🧅🌱 Sowing Onion Seeds: How Many Seeds Per Cell? (UK Guide)

🌱 Introduction: The Right Number of Seeds Makes All the Difference

How many onion seeds you sow per cell directly affects seedling strength, bulb size, and workload later. Too many seeds cause crowding, weak growth, and extra thinning. Too few can waste space or leave gaps if germination is poor.

This guide explains exactly how many onion seeds to sow per cell, based on your goal—storage onions, general crops, or exhibition onions—with clear UK-specific advice.

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✅ The Simple Answer (Quick Reference)

• 1 seed per cell → best for strong, large onions
• 2 seeds per cell → acceptable if you’ll thin later
• 3+ seeds per cell → not recommended (except for spring onions)

For most gardeners, 1 seed per cell is ideal.

🌱 Why Seed Number Matters for Onions

Onions are sensitive to early competition. When crowded, they:

• Grow thinner stems
• Develop weaker roots
• Produce smaller bulbs
• Are more likely to bolt or stall

Unlike some crops, onions don’t benefit from growing in clumps when grown for bulbs.

🧅 Best Seed Counts by Growing Goal

🏆 Exhibition Onions

• 1 seed per cell (essential)
• Any competition reduces maximum bulb size
• Weak or uneven seedlings should be discarded early

👉 Exhibition onions demand zero competition from day one.

📦 Storage & Maincrop Onions

• 1 seed per cell → best results
• 2 seeds per cell → thin to the strongest seedling

This gives you insurance against poor germination without long-term crowding.

🌿 Spring Onions

• 2–5 seeds per cell is fine
• Spring onions are harvested young
• Clumping is acceptable and space-efficient

This is the only time multiple seeds per cell makes sense.

🪴 What Cell Size Makes a Difference?

Small cells (2–3 cm wide)

• Stick to 1 seed
• Crowding happens fast

Medium cells (4–5 cm wide)

• 1 seed ideal
• 2 seeds maximum, then thin

Deep modules

• Still best with 1 seed
• Depth helps roots, not competition

Cell size doesn’t change the rule—competition always limits onions.

🌱 What If You Sow More Than One Seed?

If you sow 2 seeds per cell:

• Wait until both germinate
• Keep the strongest, upright seedling
• Snip the weaker one at soil level (don’t pull it)

Never let two onions grow together in one cell long-term if growing for bulbs.

🌱 Sowing in Trays Instead of Cells

If you’re sowing in trays:

• Space seeds 1–2 cm apart
• Avoid scattering thickly
• Thin early if crowded

The principle is the same: each onion needs its own space.

🚫 Common Mistakes with Seed Numbers

• “Just in case” sowing too many seeds
• Leaving multiple seedlings per cell too long
• Pulling seedlings instead of snipping
• Assuming onions grow well in clumps (they don’t)

Overcrowding early permanently limits final bulb size.

🧠 Key Takeaway

For onion seeds:

• 1 seed per cell = strongest plants and biggest bulbs
• 2 seeds per cell = thin early
• More than 2 = problems later

If you want reliable, high-quality onions in UK conditions, give each seed its own space from the start. Onions reward room, patience, and steady growth.