Last Updated on: December 14, 2025

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🧅🌱 Planting Onion Seeds: A Step-by-Step Guide for a Successful Harvest

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🌿 Introduction: Why Grow Onions From Seed?

Growing onions from seed takes a little more patience than sets, but it offers better variety choice, stronger plants, and improved storage quality. It’s also more economical and ideal for gardeners who want consistent, long-keeping onions.

Planting onion seeds:
✔ gives access to more varieties
✔ reduces the risk of bolting
✔ produces uniform bulbs
✔ works well for long-term storage onions

This guide walks you through exactly how to plant onion seeds, from sowing to harvest, with clear steps for success.

Below

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🧅🧠 1. When to Plant Onion Seeds (UK Guide)

Timing is key when growing onions from seed.

Indoors or under cover:

✔ January to February (maincrop onions)

Outdoors (mild areas):

✔ March to April

Earlier sowing gives onions more time to size up before bulbing begins.

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🌱🧺 2. Choosing the Right Onion Varieties

Select varieties based on use and storage.

✔ long-day onions for UK conditions
✔ maincrop varieties for storage
✔ salad onions for early harvest

Popular seed-grown onions include:
✔ ‘Sturon’
✔ ‘Bedfordshire Champion’
✔ ‘Ailsa Craig’

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🌱✋ 3. How to Sow Onion Seeds (Step-by-Step)

✔ fill trays or modules with fine compost
✔ gently firm the surface
✔ sow seeds thinly (about 1cm apart)
✔ cover lightly with compost or vermiculite
✔ water gently

Onion seeds don’t need deep sowing.

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🌡️🌱 4. Germination Conditions

✔ temperature: 10–20°C
✔ bright light once seedlings appear
✔ keep compost moist, not wet

Seeds usually germinate in 7–14 days.

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✂️🌱 5. Caring for Onion Seedlings

Onions grow slowly at first.

✔ keep in bright light to prevent legginess
✔ water sparingly but regularly
✔ trim tops to 5–7cm if they flop over

Trimming strengthens stems and roots.

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🌿🪴 6. Potting On or Thinning

Once seedlings are established:

✔ thin overcrowded seedlings
✔ or pot on small clumps
✔ handle gently by leaves, not stems

Strong spacing now leads to larger bulbs later.

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🌱🏡 7. Planting Out Onion Seedlings

✔ harden off gradually over 7–10 days
✔ plant out when seedlings are pencil-thick
✔ space 10–15cm apart
✔ rows 30cm apart

Plant firmly but don’t bury the neck.

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💧🌱 8. Watering and Feeding Onions

✔ water regularly during dry spells
✔ avoid waterlogging
✔ light feeding early on if soil is poor

Stop feeding once bulbs start swelling.

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🌞🧠 9. How Long Do Onions Take to Grow?

✔ 20–24 weeks from seed to harvest
✔ bulbing triggered by day length
✔ growth slows towards harvest

Patience is essential with seed-grown onions.

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🧅✂️ 10. Harvesting and Curing Onions

✔ harvest when tops fall over naturally
✔ lift gently and allow to dry
✔ cure in a warm, airy place

Proper curing improves storage life dramatically.

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⚠️❌ 11. Common Mistakes When Planting Onion Seeds

❌ sowing too late
❌ overcrowding seedlings
❌ planting too deeply
❌ overwatering
❌ poor hardening off

Most problems come from lack of light or space.

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🌟 FAQs

Are onions better grown from seed or sets?

Seeds give better choice and storage; sets are quicker.

Can I grow onions from seed in containers?

Yes — use deep containers and good spacing.

Why are my onions small?

Late sowing or overcrowding is usually the cause.

Do onion seedlings need trimming?

Yes — trimming helps prevent flopping and strengthens growth.

Can I overwinter onions from seed?

Some varieties can, but spring sowing is safer for beginners.

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