When to Plant Out Leeks in the UK: The Perfect Timing for a Successful Crop

Introduction

Leeks are a hardy, easy-to-grow vegetable loved by UK gardeners for their resilience, flavour, and long harvesting season. But to get the best results, you need to know exactly when to plant out your leek seedlings — the stage when they move from trays to open ground.

Timing is everything. Plant them too early and they may suffer from cold or poor root establishment. Too late, and they might not grow thick enough before autumn arrives.

In this article, we’ll explain when to plant out leeks in the UK, how to tell when seedlings are ready, and how to prepare your soil and site for the best possible results.

Quick Answer: When to Plant Out Leeks in the UK

• Best time to transplant leek seedlings:
  ✅ Late April to early July

This window gives the seedlings time to establish, grow strong, and develop thick stems before harvest time in autumn or winter.

Understanding the Leek Growing Timeline

This means leek growing is a long-term crop, with a steady progression through the year.

Signs Your Leek Seedlings Are Ready to Plant Out

You’ll know your leeks are ready to transplant when:

✅ They are 15–20cm tall (6–8 inches)
✅ Stems are roughly pencil thickness
✅ They have been hardened off (acclimatised to outdoor conditions)
✅ Soil is warm, workable, and frost-free

This usually happens 8–10 weeks after sowing, so if you sowed in February, you’ll likely be planting out in late April to May.

Factors That Affect When to Plant Out Leeks

🌡️ 1. Soil Temperature

Leeks prefer soil that’s above 10°C (50°F). Cold, wet soil will shock seedlings or stunt growth.

📍 2. Region of the UK

• Southern UK: You may be able to plant out by mid-late April
• Northern UK or Scotland: Best to wait until mid-May or later to avoid cold snaps

☁️ 3. Weather Conditions

• Choose a mild, overcast day to plant out
• Avoid hot, dry days or risk transplant shock
• Ensure soil is moist but not waterlogged

How to Harden Off Leek Seedlings Before Transplanting

Hardening off helps plants adjust to outdoor conditions.

How to Do It:

• Move trays outside during the day for 7–10 days before transplanting
• Bring them indoors at night for the first few days
• Gradually increase time outside
• Protect from wind, heavy rain, or frost

This step is crucial for strong, stress-resistant plants.

How to Plant Out Leeks (Recap of the Dibbing Method)

Once the timing is right, here’s how to plant out leeks for the best results.

Step-by-Step:

1. Use a dibber or stick to make holes 15–20cm deep
2. Space leeks 15cm apart, with 30cm between rows
3. Drop one leek seedling into each hole
4. Do not backfill with soil — instead, water into each hole
5. As the leeks grow, soil will gradually fill the hole, blanching the stem

Best Soil and Site for Planting Out Leeks

✅ Full sun or partial shade
✅ Well-drained, fertile soil
✅ Add compost or aged manure months before planting
✅ Avoid freshly manured beds — it may cause misshapen stems

Rake beds level before planting and remove weeds thoroughly.

Companion Plants for Leeks

Leeks grow well with:

• Carrots – mutual pest protection (leek moth vs. carrot fly)
• Beetroot – doesn’t compete for nutrients
• Lettuce – quick crop between rows
• Brassicas – similar water and nutrient needs

Avoid growing leeks with peas or beans, which dislike the allium family.

Seasonal Care After Planting Out

When Will My Planted Leeks Be Ready to Harvest?

Some varieties are bred for early harvest, others for overwintering. Always check your seed packet for guidance.

Common Mistakes When Planting Out Leeks

Timing and technique make all the difference in leek success.

Can You Plant Out Shop-Bought or Regrown Leeks?

Some people grow leeks from supermarket root ends. While fun and educational, they:

• May bolt early or fail to bulk up
• Aren’t as productive as seed-grown leeks
• Can still be used in containers or small beds for salad onions

What to Do If You Missed the Planting Window

If you’re past July:

• You may still sow quick-maturing leek varieties in modules
• Use baby leeks for soups or stir-fries
• Consider autumn sowing of overwintering types (e.g. Bandit) for next year’s crop

Conclusion

Late April to early July is the prime window for planting out leek seedlings in the UK. By waiting until they’re pencil-thick, the soil is warm, and the weather is stable, you’ll give your leeks the best possible start.

Use the dibbing method, ensure proper spacing, and care for them through summer for a rich harvest in autumn and winter. Whether you’re filling raised beds or allotment rows, leeks are an essential crop for cold-season self-sufficiency — and they all start with good timing.

Top 10 Questions and Answers About When to Plant Out Leeks

1. When should I plant out leeks in the UK?

Between late April and early July, depending on weather and seedling size.

2. How big should leek seedlings be before planting out?

15–20cm tall and about pencil thickness.

3. Can I plant out leeks in March?

It’s usually too early — soil is still cold. Wait until late April unless using cloches.

4. What happens if I plant leeks out too late?

They may stay small or not mature fully before frost arrives.

5. Do I need to harden off leeks before planting out?

Yes — harden off for 7–10 days to prevent transplant shock.

6. Can I transplant leeks straight from a seed tray?

Yes, once hardened off and pencil-thick.

7. Is it OK to plant out during a heatwave?

Avoid hot days — leeks transplant best on cool, cloudy days.

8. Should I water after planting leeks out?

Yes — water directly into the hole to settle the soil around roots.

9. Can I grow leeks year-round?

You can grow and harvest leeks most of the year by choosing the right varieties.

10. What if my leeks bolt after planting out?

Bolting often happens due to stress — harvest early and use the stems or flowers.