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A greenhouse can reach temperatures as high as 100°F (37.8°C) in a typical British summer, depending on the local climate. While this is normal, especially during peak summer, such heat levels can be too high for many plants. In fact, anything higher than 90°F (32°C) can cause stress or damage to some plants.
This guide covers effective temperature control methods, including ventilation, shading, and cooling systems. Learn how each works and get some tips to help protect your plants from excessive heat.
Ventilate Your Greenhouse
Ventilation is one of the cost-effective ways to cool your greenhouse without electricity. Simply opening the windows and doors or the roof vents can already help cool down the interior. Doing so invites more fresh air to circulate inside, preventing excessive heat buildup.
When the environment becomes too warm, plants will either wilt, stop growing or die. Surprisingly, they’re more at risk from intense heat than from cold.
Refer to our guide, “How to Ventilate a Greenhouse?” to explore different ventilation systems worth considering.
Use Shading Techniques
Relocating your greenhouse to a spot with less sun exposure can be a hassle. In some cases, it simply can’t be done due to the structure being anchored to a foundation or installed in a fixed location. In situations like these, shading is your most reliable option.
Techniques like shade cloth, reflective materials, and external shading can all do the trick.
Shade cloths come in various densities, offering different shade levels from 5% to 95%. They’re usually made of loosely woven polyester that allows air to flow through while blocking some of the sun’s heat. These cloths can be draped over the greenhouse or installed as sidewalls.
Reflective materials like aluminium foil and Mylar sheets work by being applied to the walls or ceiling to redirect sunlight. Aluminium foil is a common household kitchen item. Mylar sheets, in contrast, are made from polyester film with a shiny, reflective surface. Both help bounce excess sunlight away while ensuring plants get even, indirect light.
External shading involves installing panels or shade sails outside your greenhouse. This garden shade solution stops the sun’s rays from hitting the glass or plastic, keeping the inside cooler. If you want to minimise sunlight from low angles, the sides of the greenhouse are the best place to position them. Placing shading around the perimeter also helps, as it blocks the harsh sun before it even reaches the structure.
Invest in Quality Greenhouse Fans
We can’t recommend greenhouse fans enough if you’re looking for something energy-efficient. As fans exhaust the heated air, a slight vacuum is created, drawing in cooler outside air through open windows, doors, and vents. They also work best if installed on one end wall with louvres or vents placed on the opposite ends.
The number of fans your greenhouse needs depends on how big it is. For a medium-sized 9×9 polycarbonate greenhouse, two fans should do the trick. Smaller units only need one fan, while larger ones will need a few more to keep the air flowing properly.
Tip: Although not a cooling system but rather a smart tool, automatic vent openers are also worthwhile. These non-electric devices respond to temperature changes by expanding a piston that opens the vent. Pair them with a fan for better airflow, keeping the temperature steady in your greenhouse with minimal effort.
Round-up
Keep your greenhouse ventilated by opening the windows and vents to let fresh air circulate—your plants will thank you. If the sun’s too strong, shade cloths or a shade sail are a simple solution. Just place them where they’ll block the most sunlight. If you can, consider fan systems, but get professional help if installation’s beyond your skills.
Tip: Get yourself a thermometer to stay on top of temperature changes and make adjustments as needed!
Up next on your reading list: How to Keep a Greenhouse Warm in Winter
