Polytunnel During A Heat Wave

Polytunnel During A Heat Wave
In this Krosagro guide, we will advise you on how to regulate the conditions inside the polytunnel during a heat wave. The key is provide enough moisture for the plants to draw up from below. They will then remain unstressed and cool. But you should always be on the lookout for signs of heat stress.

1. Agriculture and gardening under cover

The extreme temperatures that hit Europe and the world this summer represent a huge obstacle for growers and gardeners. The obvious climate changes are leading to profound changes for hobby gardeners. This is also the case for large – scale professional production, heavily linked to weather conditions.
The polytunnel and the foil in particular form a protection against the external climatic conditions. While dynamic processes are ongoing in the structure. It is important to know why they occur and how to correct them in case of anomalies. The text below is a brief reminder for beginners and advanced gardeners.

2. Benefits of cultivation in the tunnel

During the cold months, as the soil warms up slowly in the tunnel. Warmer soil and air allow you to sow seeds and seedlings earlier. Under normal conditions, open field cultivation at the same time is impossible because there is a risk of damage caused by cold and frost. The foil applied onto the steel frame is a protection against low temperatures. That way, polytunnel owners do not have to postpone sowing and can start early with spring vegetables. With more time for vegetable and fruit development, a more mature and high quality crop can be expected.

3. Greenhouse effects

The transparent foil transmits 95% of the sun’s rays, which, as already mentioned, warms up substrate and air. Throughout the day and night, during cool or warm months when temperature fluctuations occur, the stored heat escapes much more slowly. As a result, the plant growth process is optimal. A plant that does not have to focus on protecting from the cold, dedicates its energy towards the development of the root system, the formation of green mass as well as flowering and fruiting .

4. So, what to do in case of hot weather?

Although the above information is known to all gardeners and growers who use greenhouses, they should be reminded. They will allow you to be vigilant when the growing season coincides with a heat wave. It can happen that a high temperature inside the tunnel damages the crops. It becomes essential to normalize it by ventilating the greenhouses.

5. Ventilation of crops under cover

Air circulation within the structure can be achieved in several ways. First by opening the doors on both sides of the greenhouse. However, it should be remembered that standard polytunnels are usually equipped with only one entrance, the other being optional. Hobby tunnels offered by Krosagro are an example of a well adapted structure. A tunnel with side vents, also known as multi-vegetation, is a faster and more efficient solution. On the sides of the tunnel, openings are raised thanks to cranks, contributing to the flow of air under the plastic film. The excess heat is then removed outside the installation, lowering the temperature in the facility.

6. Shading a polytunnel during a heat wave

Shading is usually required from the end of spring until early autumn. Unfortunately, shading limits light intake for plants. As plant growth depends on light, only the minimum amount of shading should be implemented to keep temperatures to a maximum required. Shade netting is usually placed inside the polytunnel and secured with clips. Shading paints are also available but often need to be refreshed  to perform well.

7. Irrigation in a garden tunnel

Watering plants is essential to limit the effects of heat, while ensuring the optimal water input. Too much or not enough can irreversibly cause damage. The ideal amount can be reached by observing humidity sensors, which determines activities to be performed. In case of low water saturation, it is necessary to provide plants with more, in case of excess, ventilation will contribute to the regulation. The process will be similar to removing excess heat: open the doors or lift the vents in the multi-vegetation tunnel. By mulching your plants early on (and refreshing the mulch 1-2 times throughout the season), you will contribute to a cool soil and limit water loss.


With rare exceptions, 35 degres is really too much for our usual harvests. Having to deal with especially hot conditions is never a pleasure for gardeners and their plants. But there are ways to ease the pressure in the polytunnel during a heat wave and mid August often indicates the end of summer. Although high temperatures are still possible outside, the growing season is nearing to an end. This is the opportune time to briefly review the successes of the current season. Evaluate yield, monitor the evolution of plantations under shelter and ensure a favorable microclimate under the facility.


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