Dealing With Extreme Heat in a Polytunnel

The impact of global warming on Britain's weather is becoming increasingly apparent. Flooding, cold snaps and heatwaves are all becoming more extreme and more frequent. For gardeners, severe weather can wreak havoc and destroy crops. Plants grown in a domestic polytunnel are protected to a certain extent from wind, rain, hail and snow but are in more danger during a heatwave. What can you do to minimise stress and damage for your polytunnel crops during a heatwave?

 

Hot, Hot, Hot

For gardeners, a domestic polytunnel offers an affordable option for growing plants such as tomatoes that would typically struggle in the British climate. The polytunnel creates a microclimate, trapping the heat and moisture that plants love. However, as daylight hours increase and summer temperatures rise, so does the heat in your polytunnel. On a sunny day, without ventilation, your polytunnel's internal temperature can easily exceed 35 degrees celsius and, during a heatwave, rise above 40 degrees Celsius. For most polytunnel crops, this is too hot and stresses the plants.

 

Beat the Heat

The easiest way to regulate your polytunnel temperature is to provide adequate ventilation, increase humidity, or prevent solar energy from reaching your tunnel to heat it up.

 

●     Ventilation: During summer, opening doors at both ends of your tunnel should provide sufficient ventilation. Depending on the design of your polytunnel, you may also be able to raise part or all of the side panels for increased ventilation. Planning planting and avoiding overcrowding will also help fresh air to circulate and prevent overheating.

 

●     Humidity - Spraying hard surfaces with water will encourage evaporation and increase the moisture within your polytunnel. It might be tempting to spray your plants as well but try to only water at the roots. Water on foliage can act as a magnifying glass on a hot day and result in leaf burn and damaged plants.

 

●     Shade - Taller plants provide natural shade for smaller plants, so if you plan your crop layout carefully, you can provide natural protection for some crops. However, as temperatures rise, you may want to consider throwing an old sheet over your polytunnel or even over plants to give relief from the sun in much the same way as protecting plants from frost.

 

Forward Planning

These three steps mentioned above will help reduce your polytunnel's overall temperature during a heatwave and give your plants the best chances of survival. There are also steps you can take in advance of the hot weather to assist your plants. These include:

 

●     Decluttering: Ensuring your polytunnel is neat and tidy helps improve the airflow through your tunnel. If you feel parts of your polytunnel are insufficiently ventilated, you can always use a solar fan to help get the air moving.

 

●     Mulching: A good layer of mulch will help the soil retain moisture and keep roots cool. It will also help prevent weeds that could contribute to overcrowding.

 

●     Heat Regulation: Installing a few large containers filled with water or large stones can help maintain a more even temperature in your polytunnel. They trap heat during the day and release heat through the night, which helps avoid extreme temperature differences.