🧅🌱 Sowing Onion Seeds: Soil Temperature Guide (UK)

🌱 Introduction: Soil Temperature Decides Success

When sowing onion seeds, soil (compost) temperature matters more than air temperature. Seeds respond to the warmth around them—not the room, windowsill, or greenhouse air. Many sowing failures happen because compost is too cold or too wet, even when everything else seems right.

This guide explains the ideal soil temperatures for onion seed germination and growth, what happens when it’s too cold or too warm, and how to manage temperature in typical UK conditions.

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🌡️ Ideal Soil Temperatures for Onion Seeds

✅ Germination (Most Important Stage)

• Ideal: 15–20°C
• Acceptable: 10–14°C (slower germination)
• Poor: Below 10°C
• Risky: Above 22°C

At the right temperature, onion seeds usually germinate in 7–14 days.

🌿 After Germination (For Strong Seedlings)

• Ideal: 10–15°C
• Too warm: Above 18°C (causes weak, leggy growth)

Cool compost after germination produces thicker stems and stronger roots, which leads to better bulbs later.

❄️ What Happens If Soil Is Too Cold?

Cold compost causes:

• Very slow or failed germination
• Seeds rotting before sprouting
• Patchy, uneven emergence
• Weak early root growth

Below 10°C, onion seeds often sit dormant or rot—especially if compost stays wet.

👉 Cold + wet is the most common cause of onion seed failure in the UK.

🔥 What Happens If Soil Is Too Warm?

Warm compost (often indoors) causes:

• Fast but weak germination
• Thin, floppy seedlings
• Poor root development
• Higher stress later when planted out

Warmth without enough light leads to leggy seedlings, which never fully recover.

🪟 Indoor Soil Temperature vs Outdoor Conditions

Indoors

• Compost can be much warmer than you think
• Windowsills above radiators are often too warm
• Trays dry faster but roots grow weaker

Outdoors / Polytunnel

• Compost warms slowly
• Night temperatures matter
• Raised trays warm faster than those on cold ground

Always think about compost temperature, not just room temperature.

🌱 Soil Temperature by Sowing Method (UK)

🌡️ How to Control Soil Temperature

To warm compost slightly:

• Move trays to a milder room for germination
• Place trays on insulation (wood, polystyrene)
• Avoid cold floors or concrete

To cool compost after germination:

• Move to a cooler, brighter location
• Avoid radiators and constant heat
• Vent greenhouses and polytunnels on mild days

Onions want warmth to start, coolness to grow.

🌱 Do You Need a Soil Thermometer?

Not essential—but useful.

A simple soil thermometer helps you:

• Avoid sowing into cold compost
• Understand why germination is slow
• Prevent overwatering in cold conditions

If compost feels cold to the touch, it’s probably too cold for fast germination.

🚫 Common Soil Temperature Mistakes

• Sowing into cold compost in January
• Keeping seedlings warm for too long
• Using heat mats without light
• Placing trays directly on cold floors
• Overwatering cold compost

Most problems come from temperature + moisture, not seed quality.

🧠 Key Takeaway

For onion seeds, soil temperature controls everything. Aim for 15–20°C to germinate, then 10–15°C to grow on. Too cold leads to failure; too warm causes weak growth.

If you manage compost temperature properly, onion seeds become one of the most reliable crops to raise in the UK.