How Much Frost Protection Does a Polytunnel Actually Give You?

A polytunnel transforms the range of crops you can grow and the length of your season. Unlike a cold frame or cloche, it gives you a large, walk-in growing space that stays significantly warmer than outdoors year-round — but understanding its limits prevents costly crop losses on exceptionally cold nights.

Temperature Gains in a Polytunnel

On a typical cold night, a standard single-skin polytunnel with closed doors stays 2–4 °C warmer than outside. On a still, clear night that is usually enough to prevent frost when outside temperatures hover around −2 to −3 °C. But on a hard frost night of −6 °C or below, the inside of an unheated polytunnel can still drop below 0 °C. A polytunnel keeps out most frosts but is not frost-proof without additional measures.

During the day, the reverse applies with dramatic effect. In spring sunshine a polytunnel can reach 35–45 °C with doors closed, which is why ventilation management is a daily task.

Boosting Frost Protection Inside a Polytunnel

To protect the most tender crops on very cold nights, add a layer inside. A double-skin bubble polythene inner curtain (hung from the hoops) creates an air gap that significantly reduces heat loss. Alternatively, drape fleece directly over plant rows inside the tunnel. Water thermal mass also helps: filling black-painted drums with water and positioning them inside the tunnel means they absorb daytime heat and release it slowly overnight, raising minimum temperatures by 1–2 °C.

Managing the Polytunnel Door and Ventilation

Open both doors on any day above 15 °C to prevent overheating and disease. Secure mesh over door openings prevents pest entry while allowing airflow. In autumn and winter, close doors each evening before temperatures fall. In spring, keep doors open progressively longer as outside temperatures rise — this is also how you gradually harden off plants that have been growing inside through winter.

Year-Round Cropping in a Polytunnel

In a UK climate, a polytunnel allows tomatoes, cucumbers, and peppers to be planted in April rather than late May. Autumn tomato harvests extend to November rather than October. Winter salads — land cress, spinach, mizuna, claytonia — grow steadily through December to February when outside crops are frozen solid. A well-managed polytunnel means you rarely go without homegrown salad at any point in the year.

The Investment Case

A basic 6 m × 3 m polytunnel costs significantly less than a glazed greenhouse of the same size, and polythene covers last 5–8 years before needing replacement. Factor in the value of crops you can grow — strawberries two weeks early, tomatoes into November, year-round salad — and most polytunnels pay for themselves within two to three growing seasons.