🌱🏡 When to Plant Vegetables in a Polytunnel (UK Guide)

🌱🏡 Introduction: Extending the Season the Smart Way

A polytunnel is one of the most powerful tools for UK vegetable growing. It warms soil earlier in the year, protects crops from wind and heavy rain, and extends harvests well into autumn and winter. However, knowing when to plant vegetables in a polytunnel is just as important as knowing what to plant.

This guide explains exactly when to sow and plant vegetables in a polytunnel throughout the year, with realistic UK timings and practical advice.

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🌡️ Why Polytunnels Change Planting Times

A polytunnel typically allows you to:

✔ Sow 4–8 weeks earlier than outdoors
✔ Protect crops from late frost
✔ Grow tender vegetables reliably
✔ Continue harvesting much later in the year
✔ Control moisture better than outdoors

However, temperatures can fluctuate fast, so timing and ventilation matter.

📅 Polytunnel Planting Calendar (UK)

❄️ January – Early Starts (Protected & Hardy Crops)

Suitable for heated polytunnels or mild areas.

✔ Chillies
✔ Sweet peppers
✔ Aubergines
✔ Onions from seed
✔ Leeks

Conditions needed:

• Minimum 15–18°C for fruiting crops
• Good light (grow lights help)

🌱 February – Early Sowing Begins

✔ Tomatoes (early or greenhouse types)
✔ Lettuce
✔ Spinach
✔ Spring onions
✔ Early brassicas

Unheated polytunnels may still need fleece overnight.

🌸 March – Main Early Polytunnel Sowing

✔ Tomatoes
✔ Cucumbers (polytunnel types)
✔ Courgettes
✔ Squash
✔ Sweetcorn
✔ Salad leaves
✔ Herbs

Ventilate well on sunny days to avoid overheating.

🌼 April – Strong Growth Phase

✔ More cucumbers and courgettes
✔ Beans (early starts)
✔ Basil
✔ Succession salads

Begin hardening off plants that will go outdoors in May.

☀️ May – Summer Crops Move In

✔ Plant tomatoes, peppers, cucumbers into final tunnel positions
✔ Continue sowing salads and herbs
✔ Start autumn brassicas

Frost risk is usually low, but cold nights can still occur.

🌻 June – August – Peak Polytunnel Production

✔ Tomatoes, peppers, cucumbers cropping heavily
✔ Basil, parsley, coriander
✔ Continuous lettuce and salad sowings

Focus on watering, feeding, and harvesting, not early sowing.

🍂 September – Autumn Polytunnel Sowing

✔ Spinach
✔ Winter lettuce
✔ Pak choi
✔ Mizuna
✔ Spring onions

Polytunnels shine at autumn and early winter production.

❄️ October – November – Overwintering Crops

✔ Spinach
✔ Winter salads
✔ Spring onions
✔ Garlic (beds or containers)

Growth slows, but crops remain harvestable.

🥬 What Vegetables Grow Best in Polytunnels?

🌶️ Heat-Loving Crops (Biggest Winners)

✔ Tomatoes
✔ Chillies
✔ Peppers
✔ Aubergines
✔ Cucumbers

🥬 Season-Extension Crops

✔ Lettuce
✔ Spinach
✔ Rocket
✔ Pak choi
✔ Spring onions

Polytunnels are ideal for both summer crops and winter harvesting.

🌱 When to Move Polytunnel Plants Outdoors

Approximate UK timings:

• Southern UK: late April–early May
• Midlands & Northern England: mid–late May
• Scotland / exposed areas: late May–June

Always harden off and check night temperatures.

🌬️ Ventilation & Temperature Tips

✔ Open doors and vents daily in spring and summer
✔ Close in the evening to retain warmth
✔ Use shade netting during heatwaves
✔ Water in the morning
✔ Avoid still, humid air

Poor airflow causes problems faster than cold.

🚫 Common Polytunnel Planting Mistakes

❌ Sowing too early without warmth or light
❌ Overcrowding plants
❌ Forgetting ventilation
❌ Overwatering in cool weather
❌ Treating a polytunnel like open ground

A polytunnel magnifies both success and mistakes.

❓ FAQs

Can I plant vegetables in a polytunnel all year?

Yes — with planning, something can be growing or harvested every month.

Is a polytunnel better than a greenhouse?

Polytunnels offer more space; greenhouses offer better structure. Both work well.

Do vegetables grow faster in a polytunnel?

Yes — warmer soil and shelter speed up growth significantly.

Do I still need fleece inside a polytunnel?

Sometimes — especially in winter or during late frosts.

Are polytunnels good for beginners?

Absolutely — they make growing far more reliable.