🧅🏆 Sowing Onion Seeds for Exhibition Onions (UK Guide)

🌱 Introduction: Exhibition Onions Are Won Early

Exhibition onions aren’t about luck—they’re about precision from day one. The size, shape, and finish of a show onion are largely decided in the first 6–8 weeks after sowing. Perfect timing, steady growth, and zero stress are what separate a good onion from a prize-winner.

This guide explains exactly how to sow onion seeds for exhibition onions, with a focus on maximum bulb size, uniform shape, and show-quality finish in UK conditions.

⭐ Check Out Our Recommended Products

• Onion Seeds

Starting onions from seed at the correct time avoids common mistakes like sowing too late or relying only on sets. Ideal for early sowing under cover to get strong plants before spring planting.
Click here to see them

---

• Seed Trays & Module Pots

Essential for avoiding early planting failures caused by cold, wet soil. Seed trays allow you to start vegetables under cover and transplant at the right moment.
Click here to see them

---

• Seed Compost & Propagator

Using proper seed compost in a propagator gives reliable warmth and drainage, preventing poor germination, damping off, and weak seedlings—a common monthly mistake for beginners.
Click here to see them

---

🧭 What Makes an Onion “Exhibition Quality”?

Exhibition onions must have:

• Very large, uniform bulbs
• Perfect round shape
• Clean, unblemished skins
• Strong necks (not thick or soft)
• No bolting or stress marks

All of this starts before the plant ever goes outside.

---

📅 When to Sow Onion Seeds for Exhibition (UK)

✅ Ideal sowing window

• Late December–January → with grow lights
• Late January–early February → most common and safest

⚠️ Exhibition onions are the exception to the “don’t sow too early” rule—but only if you can provide strong light and control conditions.

Sowing this early without proper setup leads to failure.

---

💡 Light: Non-Negotiable for Show Onions

Exhibition onions must have intense, consistent light.

Required light levels:

• 14–16 hours per day
• Strong overhead light (grow lights essential)
• Lights positioned 5–8 cm above seedlings

Without this, seedlings will stretch, weaken, and never recover enough for exhibition size.

---

🌡️ Temperature Control (Critical)

Germination:

• 18–20°C until emergence

After germination:

• Reduce to 10–14°C
• Cool growth = thick stems + strong roots

Warm conditions after emergence ruin exhibition potential.

---

🌱 Best Compost for Exhibition Onion Seeds

At sowing:

• Fine seed compost only
• Free-draining, low nutrient

Why low nutrients matter:

• Prevents soft, lush growth
• Encourages strong root systems
• Produces denser bulbs later

Rich compost early = poor skin quality at harvest.

---

🌱 How to Sow Onion Seeds for Exhibition

1️⃣ Sow Individually

• One seed per cell or spaced 2–3 cm apart
• No crowding—competition reduces size potential

---

2️⃣ Sow Shallow

• Cover with 5 mm compost or vermiculite
• Never bury seeds deeply

---

3️⃣ Water With Precision

• Compost just damp
• Never saturated
• Cold, wet compost is disastrous at this stage

---

4️⃣ Position Under Lights Immediately

• Do not wait for germination to move under lights
• Light must be ready from day one

---

⏳ Germination Time

• 7–12 days in ideal conditions
• Do not disturb or resow early

Uniform germination is key for even development.

---

✂️ Trimming: Essential for Exhibition Size

Trimming is mandatory, not optional.

When:

• At 12–15 cm height
• Repeat regularly

How:

• Trim to 8–10 cm
• Use clean, sharp scissors

Why trimming matters:

• Thickens the stem base
• Prevents bending
• Encourages stronger leaf production
• Improves final bulb diameter

Well-trimmed seedlings outperform untrimmed ones every time.

---

🪴 Potting On (Done in Stages)

Exhibition onions require multiple potting stages.

Typical progression:

1. Seed tray or cells
2. Small pots (5–7 cm)
3. Larger pots (9–10 cm) before planting out

Never allow roots to become restricted—root checks permanently limit bulb size.

---

🌬️ Airflow & Hygiene

Disease or stress ruins show onions.

• Excellent airflow at all times
• Spotlessly clean containers
• Never reuse compost
• Remove weak or uneven plants early

Only keep the best-looking seedlings—be ruthless.

---

🌤️ Hardening Off (Slow and Careful)

Start hardening off earlier than normal, but gently.

• Begin in March
• Gradual exposure over 10–14 days
• Avoid cold winds and frost

Any shock at this stage increases bolting risk.

---

🌱 Planting Out for Exhibition Onions

Plant out when:

• Seedlings are pencil-thick
• Soil is warming
• Frost risk is minimal

Final spacing:

• 30–45 cm between plants
• Plenty of airflow and root room

Crowding kills exhibition potential.

---

🚫 Common Exhibition Onion Mistakes

• Sowing early without grow lights
• Keeping seedlings too warm
• Skipping trimming
• Allowing root restriction
• Overfeeding early
• Keeping weak plants “just in case”

Show onions reward discipline and consistency.

---

🧠 Key Takeaway

Exhibition onions are made by early sowing, intense light, cool steady growth, regular trimming, and zero stress. Every decision you make in the first few weeks affects final bulb size and quality.

If you treat exhibition onions like ordinary onions, you’ll grow ordinary onions.
If you grow them with precision, you give yourself a genuine chance at the show bench.

---