🧅🌱 Sowing Onion Seeds in Polytunnels (UK Guide)

🌱 Introduction: Why Polytunnels Are Ideal for Onion Seeds

Polytunnels give you control—more light, shelter from wind and rain, and warmer daytime temperatures—making them an excellent place to start onion seeds in the UK. Used correctly, a polytunnel lets you sow earlier without stressing seedlings, producing stronger plants and better bulbs than many indoor setups.

This guide explains when to sow, where to place trays inside the tunnel, and how to avoid the common polytunnel mistakes that lead to leggy or checked growth.

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• Onion Seeds

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📅 When to Sow Onion Seeds in a Polytunnel (UK)

✅ Best sowing window

• Late January–February → ideal in most polytunnels
• Early March → very reliable
• January → only if the tunnel stays frost-free at night

Polytunnels allow earlier sowing than outdoors, but light still matters more than heat.

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🌡️ Temperature Management (The Polytunnel Balance)

During germination

• 15–20°C is ideal
• Daytime warmth is helpful

After germination (very important)

• 10–15°C produces the strongest seedlings
• Avoid constant warmth above 18°C

⚠️ Polytunnels can heat up fast on sunny days—ventilation is essential.

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☀️ Light Levels in Polytunnels

Light is one of the biggest advantages of a polytunnel.

• Clear covers give strong natural light
• Much brighter than most windowsills
• Even winter sun is effective on bright days

Place trays where they get maximum light, not tucked into shaded corners.

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🌱 Containers & Compost

Containers

• Seed trays or shallow modules
• Recycled containers work if they have drainage
• 8–10 cm depth is ideal

Compost

• Fine seed compost only
• Lightly moisten before sowing
• Never compact it

Avoid rich compost early—soft growth stores poorly later.

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🌱 How to Sow Onion Seeds in a Polytunnel

1️⃣ Sow Thinly

• Space seeds 1–2 cm apart
• Crowding causes weak growth in low winter light

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2️⃣ Sow Shallow

• Cover with 5–10 mm of compost or vermiculite
• Do not bury seeds deeply

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3️⃣ Water Gently

• Compost damp, not wet
• Cold, soggy compost is the main cause of failure

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4️⃣ Position Trays Correctly

• On benches or raised staging if possible
• Avoid placing directly on cold ground
• Keep away from draughts near doors

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🌿 Seedling Care Inside the Polytunnel

Once seedlings emerge:

• Vent on mild days to control heat
• Close up before temperatures drop
• Water sparingly but consistently
• Ensure good airflow

Healthy seedlings should be upright, compact, and dark green.

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✂️ Trimming Onion Seedlings (Still Important)

When seedlings reach 12–15 cm tall:

• Trim back to 8–10 cm
• Use clean scissors

Trimming:

• Thickens stems
• Prevents tangling
• Keeps growth steady

This is especially helpful if days are warm and nights are cool.

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🌤️ Frost Protection in Polytunnels

Even in a polytunnel:

• Severe frosts can damage seedlings

Simple protection:

• Horticultural fleece over trays
• Bubble wrap around staging (not touching plants)
• Move trays closer to the centre of the tunnel at night

Onions tolerate cool conditions—but not freezing compost.

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🌱 Potting On or Holding Back

Pot on only if:

• Seedlings are crowded
• Roots fill the tray
• Growth slows despite good light

Otherwise, polytunnel-grown onions often go straight from tray to ground.

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🌤️ Hardening Off (Usually Shorter)

Polytunnel-grown seedlings are already semi-hardened.

• Harden off 5–7 days
• Gradually increase exposure to outdoor wind and sun
• Avoid frost

This step is quicker than for indoor-grown plants.

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🌱 Planting Out

Plant out when:

• Seedlings are pencil-thick
• Soil is workable and warming

Spacing:

• 10–15 cm between plants
• 25–30 cm between rows

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🚫 Common Polytunnel Mistakes

• Letting trays overheat on sunny days
• Poor ventilation
• Overwatering in cool weather
• Placing trays on cold ground
• Forgetting frost protection at night

Polytunnels magnify mistakes—check conditions daily.

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🧠 Key Takeaway

Sowing onion seeds in a polytunnel gives you earlier sowing, stronger light, and better control—perfect for UK conditions. Grow seedlings bright and cool, ventilate well, and protect from frost, and you’ll raise compact plants that establish quickly outdoors.

Used properly, a polytunnel is one of the best environments for starting onions from seed.

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