Dealing With Extreme Heat in a Polytunnel

Unfortunately, it is not just extreme cold or wet conditions that can pose a problem for polytunnel gardeners anymore. Extreme heat can be just as damaging to your plants as a storm or deep freeze. Luckily, with top notch polytunnel ventilation and a few precautions, you can fight through the extreme heat for a successful harvest in a heat wave. Here are our tips on ensuring you have adequate polytunnel ventilation, and other ways to deal with extreme heat in a polytunnel.

Extreme Heat in A Polytunnel – How Hot Can It Get?

If it is sweltering outside, inside a polytunnel it will, of course, be even warmer! The sun will raise the internal temperature of a polytunnel considerably. A polytunnel that is sealed and shut up can see temperatures soar to well over 30 degrees C – above the ideal temperature range for many common polytunnel plants. Some gardeners have chosen to avoid polytunnel ventilation and embrace the heat – choosing to use their polytunnel as a sort of sauna, or for a hot tub or swimming pool.

What’s The Ideal Greenhouse Temperature In The UK?

If a polytunnel sauna isn’t for you, it’s important that you keep the temperature just right to allow your plants to thrive.

According to the RHS, the ideal temperature for a greenhouse or polytunnel here in the UK is 25 – 27 degrees Celsius, or 77 – 81 degrees Fahrenheit. If you’re in the middle of a heatwave, this may seem like a daunting task, however there are several ways you can improve polytunnel ventilation and bring the temperatures down.

How To Bring Temperatures Down

Regulating temperatures in a polytunnel is largely a matter of regulating the airflow through the structure, and the humidity (moisture levels) in the air. You can also bring temperatures down by reducing the quantity of solar energy that is allowed to enter the structure (or reach certain plants).

To bring temperatures down, you can:

• Improve polytunnel ventilation.
• Damp down (water hard surfaces to raise humidity).
• Create shade.

You can use planting to create shade, or add shading by means of a additional area of cover on the south side of your polytunnel.

Improving Polytunnel Ventilation

Polytunnel ventilation is vital to a thriving polytunnel garden. Having doors at both ends of a polytunnel is a good idea as this will allow for a through breeze which will aid in natural ventilation. If you have yet to purchase a polytunnel and think you may experience extreme heat in the summer, you may also consider having double doors, or larger sliding doors on the ends of your polytunnel, in order to maximise air flow through the structure. Certain planting outside your polytunnel can also help to funnel summer breezes through it to keep it cool.

When planting, take care not to try to cram in too much. Overcrowding your plants can lead to problems with air flow and ventilation which can make plants more prone to disease and more likely to succumb to various problems.

To ensure your polytunnel ventilation is adequate, you can also:

• Consistently monitor the temperature of your polytunnel
• Open all the doors and vents on warm days
• Look out for signs that shade or ventilation are necessary, such as leaf scorch and partial collapse of plants
• Fit automatic vent openers

Growing Plants With Polytunnel Ventilation In Extreme Heat

Even with polytunnel ventilation, growing and harvesting in your polytunnel can be difficult in extreme heat. Here our some tips to help you grow in your polytunnel in even the warmest of weathers.

Water At The Right Time

The last thing you want to do is waste water, yet if you get the hosepipe out in the middle of the day, I’m afraid that’s what’ll happen, as evaporation of your precious resource will surely occur.  Early in the morning or later at night, when it’s cooler is therefore the best time to ensure that your plants are sufficiently quenched.

Water For Longer, Less Often

A surface watering is just that, what you really want is to give your plants (and the ground around them) a proper soaking so the soil can absorb and retain the moisture deeper down. With the other measures outlined, this will enable you to water every couple of days even in the likes of the heatwave we’ve just experienced.

Leave Doors & Windows Open Overnight

Even if it cools down dramatically at night-time, it’s essential to leave at least some airflow in your polytunnel. This easy form of polytunnel ventilation ensures that, when the sun rises, it’ll take longer for your tunnel to heat up and it’s less of a stress on your plants all-round.

Compost, Compost, Compost

To save time watering and help prevent your hungrier plants from suffering, a mulch of compost around their base (and the soil surrounding) will also help to keep water in for longer. It’s incredibly effective for the likes of tomatoes, cucumbers and so forth which have greater demands than other produce. The key when mulching in this way is to water the ground thoroughly first before adding your gardener’s black gold, which then slows down the drying out process. If you have plants in pots then this is also an effective technique, although I would also strongly suggest removing any potted plants outside your tunnel and also adding a layer of weed-supressing membrane over the top, to add further protection from extremes of temperature.

Using Polytunnel Ventilation To Fight Extreme Heat

Keeping everything ship-shape with your polytunnel ventilation helps to ensure that your air flow is maintained over time. Stacked pots, piles of tools and general clutter and debris can reduce the efficiency of natural ventilation patterns in your polytunnel. Keep everything tidy and in order and you will find that everything in your polytunnel goes a lot more smoothly.

How do you keep your polytunnel cool in summer? Let us know in the comments below.