How to Keep Your Greenhouse Cool in Summer

, written by Benedict Vanheems gb flag

Greenhouse shading

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.

Automatic greenhouse vent

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 louvered) 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.

Greenhouse shading

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

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 favorable 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.

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Show Comments


Fernand Tasse on Thursday 27 April 2017
"Keeping the doors open for airflow and adding shading will certainly help. I hadn't heard of a swamp cooler - not much need for that in our chilly summers!"
Ben Vanheems on Thursday 27 April 2017
"Sirs: I'm an elderly person. Trying to raise some garden stull in a greenhouse I built. Am in Tucson Arizona keep greenhouse cool from Oct to next spring is a battle. Have green shade cloth on south and west end. Solex on roof, can't afford to run misters all the time; am on limited social security Would putting shade cloth on part of roof cut too much light from plants. Am at loss on how to keep it cooler. "
roh h on Wednesday 18 October 2017
"Putting shade cloth on the south or west-facing roofs would certainly help to cut out some of the sun and hence reduce the temperature inside. There should still be enough light getting into the greenhouse, so yes, go ahead and do that as it may help to further cool your greenhouse."
Ben Vanheems on Thursday 19 October 2017
"Would a greenhouse and this procedure also include Phoenix Arizona where the summer gets extremely hot. I always plant flowers in pots in Oct and they last until June or July. I would like to move all my pots to a greenhouse on the shady side of the house and try to help them survive the summer."
Wendy on Wednesday 14 November 2018
"Hi Wendy. I think anywhere that is shadier and a little cooler than parts of the garden fully exposed to the sunshine is best. You may find that greenhouses get even hotter, even in the shadier parts. You might be better off setting up a small outdoor shade tunnel or cage for your plants, using shade cloth to cast a cooling shade below. "
Ben Vanheems on Thursday 15 November 2018
"Hello I have made a greenhouse/glasshouse I have opening windows on three sides aswell as a whirlybird on the roof and recently a solar powred fan plus uv blocking shade cloth under uv rated polycarbonate roof sheets but with 40+ degree summers here in Sydney it is causing problems for my wife’s orchids don’t know what else i can do"
Michael on Tuesday 8 January 2019
"Hi Michael. Yes - the hot summers must be a particular challenge. If you're doing all of what you say, plus the above, as detailed in the article, then short of putting in air conditioning (!) there isn't much else you could do. Can the orchids be relocated to another cooler, but still-bright, location?"
Ben Vanheems on Monday 14 January 2019
"RE: Damping Down. Evaporation is a physical process that USES heat, typically found in the air around the process, thereby LOWERING the temperature. That's how swamp coolers work. The increased humidity is a byproduct, not the reason for the temperature drop."
Maria de Jesus Gutierrez on Tuesday 29 January 2019
"Many thanks for clarifying that - yes, your right - damping down helps to directly lower the heat, helping plants cope in hot weather."
Ben Vanheems on Wednesday 30 January 2019
"My porch has. Roof of greenhouse velux. How do I stay comfortably cool in the summer. The sides are open to the air. J have North West expisure"
Janielle Nathan on Sunday 7 April 2019
"My porch has. Roof of greenhouse velux. How do I stay comfortably cool in the summer. The sides are open to the air. J have North West expisure"
Janielle Nathan on Sunday 7 April 2019
"Keep all of your windows, including roof windows, open during hot weather to encourage a breeze to pass through. Shading on the sunny side will help to stop temperatures rising too fast."
Ben Vanheems on Monday 8 April 2019
"Thank you for taking the time to this information very useful! "
Arya Steve on Monday 26 August 2019
"My first year of using a green house in our Australian hot summers was a disaster, everything died. But this year I used a sweet potato vine and Bromeliads to cool the green house as they are hardy and keep the moisture in the air. It is working so far as we have already had temps of over 45* C."
Miranda Paynter on Friday 10 January 2020
"Wow Miranda - that is a really incredible result giving the record-breaking temperatures you guys have been enduring. I hope it cools down and rains for you soon - we're horrified to see all the bushfires and are thinking of you guys down there."
Ben Vanheems on Monday 13 January 2020
"Hi...Great article...very informative...I am practicing greenhouse farming in a hotter area. Just one question though:- You say that Damping Down a greenhouse will help plants in maintaining their temperature as it will cool the environment by increasing the humidity and we should water the plants more and more which will help in maintaining the plant temperature by transpiration as transpiration rate will be higher in a drier atmosphere. However, more the relative humidity lesser the transpiration rate, so won't damping down the greenhouse decrease the transpiration rate of the plant which could affect its growth. How can these two be balanced?"
Tushar Bajaj on Wednesday 29 January 2020
"There's a balance to be had Tushar. Ideally you would damp down but have all the vents and doors open so that the water vapour is able to move out of the greenhouse. It is the act of the water turning to water vapour that cools the air, through evaporative cooling. You don't necessarily want the humid air hanging around - you want it to move out of the greenhouse, taking the embodied heat with it. That way transpiration from the plants is relatively unchanged. But yes, the two can play against each other."
Ben Vanheems on Monday 3 February 2020
"Informative and easy to read. Thank you!"
Michael Dedmon on Monday 24 February 2020
" H Hi there,i enjoyed all discussions herein. As a first timer in summer vegetables planting where the temp at dry and north windy weather is + 120 F,i made a tunnel oriented N-S of size 160 cm W x 110 cm H x 600 cm L,covered with 80% net and doubled on south west side and misted aprox every 2-3 hours. The result is nobody would like.However i m thinking to cover it in a plastic sheet. with aeration please I need your advice in way forward in particular suction fan size and type of humidifier {wet jute} on the opposite side,or however u may thing. i would appreciate a reply."
hamoud on Sunday 14 June 2020
"Hi Hamoud. I'm afraid this is out of my area of specialism. I would suggest contacting a greenhouse/polythene tunnel supplier who will be able to advise on this. Sorry I can't help."
Ben Vanheems on Monday 15 June 2020
"Thanks for the reply. i would contact them. Thanks again"
hamoud on Monday 15 June 2020
"Does damping down add to powdery mildew?"
Eloise Levy on Tuesday 28 July 2020
"High humidity can increase the risk of diseases like powdery mildew, but this is greatly reduced if plants are kept well watered. So damp down (paths and hard surfaces only) while also ensuring plants are kept properly hydrated and you should minimise the risk of a powdery mildew attack."
Ben Vanheems on Monday 3 August 2020
"This is the best write up on how to keep your greenhouse cool in the summer. I really loved it and thank you very much for sharing this with us. You have a great visualization and you have really presented this content in a really good manner."
Sound Insulation on Tuesday 8 September 2020
"Delighted you found the article useful, thank you."
Ben Vanheems on Wednesday 9 September 2020

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