Interactive lesson

Planning a polytunnel, step by step

Interactive lesson: decide before you buy the first hoop — what a polytunnel gives, which type, how big, where, what cover and what to ask.

Free sample Chapters 1 and 2 (8 steps) are free. The full lesson comes with the Polytunnel planning module. Already bought? Unlock

This lesson is general guidance, not professional training. Local regulations and expert advice always take precedence. Read our disclaimer.

The whole lesson as text

1. What a polytunnel gives

A polytunnel doesn't heat. It holds on to the sun's warmth and protects against wind, rain, hail and light frost.

Holds the sun — On a sunny March day a closed polytunnel reaches 30 °C (86 °F) while it is 8 °C (46 °F) outside.
A night without heating — On a clear night it is only 1–3 °C warmer inside than out, sometimes even colder. So in winter it gives lettuce, spinach and lamb's lettuce.
Two to three times more — Tomatoes yield 10 kg/m² under cover and 4 kg/m² outdoors. Cucumbers 10 against 3 kg/m².
What stays outside — Courgettes, potatoes, sweetcorn, beans and pumpkins don't go in the tunnel: they take up a lot of space and do well outdoors.

Why: Tomatoes under cover yield two to three times more than outdoors, and salad can be picked from March to December. But an unheated polytunnel won't give you tomatoes in January: it gives winter greens, and tomatoes from June to November.

In the cool summers of the UK and Ireland, and in upland areas, a polytunnel is close to essential for tomatoes, peppers and cucumbers.
A polytunnel doesn't replace the garden, it extends it: inside go only the crops that really need warmth.

2. Types

Not every homestead needs a big polytunnel straight away.

Fleece and low tunnel — Fleece protects against light frost and insects. A low tunnel 50–80 cm high gives early lettuce and carrots.
Small plastic greenhouse — A plastic greenhouse of 3 × 4 to 3 × 8 m makes a good first tunnel: seedlings and 10–20 tomato plants.
Permanent hooped polytunnel — Hoops of galvanised steel tube covered in polythene. Cheap per square metre, and the frame lasts 15–25 years.
Polycarbonate and a south wall — Polycarbonate holds heat best of the light types. A lean-to against a south wall also uses the heat stored in the wall.

Why: Many start with fleece and low tunnels and build a permanent polytunnel in the third or fourth year, when they know what they need. The type decides how long it lasts, what it costs and how it copes with winter.

A straight-sided tunnel has more usable space along the edges: you can grow tall tomatoes there.
A gothic (pointed) profile sheds snow better than a round hoop.