How to Keep Your Greenhouse Cool in Summer

23 July 2015, written by Benedict Vanheems

The height of summer can be a hot and steamy affair, particularly if you’re working inside a greenhouse! Temperatures in this cosseted environment are often several degrees higher than outside – just what we’re after in winter, but not so welcome on a sunny day in the middle of the growing season when this mercury-pushing effect places strain on plants and gardeners alike!

Greenhouses, also known as glasshouses or, perhaps more aptly, hothouses, are invaluable to the kitchen gardener, enabling reliable production of warmth-loving crops such as tomatoes, cucumbers and melons. But even these sun seekers have a limit. Tomatoes, for example, see poor pollination and damage to immature fruits as temperatures climb above 32°C (90°F) by day and 24°C (75°F) by night.

It illustrates the importance of keeping your cool as the heat builds. A canny combination of shade, ventilation and humidity will help.

Ventilating a Greenhouse

One of the best ways to combat heat is to provide plants with a good through-flow of air. Ventilation, courtesy of roof vents, side vents (usually louvred) and the greenhouse door can create the necessary movement of air to cool down overheated plants.

As a very approximate rule of thumb, an area of roof vents equivalent to one fifth of the floor area will provide a complete air change every two minutes. This proportion of roof vents is a luxury in most greenhouses, but open up side vents and doors, and it’s possible to get the air moving sufficiently.

Temperatures above about 27°C (81°F) can begin to cause damage to some plants, so have a maximum-minimum thermometer on hand to monitor the situation. On sunny days, head out as early as you can to open all doors and vents, keeping them open on warm nights. You can stop the local wildlife and cats from entering by pegging netting over the door, but do make sure the netting allows pollinators through.

Automatic vent openers can be fitted to somewhat automate the process, but as they usually lag in response you’ll still need to be around to open the door to get a head start on the cooling off process.

Shading a Greenhouse

Shading is the second weapon you have to attack heat head on. Use it wisely brave gardener, as plants obviously depend on good light levels to grow to their full potential!

Shade paints are a quick and cost-effective way of filtering out some of the sunlight’s strength. You can add more layers as summer progresses, then wash and brush off as it cools back down. Shade paint may not be suitable for all greenhouses, for example those with unpainted timber, which is where netting and blinds come into their own.

Blinds can be internal or external. External blinds are the most effective as they filter the sunlight before it passes through the glass and becomes trapped inside. On cooler days they can be removed. However, they can hinder efficient vent operation, so you’ll need to work around that. Or splash the cash and fit internal blinds, which can also be automated.

Mesh or shade netting offers a cheaper alternative to blinds and is simply secured into position with clips.

Damping Down a Greenhouse

In really hot weather there is another trick to keep plants cool: damping down. Damping down is the process of raising the humidity inside the greenhouse by wetting hard surfaces such as paths and staging. As the water evaporates, it increases the moisture level of the air, which helps plants to cope with the heat. A happy side effect of raising humidity is that conditions are made less favourable for pests that thrive in dry conditions, such as the red spider mite.

How often should you damp down the greenhouse? As often as you can really – it’s hard to overdo it when it’s very hot! Assuming you’re unlikely to want/be able to stand on guard with a watering can all day long, aim for once in the morning and once in the evening. If you are at home and have the energy, another damp down at lunchtime is just peachy.

Avoiding Water Stress

It may seem obvious, but plants with enough moisture at root level are dramatically happier than those without in hot weather, so keeping plants watered is essential.

By far the most effective tool plants use to keep cool is transpiration – the loss of moisture through leaf pores (or ‘stomata’). This loss of moisture cools down the leaf surface in exactly the same way we sweat. Reduce a plant’s ability to ‘sweat’ and it can overheat and wilt. By providing enough moisture for the plants to draw up from below, they will remain unstressed and cool as…well, a cucumber!

It goes without saying that you should always be on the lookout for telltale signs of heat stress – wilting plants, scorched leaves and young foliage drying out. Diligent application of the above steps should prevent these desperate side-effects from ever showing.

As always, we’d be keen to hear about how you keep greenhouses – and tunnels – cool in summer, particularly if you’re gardening in a hotter part of the world. Let us know by dropping a comment below.