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
"Good article and even thou I have had greenhouses off and on over the past 35+ years, I have learned some things. I have double and triple polycarbonate panels on the sides and roof, respectfully. We are looking into putting some vents on the roof to help release the heat in the summer. My greenhouse is a lean-too style on the back side of our garage and is 14 x 20. I have a white shade cloth that covers the roof only. I had not read about the shade cloth going down sides until this article.I do have an exhaust fan, 6 windows and the door has a sliding window with a screen. I live in SE Texas and the heat in the greenhouse can get up to 110 degrees F in summers with fan running. We normally average low to mid 90’s in summer with about the same in humidity. I remove all plants during spring and it holds my succulents then. My question is regarding using roof vents with the shade cloth on top. With the vents open and the shade cloth is over them, will that hamper the heat leaving my greenhouse?. I don’t want to cut the shade cloth to fit the vents. Thanks in advance for suggestions.......Sheri"
Sheri Bethard on Saturday 21 November 2020
"Hi Sheri. The best way to get around opening of the vents is to use netting that is in strips, as in the third photo in this article. That way you can open the vent and the air can get in and out unhindered through the sides. The option is internal netting/shading blinds. "
Ben Vanheems on Friday 27 November 2020
"Last summer my greenhouse got so hot that the display on LCD Min-Max thermometer gave up the ghost - so well in excess of 50C even with door and roof vent open and I'm in Leeds, UK A friend suggested using a PC fan run off a small solar panel - wondering if its worth giving this a go. My greenhouse is a small 8ft x 6 ft lean-to. "
John on Monday 11 January 2021
"That sounds inordinately hot for a Yorkshire greenhouse John! Assuming you are doing all the recommended things mentioned in the article, you could try adding a fan., I'm not sure if a PC fan would be up to the job of cooling off a whole greenhouse, given it's usual function. But if you're into a bit of DIY it could be a fun project that's worth a try."
Ben Vanheems on Monday 11 January 2021
"Hi Ben Thought I'd update on my hot Yorkshire greenhouse. Today's early afternoon sun - midday till about 2 got the temperature in the greenhouse up to 40C and a 5v 4" USB fan powered from a phone charger didn't make much difference. Oh well, back to the drawing board and remembering to open the door and putting a barrier in place to stop the cats. "
John on Wednesday 17 March 2021
"Hi John. Are you saying your greenhouse in Leed, West Yorkshire, UK reached 40 Celsius this afternoon? That does seem extraordinary given the time of year!"
Ben Vanheems on Wednesday 17 March 2021
"Hi Ben Yes, either that or my thermometer is broken although having stood in it for a while it was uncomfortably hot. Cheers John"
John on Thursday 18 March 2021
"Hi John. I think your thermometer may be broken. Assuming it reached 12 Celsius or so, I wouldn't have expected the greenhouse to go much about the mid-20s max. It may be worth using another thermometer just to check the one you currently have in there, to see if they match. I suspect they may not!"
Ben Vanheems on Thursday 18 March 2021
"I am a newbie to this. I live in the Central Florida area. So needless to say it gets VERY hot in the summer. I am thinking about installing a "mister system" inside my greenhouse that I can run as often as needed. I am sure this will help dramatically with the temperature control, but I am worried it will cause other problems. Plant disease? Mold growth? other issues. Do you see this as a good idea to help control the temperature? Please let me know, Thanks!"
Doug McAllister on Thursday 6 May 2021
"Hi Doug. It would help cool things down because of evaporative cooling. I've seen mister units in outside areas of southern California and it does seem to work in cooling things down a bit! My big concern is the cost of the water to do this, and also the potential for disease. Some plants like it nice and humid, but for others it could create the ideal conditions for disease. I would stick to excellent ventilation and denser shading."
Ben Vanheems on Saturday 8 May 2021
"I have a 9x16 cattle panel greenhouse I made covered in a decent greenhouse plastic. I built it over 2 3x16 raised beds. Love it. I want to use it as a greenhouse but also a trellis, as if it had no plastic. This is my first summer with it and have planted many warm weather vegetables in it. I have roll up sides along the 16 foot walls, and doors on either end. All open of course for the summer heat but I want to encourage my vining plants; cucumbers, pole beans, winter squash and melons to climb the cattle panels which are essentially my greenhouse walls under plastic. Will they grow ok, and not burn if they are literally touching the plastic? "
Carol on Tuesday 25 May 2021
"They shouldn't burn if touching the plastic, but I would wonder if they have enough space between the cattle panel and plastic to gain a proper grip. But burning shouldn't be an issue I'd have thought."
