How to Grow Onions from Sets and Seeds: A Complete Guide

Introduction

Onions are one of the easiest and most rewarding vegetables to grow. Whether you use them in stews, stir-fries, or sandwiches, having a supply of homegrown onions in your kitchen is both practical and satisfying. You can grow onions from either sets (immature bulbs) or seeds, and each method has its own advantages.

In this step-by-step guide, we’ll cover everything you need to know about how to grow onions from sets and seeds—ideal for beginners and seasoned gardeners alike. Learn the differences between the two methods, when and how to plant, how to care for your crop, and when to harvest and store your onions for year-round use.

Why Grow Onions at Home?

• 🧅 Cost-effective: One bag of sets or seeds can yield dozens of onions.
• 🌿 Easy to grow: Onions are hardy and low maintenance.
• 🍲 Tastes better: Homegrown onions are fresher and more flavourful.
• 🌍 Versatile: Use in endless culinary dishes or store long-term.

Onion Growing Methods: Sets vs Seeds

Growing Onions from Sets

• What are sets? Small, immature onion bulbs.
• Best for: Beginner gardeners.
• Pros: Easy to plant, reliable, quicker to harvest.
• Cons: Slightly more expensive, fewer variety options, prone to bolting in warm weather.

Growing Onions from Seeds

• What are they? True onion seeds started from scratch.
• Best for: Experienced gardeners or those wanting unique varieties.
• Pros: Greater variety, less chance of bolting, cheaper.
• Cons: Slower to grow, needs early sowing and more attention.

Best Onion Varieties to Grow

When to Plant Onions

From Sets:

• Spring planting: Mid-March to April
• Autumn planting (overwintering varieties): September to November

From Seeds:

• Indoor sowing: December to February
• Outdoor sowing: March to April

Note: Autumn-sown onions mature earlier in summer but may need protection from severe frosts.

How to Grow Onions from Sets

Step 1: Prepare the Soil

• Choose a sunny, open site with well-drained, fertile soil.
• Avoid heavy clay; improve with compost or sand.
• Remove stones and weeds.
• Add general-purpose fertiliser (e.g., Growmore) a week before planting.

Step 2: Plant the Sets

• Push each set into the soil with the tip just above the surface, about 10–15cm apart.
• Space rows 25–30cm apart.
• Water well after planting.

Top tip: Cover with mesh or fleece to protect from birds who may pull up the bulbs.

How to Grow Onions from Seeds

Step 1: Sow Seeds Indoors

• Sow from December to February in seed trays or modules filled with seed compost.
• Sow thinly and lightly cover with compost.
• Keep in a cool greenhouse or windowsill around 10–15°C.
• Once seedlings are large enough, thin to one per cell.

Step 2: Harden Off and Transplant

• After 6–8 weeks, harden off seedlings by gradually exposing them to outdoor conditions.
• Transplant to the garden in March or April, spacing plants 10cm apart in rows 25–30cm apart.

Ongoing Care and Maintenance

Watering:

• Water during dry spells.
• Avoid waterlogging, which can cause rot.

Weeding:

• Keep onion beds weed-free—onions don’t compete well.
• Weed by hand to avoid damaging shallow roots.

Feeding:

• Apply a high-potash liquid feed every few weeks during bulbing.

Mulching:

• Optional, but helps retain moisture and reduce weeds in dry summers.

Protecting Against Pests and Diseases

When and How to Harvest Onions

Harvesting Time:

• Spring-planted onions: July to August
• Autumn-planted (overwintered): June to July

Signs They’re Ready:

• Foliage starts to yellow and fall over.
• Bulbs swell above soil level.

How to Harvest:

• Loosen the soil with a fork.
• Gently lift onions and lay them on the surface to dry.
• Let them cure in the sun for 1–2 weeks or under cover if wet.

Storing Your Onions

How to Cure Onions:

• Let skins dry and necks wither.
• Remove loose soil, but don’t wash.

How to Store:

• Store in mesh bags, slatted trays, or hang in braided ropes.
• Keep in a cool, dry, well-ventilated area.
• Check regularly and remove any soft or sprouting onions.

Note: Red onions and shallots don’t store as long as brown varieties.

Growing Onions in Containers or Raised Beds

Perfect for small gardens or patios.

Tips:

• Choose a deep container (25–30cm) with good drainage.
• Use multi-purpose compost mixed with grit.
• Plant sets or seedlings with proper spacing.
• Water regularly and feed with liquid fertiliser during growth.

Successive Planting and Crop Rotation

• Plant a new batch every few weeks for extended harvest.
• Don’t grow onions in the same place year after year.
• Rotate with legumes or brassicas to reduce soil-borne diseases.

Onion Companion Planting

Good Companions:

• Carrots (repel onion flies)
• Beets
• Lettuce
• Tomatoes

Avoid Planting Near:

• Beans
• Peas
• Asparagus

Conclusion

Growing onions from sets or seeds is a rewarding process that suits both beginners and expert gardeners. With a little preparation, the right planting time, and consistent care, you’ll be rewarded with a generous harvest of flavour-packed onions ready to enjoy or store for months. Whether you’re growing in the ground or a container, from seed or set, follow this guide and enjoy fresh, homegrown onions all year long.

Top 10 Questions and Answers About Growing Onions

1. What’s the difference between onion sets and seeds?

Sets are small bulbs that grow faster and are easier to plant, while seeds offer more variety and are less prone to bolting.

2. When should I plant onion sets?

In early spring (March–April) or autumn (September–November) depending on the variety.

3. How long do onions take to grow?

From sets: 4–5 months. From seeds: 5–7 months depending on variety and growing conditions.

4. Do onions need full sun?

Yes, onions grow best in full sun with well-drained soil.

5. How deep should I plant onion sets?

Plant so that the tip of the bulb is just above the surface.

6. How do I stop onions from bolting?

Choose bolt-resistant varieties and avoid planting too early in spring.

7. Can I grow onions in containers?

Yes! Use deep containers with well-draining compost and ensure regular watering.

8. How do I know when onions are ready to harvest?

Leaves will yellow and flop over. Wait a week, then lift and cure them.

9. How long do harvested onions last?

If stored properly, cooking onions can last 3–6 months; shallots up to 9 months.

10. What can I do with a large onion harvest?

Store, pickle, freeze chopped onions, or use in bulk cooking and sauces.