Onion Planting Guide | How to Grow Big, Flavourful Onions from Sets or Seed
Introduction: The Staple Crop for Every Garden
Onions are a kitchen essential — from stews and roasts to sandwiches and salads, they’re used in almost every meal. Growing your own onions not only saves money but also rewards you with fresher flavour, longer storage life, and the satisfaction of harvesting your own homegrown produce.
Whether you’re planting from sets or seed, this guide will walk you through everything you need to know about how to plant onions successfully, care for them, and harvest a bountiful crop.
1. Choosing Between Onion Sets and Onion Seed
There are two main ways to grow onions:
Onion Sets
- Small, immature bulbs grown the previous year
- Easier to handle, faster growing, and more reliable
- Less prone to poor germination
- Ideal for beginners or busy gardeners
Onion Seeds
- Cheaper and offer a wider range of varieties
- Take longer to mature
- Require earlier sowing and more attention at the seedling stage
- Useful for exhibition growers or organic seed lovers
Best choice?
Start with sets for simplicity. Switch to seeds if you want to try unique varieties or have more time.
2. When to Plant Onions (UK Guide)
Onion Sets
- Spring planting: March to April
- Autumn planting: September to November (for overwintering varieties)
Onion Seeds
- Sow indoors in January to February for spring planting
- Direct sow March to April in milder regions
Choose planting time based on variety and climate. Spring-planted onions typically mature in summer, while autumn sets are ready in early summer the following year.
3. Best Onion Varieties to Grow
Here are some reliable varieties for UK gardeners:
Spring-Planted Sets
- Sturon – Reliable, firm bulbs with good flavour
- Centurion – Early cropping and bolt-resistant
- Red Baron – Deep red colour and sweet taste
Autumn-Planted Sets
- Radar – Hardy overwintering white onion
- Electric – Vibrant red overwintering variety
From Seed
- Ailsa Craig – Large exhibition-style onions
- Bedfordshire Champion – Traditional favourite
- Hi Keeper – Great for long storage
Choose varieties based on flavour preference, bulb size, colour, and harvesting time.
4. How to Prepare the Soil
Onions thrive in:
- Full sun — choose a site with at least 6 hours of direct light
- Free-draining, fertile soil
- Soil that is light and friable — avoid heavy clay
Soil Prep Tips:
- Clear all weeds and rake soil to a fine tilth
- Add well-rotted compost or garden compost a few weeks before planting
- Avoid fresh manure — it can cause rot
- Add general-purpose fertiliser before planting, such as blood, fish & bone or growmore
Onions prefer soil with a pH between 6.0 and 7.0. If soil is too acidic, apply garden lime.
5. How to Plant Onion Sets
- Space sets 10–15cm apart in rows 25–30cm apart
- Push each set into the soil so the tip is just visible
- Firm the soil gently around the bulb
- Water lightly to settle soil
Birds sometimes pull out newly planted sets, mistaking them for worms — place mesh or fleece over the bed until shoots emerge.
6. How to Sow Onion Seeds
Indoors (January–February):
- Sow in modules or trays using seed compost
- Cover lightly and place in a cool, bright location
- Harden off seedlings in April and transplant outdoors
Direct sowing (March–April):
- Sow thinly in drills 1cm deep
- Space rows 25–30cm apart
- Thin seedlings to 10cm apart once they reach 10cm high
Onion seeds are slower to establish but give strong, uniform crops when well cared for.
7. Watering, Feeding & Weeding
Onions don’t need much water — but consistency is key.
Watering:
- Water during dry spells, especially during bulb swelling (May–June)
- Avoid waterlogging, which leads to rot
Feeding:
- Feed with a high-potassium fertiliser (e.g. tomato feed) every few weeks from late spring
Weeding:
- Keep beds weed-free — onions don’t compete well
- Weed by hand to avoid disturbing shallow roots
8. Common Onion Problems and How to Prevent Them
Bolting (premature flowering):
- Caused by cold stress or planting too early
- Use bolt-resistant varieties and don’t disturb plants
Onion Fly:
- Larvae feed on bulbs, causing plants to wilt
- Use fleece to protect seedlings and sets
White Rot:
- Fungal disease causing yellowing and rot
- Rotate crops and avoid replanting onions in the same spot for at least 3 years
Downy Mildew:
- Yellow patches and grey mould on leaves
- Improve airflow, avoid overhead watering, and don’t overcrowd
With good hygiene and care, most onion issues are easily managed.
9. When to Harvest Onions
Harvest time depends on the variety and planting date.
- Spring-planted onions: Ready July to August
- Autumn-planted onions: Ready June to July
Signs of readiness:
- Leaves turn yellow and flop over
- Neck softens and bulbs feel firm
Gently lift bulbs with a fork and let them dry (cure) in the sun for 1–2 weeks before storing.
10. How to Store Onions
Once dried, onions store well for months if kept properly.
Storage tips:
- Trim leaves and roots
- Store in a cool, dry, well-ventilated place
- Use net bags, mesh trays, or old tights
- Check regularly for soft or sprouting bulbs and remove any affected
Avoid plastic bags or containers — onions need airflow to stay firm and mould-free.
11. Companion Planting with Onions
Onions can be helpful neighbours in the garden.
Good companions:
- Carrots (onions deter carrot fly)
- Beetroot
- Lettuce
- Strawberries
Avoid planting near:
- Beans and peas (onions can stunt their growth)
Onions also benefit from being rotated each year to avoid soil-borne diseases.
12. Growing Onions in Containers
If space is limited, you can grow onions in large pots or deep trays.
Tips:
- Use a container at least 20–25cm deep
- Fill with quality compost mixed with grit for drainage
- Space sets or seedlings 10cm apart
- Water regularly and feed monthly
- Position in full sun
Great for patios, balconies, and small gardens.
Conclusion: A Reliable, Rewarding Crop for Every Garden
Onions are an excellent crop for gardeners of all skill levels. They’re low-maintenance, versatile, and store brilliantly — making them a true staple for kitchen gardens.
Whether you grow them from sets or seeds, in beds or containers, planting onions is simple and satisfying. With the right care and timing, you’ll be harvesting armfuls of fresh, flavoursome onions to enjoy all year round.
Top 10 FAQs: Onion Planting Guide
1. When is the best time to plant onion sets?
Spring (March–April) or autumn (September–November) depending on the variety.
2. How deep should I plant onion sets?
Push into soil so just the tip is visible — about 2–3cm deep.
3. How far apart should I space onions?
Space 10–15cm apart in rows 25–30cm apart.
4. Should I soak onion sets before planting?
Not necessary, but soaking for 1 hour can encourage quicker growth.
5. Why are my onions going to seed?
This is bolting — often due to cold stress. Use bolt-resistant varieties and plant at the right time.
6. Can I plant onions in containers?
Yes — use deep containers and well-drained compost, spacing 10cm apart.
7. How long do onions take to grow?
From sets: about 4–5 months. From seed: 5–6 months.
8. What causes my onions to rot in the ground?
Usually waterlogged soil or fungal infections like white rot.
9. Do onions need a lot of watering?
Not much — water during dry spells and when bulbs are swelling.
10. How do I cure and store harvested onions?
Lay them out in the sun or a dry place for 1–2 weeks, then store in a cool, dry, airy location.