How Do You Keep A Greenhouse or Polytunnel Cool in Summer

At a Glance

Summer temperatures make heat management increasingly challenging for gardeners. Excessive heat can reduce crop yields, increase plant stress and make watering needs harder to predict. Automated greenhouse watering systems help maintain more consistent moisture levels during heatwaves, supporting healthier growing conditions when traditional watering routines struggle to keep pace.

Explore Harvst's irrigation solutions and build your own setup tailored to your greenhouse, growing space and crop requirements.

Why Summer Heat Is Becoming a Bigger Problem for Polytunnels and Greenhouses

For many growers, summer used to be the season when crops really took off. Longer daylight hours, warmer temperatures and steady growth created ideal conditions for tomatoes, cucumbers, peppers and other productive crops. Currently, that balance is becoming harder to maintain, as in recent years there have been more frequent heatwaves, longer dry spells and periods of unusually high temperatures recorded than ever before. 

According to the UK Met Office, 2025 was the UK's warmest year on record, with an average temperature of 10.09°C and all of the UK's ten warmest years have now occurred within the last two decades. As these conditions become more common, growers are finding that excess heat can be just as challenging as cold weather. 

A greenhouse or polytunnel that feels pleasantly warm in the morning can become uncomfortably hot by mid-afternoon. On sunny days, temperatures inside can rise far beyond the conditions plants need for healthy growth. This is why many gardeners commonly ask, “How do you keep a greenhouse cool in summer?” or “What measures should you take to ensure polytunnels maintain the right temperature?” They are not simply looking for an answer tos these questions, but for a solution to protect the crops they have spent months growing and nurturing.

This challenge is not limited to large greenhouses. It is also relevant to small hobby gardens, compact grow houses and polytunnels, as all of these experience rapid temperature increases during hot weather. This reason is that such structures trap heat, during spring and summer, this creates ideal growing conditions, but during summer, mainly during heatwaves, the same design can cause temperatures to rise significantly above outdoor temperatures.

In many cases, the issue develops gradually, so plants appear healthy one week and then suddenly begin to show signs of stress. At that point, don’t simply try to find a remedy for overheating greenhouse plants. Instead, try to understand how heat affects crops, which can actually be a more effective step towards managing this stress.

What Happens to Crops When They Get Too Hot?

Plants rely on a balance of light, warmth, moisture and airflow, but when temperatures climb too high, that balance begins to break down. 

One of the first signs of overheated greenhouse plants and polytunnel crops is wilting. Many gardeners assume this means plants need more water, but heat stress can affect plants even when the soil is still moist. High temperatures increase the rate at which plants lose water through their leaves. When the loss happens faster than roots can replace it, growth starts to suffer.

Pollination can also become less reliable, with crops such as tomatoes failing to set fruit or leafy crops bolting earlier than expected. This results in lower yield despite putting in the same amount of effort. 

Heat also affects the growing environment as soil dries faster, containers lose moisture more quickly and water demand becomes less predictable. A watering routine that worked perfectly in May may no longer be sufficient due to a week of intense heat and sunshine. 

This is a common problem, leaving many growers wondering how to keep their greenhouses or polytunnels cool and ensure plants receive more responsive care. Unfortunately, traditional watering methods are not always able to keep pace with changing conditions.

Why Traditional Watering Methods Struggle During Heatwaves

Most gardeners begin with a simple watering routine like watering cans, hoses and manual schedules, which work well when conditions remain fairly consistent. 

Heatwaves change that, because during such times, a greenhouse or polytunnel may require significantly different amounts of water from one day to the next. Cloud cover, humidity levels, airflow and temperature all influence how quickly plants use moisture. 

During particularly hot periods, crops can need attention multiple times throughout the day. For hobby growers, balancing work, family commitments and holidays while maintaining consistency can be difficult. This pattern is repeatedly evident in growing, as the issue is not usually a lack of effort but rather that environmental conditions change faster than manual routines can adapt to.

Many growers also find themselves overcompensating after noticing signs of stress, then apply extra water without knowing whether plants genuinely need it. This can create a different set of problems, including waterlogging, poor root development and inconsistent growth.

Can Watering Systems Help Greenhouses and Polytunnels 

Automated greenhouse watering systems can help in cooling greenhouses and polytunnels as well as support more stable growing conditions. 

