How to Plant Autumn Onions: Step-by-Step Guide for UK Gardeners

Introduction

Onions are a staple in most kitchens, and growing your own is one of the easiest ways to ensure a steady supply of this versatile vegetable. While many gardeners think of onions as a spring crop, planting onions in autumn is an excellent way to get ahead of the game. By sowing or planting onion sets in autumn, you give them a head start over winter, allowing them to establish strong roots and grow into larger, healthier bulbs the following summer.

This guide provides a step-by-step method for planting autumn onions in the UK, along with tips on soil preparation, spacing, care, and harvesting. Whether you’re a beginner or an experienced gardener, these instructions will help you enjoy a bumper onion harvest.

Why Plant Onions in Autumn?

• Earlier harvest: Autumn-planted onions are ready up to 4 weeks earlier than spring onions.
• Larger bulbs: They have a longer growing season, leading to bigger bulbs.
• Efficient use of space: Grow over winter when beds would otherwise be empty.
• Low maintenance: Onions are hardy and require little attention through winter.

Best Onion Varieties for Autumn Planting in the UK

Not all onions are suitable for autumn planting — you’ll need hardy varieties that can withstand cold weather. Popular choices include:

• Radar – A reliable, cold-hardy variety with good flavour.
• Electric – Produces beautiful red onions.
• Shakespeare – Excellent for overwintering with strong disease resistance.
• Senshyu Yellow – A Japanese variety known for hardiness.

Step 1: Prepare the Soil

Onions grow best in fertile, well-drained soil.

• Choose a sunny spot: Onions need full sun for strong growth.
• Improve drainage: If your soil is heavy clay, consider raised beds or add grit.
• Add compost: Dig in well-rotted compost to enrich the soil.
• Check pH: Aim for neutral to slightly alkaline soil (pH 6.5–7.5). If soil is too acidic, add garden lime.

Step 2: Choose Onion Sets

Onion sets (small immature bulbs) are the easiest and most reliable way to grow autumn onions.

• Buy from reputable suppliers to avoid disease.
• Avoid supermarket onions, which may bolt or carry diseases.
• Select firm, healthy sets with no signs of rot.

Step 3: Planting Onion Sets

When to Plant

• Plant onion sets in September–November before heavy frosts set in.
• In milder areas, you can plant right up to late November.

How to Plant

1. Rake soil to a fine, crumbly texture.
2. Make shallow holes or a trench about 2–3cm deep.
3. Place sets with the pointed tip up and flat base down.
4. Space sets 10cm apart in rows, with 25–30cm between rows.
5. Cover lightly with soil and firm gently.
6. Water in lightly to settle the soil.

Step 4: Protecting Autumn Onions

• Birds: Birds may pull sets out of the ground. → Cover with fleece or netting until they root.
• Frost: Hardy onion varieties survive frost, but in very exposed sites, fleece can add protection.
• Weeds: Onions dislike competition. Keep beds weed-free by hand weeding or mulching.

Step 5: Caring for Onions Over Winter

• Minimal care is needed through winter.
• Check occasionally to re-firm any sets lifted by frost.
• Avoid overwatering — rainfall is usually sufficient.

Step 6: Spring and Summer Care

As temperatures rise, onion growth will accelerate.

• Watering: Water during dry spells in spring and summer, keeping soil evenly moist.
• Feeding: Apply a nitrogen-rich fertiliser in early spring, then switch to high-potassium feed (like tomato fertiliser) in late spring to boost bulb swelling.
• Weeding: Keep beds free of weeds to prevent competition.

Step 7: Harvesting Autumn Onions

• Onions planted in autumn are usually ready to harvest in June–July.
• Signs onions are ready: leaves yellow and flop over.
• Stop watering 2 weeks before harvest.
• Lift onions gently with a fork and leave to dry on the surface if the weather is dry.

Step 8: Curing and Storing Onions

• Cure onions by leaving them in a dry, airy spot for 2–3 weeks.
• Once skins are papery, trim roots and leaves.
• Store in mesh bags, netting, or braids in a cool, dry, ventilated space.
• Autumn onions store for around 3–5 months (less than spring onions).

Common Mistakes When Planting Autumn Onions

• Planting sets too shallow → birds pull them up.
• Planting in waterlogged soil → bulbs rot.
• Planting too late → bulbs don’t establish before winter.
• Overfeeding with nitrogen → large leaves, small bulbs.

Conclusion

Planting onions in autumn is a simple way to secure an early, plentiful harvest the following summer. By preparing the soil well, choosing hardy varieties, spacing correctly, and caring for your crop through the seasons, you’ll enjoy large, tasty onions long before your spring-planted sets are ready.

With minimal effort over winter, autumn onions are one of the easiest and most rewarding crops for UK gardeners.

Top 10 Questions About Planting Autumn Onions

1. What month do you plant autumn onions in the UK?
September–November is the ideal window.

2. What onions are best for autumn planting?
Varieties like Radar, Electric, Shakespeare, and Senshyu Yellow.

3. Can autumn onions survive frost?
Yes, hardy varieties are designed to withstand UK winters.

4. Do autumn onions need netting?
Yes, to protect from birds pulling up sets.

5. How deep do you plant onion sets?
2–3cm deep, just covered with soil.

6. How far apart should onion sets be planted?
10cm between sets, 25–30cm between rows.

7. Do autumn onions need watering in winter?
No, rainfall is usually enough.

8. When are autumn onions ready to harvest?
Typically June–July.

9. How long do autumn onions store for?
Around 3–5 months.

10. Can I plant autumn onions in pots?
Yes, but ensure pots are at least 25–30cm deep with good drainage.