How do I shade and cool a polytunnel?

Polytunnels are often treated as cheap greenhouses, and for temperature control they behave differently in three ways that matter. There is less thermal mass, so they heat and cool faster. The covering cannot take shade paint. And the ventilation is concentrated at the two ends rather than spread around the structure, so the airflow pattern is a tunnel rather than a stack.

Safety

Step by step

1. Use netting, never shade paint.Shade paint is formulated for glass and is removed at the end of the season by scrubbing. On polythene it is difficult to apply evenly, difficult to remove, and the scrubbing needed to shift it damages the cover and shortens its life. Purpose-made shade netting is the correct material.
2. Fit the netting outside the polythene.Outside stops the radiation before it enters, which is worth considerably more than intercepting it inside. It also keeps the net away from the crop and lets air move between the net and the cover, which carries heat away.
3. Choose a shade percentage for the crop, not the maximum.Netting is sold by the proportion of light it blocks. Lighter shading suits fruiting crops like tomatoes and peppers, which still need strong light to crop well. Heavier shading suits leafy salads, seedlings, ferns and anything being protected through a heatwave. Too much shade costs yield, so match it to what is inside rather than buying the densest available.
4. Fix it so it cannot flap.A loose net rubbing on polythene will wear a hole in a season, and in wind it can tear the cover or pull fixings out. Tension it properly, fix it to the frame or to ground anchors rather than to the polythene itself, and check it after every windy spell.
5. Ventilate through both ends.The doors at each end are the main ventilation on most tunnels, and opening both creates a through-draught along the length that clears the volume quickly. Opening one end alone does very little. Mesh doors let you ventilate while keeping animals and larger pests out.
6. Use side ventilation if the tunnel has it.Roll-up or hinged side vents transform hot-weather performance, because they let air in low along the whole length rather than only at the ends. If you are buying or re-covering a tunnel, side ventilation is the option most worth having in a warming climate.
7. Damp the paths down.Wetting the paths and any bare soil in the morning and again at midday cools by evaporation and raises humidity. In a tunnel, where air is often still, this makes a bigger difference than it does in a well-ventilated glasshouse.
8. Move the vulnerable out.Seedlings, small pots and anything shade-loving should spend a heatwave outside under a bench or a tree rather than in the tunnel. A tunnel in a heatwave is not a place to keep anything delicate.
9. Take the netting off promptly in autumn.Light becomes the limiting factor quickly as the season turns, and shading left on into September costs late crops. Take it off, clean it and store it dry so it lasts several seasons.

Tips

• Netting also reduces wind damage to plants inside and, in a fine mesh, keeps some pests out — a useful secondary benefit for brassicas and carrots.
• A tunnel sited with its length running roughly north to south receives more even light and heats less viciously at one end than an east-west one.
• Whitewash and diluted emulsion are sometimes suggested for polythene. Do not — they degrade the cover and are extremely difficult to remove cleanly.
• Polythene degrades under ultraviolet light and has a service life measured in years. Shade netting over the top slightly extends the cover life as well as cooling the tunnel.
• If the tunnel is unbearable every summer and the crops inside are salads and seedlings rather than tomatoes, consider whether it should be shaded permanently through the summer months rather than in heatwaves only.

Common mistakes

• Painting shade paint onto polythene — It is formulated for glass, is very hard to remove from polythene, and the scrubbing required damages the cover and shortens its working life.
• Fitting shade netting inside the tunnel — The radiation has already passed through the cover, so most of the heat is already inside. Outside, with an air gap, is far more effective.
• Opening only one door — The ventilation on most tunnels works as a through-draught end to end. One open door gives almost no airflow along the length.
• Fixing netting to the polythene itself — It concentrates load on the cover and tears it. Fix to the frame or to ground anchors.

Questions people ask