Interactive lesson

The winter tunnel, step by step

Interactive lesson: a tunnel kept above freezing without electricity — inner tunnel, insulation, water, compost, carbon monoxide safety and snow.

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The whole lesson as text

1. What a winter tunnel can do

A winter tunnel is not a warm tunnel: in January it grows greens, not tomatoes.

One film is not much — A tunnel with a single film is only 1–3 °C warmer than outside at night. In a cloudy week there is nothing to give back.
Lamb's lettuce and spinach — Under cover lamb's lettuce tolerates roughly −15 °C (5 °F), and spinach −10 to −12 °C. Lamb's lettuce is the safest choice.
Kale, onions and lettuce — Kale tolerates −10 °C and lower, spring onions −10 °C. Winter lettuce −5 to −8 °C, and only winter varieties.
Don't touch it frozen — Greens that freeze overnight usually recover when they thaw slowly. So don't touch them while they are frozen.

Why: A tunnel doesn't produce heat; it holds part of the sun's heat and gives it back at night. Greens that tolerate frost don't need 15 °C, only protection from the worst nights, wind and snow.

On a clear, still night the tunnel can be colder than the air outside (inversion).
A plant that gets used to the cold slowly tolerates more than one exposed suddenly.

2. A tunnel inside a tunnel

The strongest tool of a winter tunnel is a second cover directly over the plants.

Hoops over the bed — Put hoops of 5–6 mm wire over the bed, 40–50 cm high.
Fleece over the hoops — Lay fleece of 30–50 g/m² or film over the hoops.
Cover while it's warm — Cover at 3–4 pm, while it is still warm. Uncover in the morning when the sun has warmed the tunnel.
Edges under boards — Weight the edges with boards so cold air can't get in. The tunnel and inner tunnel together give roughly 5–8 °C above the outside night temperature.

Why: Still air between the two layers insulates, and the heat from the soil stays with the plants. Heat the plant, not the whole tunnel: otherwise the heat goes out through the roof.

Fleece lets air and moisture through and doesn't drip; film on hoops warms more but makes damp and mould.
Fleece of 50 g/m² lets little light through, so remove it by day.