Instead of relying entirely on fixed watering times, modern systems can adjust irrigation based on environmental factors such as temperature and sunlight levels. With an automatic irrigation system, plants receive additional support during hotter days, while watering is automatically reduced during cooler periods. The goal is not simply to maintain a consistent watering schedule but to match water delivery more closely to the plants’ changing needs throughout the season.

At Harvst, this system-based approach is at the centre of greenhouse irrigation design. For a smaller space up to 6ft by 8ft, the WaterMate Mini is designed to automate watering while adapting to changing weather conditions. It can run from a water butt or mains supply and uses solar power for off-grid growing. Many growers use it in greenhouses, cold frames, vegetable plots and allotments where daily access is not always possible. 

Larger greenhouses, polytunnels and multi-zone growing spaces usually require more flexibility. Systems such as WaterMate Pro allow growers to create different watering zones and respond to varying crop requirements across a larger area. What matters most is maintaining reliable conditions through changing weather patterns. That consistency becomes increasingly valuable as summer temperatures continue to rise. Interested in seeing how different setups are managed? Harvst’s YouTube channel includes practical demonstrations and real-world growing examples that show how growers adapt systems to different spaces and conditions.

Additional Cooling Strategies To Adapt During Heatwaves

While watering plays a vital role in helping plants cope with heat, it works best alongside measures that actively reduce temperatures. Let's look at some strategies, greenhouse cooling tips and polytunnel temperature-control ideas that can be adapted during the hottest time of the year.

1. Automatic Vent Openers

Investing in automatic vent openers is a great way to ensure your greenhouse is well-ventilated without manual intervention. These openers use a temperature-sensitive mechanism to open and close vents as needed, keeping your plants cool and comfortable. Our Automatic lid lifters work by heating and cooling the wax inside to lift your greenhouse window when temperatures soar, so no power is needed.

2. Fans and Circulation

Proper ventilation is crucial for maintaining a stable temperature, which can also be achieved with fans and air circulation. By allowing hot air to escape and cooler air to enter, you can significantly reduce the internal temperature with these measures. For larger structures, installing oscillating fans can help improve air circulation, preventing hot spots and ensuring uniform temperature distribution.

3. Shade Cloth

Shade cloths are an excellent way to reduce the amount of direct sunlight entering your greenhouse. They come in various densities, allowing you to control the amount of light and heat they block. External shade cloths are more effective at blocking heat before it enters the greenhouse, whereas internal options are easier to install but may trap some heat inside. You may also consider an automated shade system for optimal convenience. These systems can be programmed to deploy the shade cloth based on temperature or time of day, offering protection when it’s needed most.

4. Damping Down 

Increasing humidity through damping down can help lower the temperature in your greenhouse or polytunnels. This process involves wetting hard surfaces like paths and shelves, which then evaporate, cooling the air. To obtain accurate humidity measurements in your growing spaces, use the wireless humidity/temperature sensor from Harvst.

5. Automatic Misting Systems

To make damping down hassle-free, consider installing an automatic misting system. These systems can be set to mist at specific intervals, maintaining ideal humidity levels and cooling the greenhouse without constant oversight. Automated greenhouse watering systems like WaterMate can also help mist your greenhouses and polytunnels. When paired with a humidity sensor, the system will automatically turn on if the humidity drops too low or the temperature rises too high. 

Prepare Your Growing Space for Longer and Hotter Summer With Harvst

Good ventilation, sensible shading and careful water management all play a role in keeping greenhouse plants cool and maintaining polytunnel temperatures during summer. 

When automated watering systems and cooling strategies work together, plants are better equipped to cope with periods of intense heat. Some growers may only need a few adjustments to their existing setup. In contrast, others may benefit from tools that help manage watering automatically as temperatures rise and fall throughout the day.

Harvst develops automatic irrigation solutions designed for a range of growing environments, from small greenhouses and cold frames to larger allotments and off-grid spaces. Features such as solar power, environmental sensors, flexible watering zones, easy DIY installations and remote control via a smartphone app help growers maintain healthier conditions without constant manual intervention.

To experience it yourself, purchase a greenhouse installation kit or build your own setup suited to your greenhouse and growing goals.