Overwintering Onions: How to Grow Onions Successfully Through Winter

Introduction

Overwintering onions is a great way to get an early harvest in spring, giving you stronger, healthier onions before the summer heat arrives. Planting onions in autumn allows them to establish roots before winter, go dormant, and then resume growth as temperatures rise.

But how do you successfully grow onions over winter, and which varieties work best for overwintering?

In this comprehensive guide, we’ll cover:
✅ The best onion varieties for overwintering
✅ How to plant and care for onions through winter
✅ How to protect onions from frost and pests
✅ Common mistakes to avoid
✅ Expert tips for a successful early harvest

Why Overwinter Onions?

Onions planted in autumn develop stronger roots and mature earlier than spring-planted onions. Overwintering onions allows you to:
🌱 Get an earlier harvest – Ready to pick by May or June.
💪 Grow stronger plants – Overwintered onions handle drought and pests better.
🌍 Maximize garden space – Harvest early, leaving room for summer crops.

💡 Tip: Overwintering onions is best for mild to moderate winters, but with protection, they can survive colder climates too.

Best Onion Varieties for Overwintering

Some onions handle winter conditions better than others. The best overwintering onions include:

💡 Tip: If you live in a cold climate (-5°C or lower), choose very hardy varieties like Radar or Senshyu Yellow.

How to Plant Onions for Overwintering

1. When to Plant Overwintering Onions

✅ Best time: September – November (before the first hard frost).
✅ Soil temperature: Above 7°C (45°F) for proper root development.
✅ Planting too late? If the ground is frozen, wait for a warm spell or use raised beds.

2. Choosing the Right Planting Site

✅ Full sun – Onions need at least 6 hours of sunlight per day.
✅ Well-drained soil – Avoid waterlogged areas to prevent rot.
✅ Raised beds or containers – Improve drainage in wet, clay-heavy soil.

💡 Tip: Adding compost or well-rotted manure before planting boosts soil fertility.

3. How to Plant Onions for Overwintering

✅ Step 1: Space onion sets 10 cm (4 inches) apart in rows.
✅ Step 2: Plant just deep enough to cover the top with soil.
✅ Step 3: Leave 30 cm (12 inches) between rows for air circulation.
✅ Step 4: Water lightly to help roots establish before winter.

💡 Tip: Don’t bury onions too deep—the tops should be slightly visible above the soil.

How to Protect Overwintering Onions from Frost & Pests

1. Mulching for Winter Protection

✅ Apply straw, shredded leaves, or compost around onions.
✅ Mulch 5 cm – 10 cm (2 – 4 inches) thick for insulation against frost.

💡 Tip: Remove excess mulch in early spring to allow onions to grow faster.

2. Protecting Onions from Heavy Rain & Snow

✅ Use fleece or row covers in harsh winters (-5°C or colder).
✅ Raised beds help prevent waterlogging from heavy rain.

💡 Tip: If snow is forecast, cover young onions with fleece to prevent freezing damage.

3. Preventing Pests & Disease

✅ Use netting or fleece to protect against birds pulling up young sets.
✅ Rotate crops yearly to reduce onion diseases like white rot.
✅ Avoid overcrowding—good air circulation prevents mold and fungal issues.

💡 Tip: Companion planting with garlic or carrots helps deter onion flies.

Spring Care & When to Harvest Overwintered Onions

1. Encouraging Growth in Early Spring

✅ Remove excess mulch when temperatures rise above 5°C (41°F).
✅ Start watering regularly in March – April as growth picks up.
✅ Feed onions with a nitrogen-rich fertilizer to boost growth.

💡 Tip: Bolting (flowering too early) happens if temperatures fluctuate too much—remove flower stalks to save the bulb.

2. Harvesting Overwintered Onions

✅ Onions are ready when the tops yellow and fall over (usually May – June).
✅ Loosen soil and pull onions out gently.
✅ Cure onions by drying them in a warm, airy place for 7-10 days.

💡 Tip: Store onions in a cool, dry place with good ventilation to keep them fresh for months.

Common Mistakes to Avoid When Overwintering Onions

🚫 Planting too late – Onion sets need time to establish roots before winter.
🚫 Burying sets too deep – They may rot in wet conditions.
🚫 Not mulching in cold climates – Without insulation, onions may freeze and die.
🚫 Waterlogging – Use raised beds or improve drainage in wet areas.
🚫 Harvesting too early – Let tops fall over naturally before pulling onions.

💡 Tip: If your onions don’t reach full size, leave them in the ground for another few weeks.

Conclusion

Overwintering onions is an easy and rewarding way to get an early harvest and stronger, healthier onions.

By choosing the right variety, planting at the right time, and using proper winter protection, you’ll enjoy large, flavorful onions ready for harvest months before spring-planted varieties.

Whether you’re growing in beds, containers, or raised beds, following these steps will help your onions thrive through winter and into spring!