching with compost or straw can help suppress weeds and retain moisture.
9. Companion Planting for Onions
Good companions:
- Carrots (onions deter carrot fly)
- Lettuce
- Beets
- Cabbage family
Avoid planting onions near:
- Peas and beans (they don’t get along!)
- Sage (may stunt onion growth)
Rotate crops yearly to reduce disease risk and soil depletion.
10. Common Onion Problems and How to Prevent Them
| Problem | Cause | Solution |
|---|---|---|
| Bolting (flowering) | Cold weather or stress | Use bolt-resistant varieties; cover in spring |
| Onion fly | Eggs laid at base, larvae eat roots | Use fleece, plant in raised beds |
| White rot | Fungal disease in soil | Rotate crops, don’t replant in same spot |
| Mildew | Humid, overcrowded conditions | Space properly, ensure good airflow |
Healthy soil, good spacing, and crop rotation go a long way toward prevention.
11. When to Harvest Onions in the UK
Spring-planted onions:
- Harvest July–August when foliage yellows and falls over
Autumn-planted onions:
- Harvest June–July
How to harvest:
- Loosen soil gently with a fork
- Pull onions when tops flop and dry out
- Lay out in the sun or under cover to dry for 1–2 weeks
Proper drying is key to long storage.
12. How to Store Onions After Harvesting
Once dry:
- Trim off tops and roots
- Store in net bags, baskets, or tied bunches in a cool, dry, well-ventilated area
- Avoid plastic bags or containers — they trap moisture
Well-cured onions can store for 4–6 months. Use any damaged or soft onions first.
13. Growing Onions in Containers or Raised Beds
If space is limited, onions grow well in:
- Deep containers (20–30cm)
- Troughs or raised beds
Use:
- Peat-free multi-purpose compost mixed with garden soil or loam
- 10–15cm spacing
- Regular watering and feeding
Great for patios or balcony gardens, especially with compact varieties.
14. Can You Grow Onions All Year Round in the UK?
You can grow autumn onions that overwinter for an earlier crop next year, but onions don’t grow actively in winter.
Year-round growing tips:
- Plant autumn sets (e.g. ‘Radar’, ‘Electric’) in September
- Protect young shoots from frost with fleece
- Harvest in early summer
Avoid sowing or planting between November and February.
15. Using Homegrown Onions
Your onions are ready — now what?
Ideas:
- Roast whole with olive oil
- Add to soups, stews, and sauces
- Pickle smaller onions for chutneys
- Caramelise for burgers or quiche
- Store for use all winter
Homegrown onions have better flavour and texture than shop-bought — and store for months.
Conclusion: Start Growing Onions with Confidence
Planting onions in the UK is one of the easiest ways to get into food growing. Whether you use sets or seeds, in the ground or in containers, onions are low-maintenance and high reward.
With the right timing, good soil preparation, and consistent care, you’ll enjoy a strong harvest of delicious, versatile onions that last well into winter.
Top 10 FAQs: Planting Onions in the UK
1. When is the best time to plant onions in the UK?
Spring (March–April) or autumn (September–November) depending on variety.
2. Should I use sets or seeds?
Sets are easier for beginners; seeds offer more variety and are cheaper.
3. How far apart should I plant onion sets?
10–15cm between sets, with 25–30cm between rows.
4. Do onions need full sun?
Yes — plant in a sunny spot with good drainage.
5. Can I plant supermarket onions?
Not recommended — they may carry disease and are not bred for growing.
6. How long do onions take to grow?
Spring onions take 4–5 months; autumn onions grow over winter and are ready by June.
7. How do I know when onions are ready to harvest?
When the leaves turn yellow and fall over, and the bulbs are firm.
8. Can I grow onions in containers?
Yes — use deep pots, quality compost, and water regularly.
9. What should I do if onions bolt?
Harvest immediately and use — bolted onions won’t store well.
10. How long can I store onions?
When dried and stored correctly, up to 6 months.