Ben Vanheems on Tuesday 25 May 2021
"Hi Ben - I have a greenhouse which had really messy green glass for the last two summers - so I didn't haven't any problems with the sun and actually grew great tomatoes! But having cleaned and scrubbed them up last winter, I'm worried my tomatoes will suffer this summer. I have quite a lot of netting left over from a fruit cage roof. It's black, and reasonably fine gauge (maybe 1cm square holes). If I drape this over the greenhouse roof will this be enough to help at all, do you think? I like to recycle where I can :-)"
Kathryn on Wednesday 26 May 2021
"Hi Kathryn. Sometimes that mould on the greenhouse glass can be quite handy can't it! I had the same on my greenhouse roof - it makes for perfect shading! I think your idea of using fruit cage netting could work. You're simply looking to reduce the amount of sunshine getting into the greenhouse - by whatever means - and that would do the trick. Whether it will be enough (or even too much) is hard to know, but it seems like an intelligent thing to try. I'd just be sure you anchor it down so it doesn't blow off and away."
Ben Vanheems on Thursday 27 May 2021
"I planted squash I grew from seed in my basement. Awesome looking plants. I hardened them in the greenhouse and then a few days to a week outside. They have been in my raised bed now for not even 2 weeks and all I do is rescue it from limp leaves, looks like it is dying. I first thought watering but then came to realize its the sun. It has been a bit hot for the season but boy. I have them under cover (old pair wood shutters leaning over them for shade, and within 40 mins to 1 hour they look great. Geez,I cant do that forever. your thoughts? "
Carol on Thursday 27 May 2021
"All my planting is done in my raised beds and now I am finding that the makeup of my soil is draining water too fast. I have to water daily. And I am giving it several inches at a time. Finger test (whole finger) finds it very moist, yet the following day it is barely damp. Certainly won't last the day. This year I added 3 inches of store bought organic compost on top of each bed, barely mixing it in with 2 inches of the old soil. Looked great but water just drains through it. Is there anything I can do now to ammend with all my plants already in?"
Carol on Thursday 27 May 2021
"Hi Carol. Squashes do go limp quite quickly with lack of moisture and also in hot sun (even if there's moisture to draw on - it's there natural response to heat but they soon perk up again in the evening). It sounds like soil moisture is definitely the issue here. You're doing the right thing - adding plenty of organic matter. Really, the only thing you can do is to keep adding well-rotted organic matter to improve moisture retention. Add it regularly (once or twice a year) and conditions should gradually improve. You could also try shading the soil with a mulch of something like grass clippings, shredded prunings etc. to keep the soil cooler and thereby slow transpiration from it. The clippings are great around most crops, and larger chippings fine around the likes of tomatoes, squashes, beans etc."
Ben Vanheems on Friday 28 May 2021
"Regarding "green grass clippings make great mulch", I've also heard not to use them as they eventually get very slimy. Your thoughts? "
Carol on Saturday 29 May 2021
"Hi Carol. Add them in thin layers (not dumped on thickly) and this avoids them getting all slimy - they break down into the soil quicker and then you can add another thin layer."
Ben Vanheems on Monday 7 June 2021
"I'm planning to build a polytunnel in an area in Australia where the summers are hot 35-40 degrees celsius and cold in winter, below freezing at times. I want to propagate cuttings and germinate seeds. I'm thinking of using plastic as the first layer (good for winter) and then overlay with shade cloth in summer. I will have screen doors at both ends of the tunnel. I'm wondering if the shade cloth alone will be enough for the extreme heat? What roof venting options are available for plastic sheeting?"
Suzanne on Monday 28 June 2021
"Hi Suzanne. Polytunnels usually have vents that run along the bottom as an option, with obviously the door/s at each end. This should be good at keeping the air moving. I think your idea of shade cloth draped over the tunnel would work well, or you could look at shade paint, which can be added or washed off according to the season and strength of sun/heat."
Ben Vanheems on Friday 2 July 2021
"Great article! I am hoping you can give me some tips on my unique setup. I have a small greenhouse, 8' X 6'. Where I live on the East coast in the USA, it regularly gets into the low 90s and during the hottest part of the day, my greenhouse can get as high as 109 degrees! This is with a coat of shading paint. I have also caulked up all the seams and have weather stripping that seals the bottom. the greenhouse is on a paver patio with full sun 80% of the day. I have a 8,000 BTU dual hose portable AC running to keep it that cool and it cools down pretty quickly when the sun goes behind the clouds. Before the shading paint and the AC, I couldn't get any plants to grow with just the door open and the roof vent. It was way too hot. So, I am still needing to cool it down during the hottest days. I am wondering if I should install an intake vent and an exhaust vent on the opposite sides to move air through during the hottest times of day. The AC has an exhaust that blows hot air out the side of the greenhouse, but it also removes humidity and as it runs, the humidity goes down in the greenhouse. Or should I simply apply further shade? Maybe another coat of the shading paint? "
Nowell on Friday 9 July 2021
"Hi Nowell. It sounds like you've got a great setup there - nice work! Anything that keeps the air moving through is going to be a good thing, and this will avoid stagnant air that could, in theory, promote disease. Not living in a hot climate myself, I'm not sure what effect adding an intake and exhaust vent might have on the effectiveness of your AC though. If it seems to be working okay as it is currently, then perhaps another layer of shading paint might be the safest/best solution. I would perhaps give that a try first - see how effective it is - and then move to the vents if you still need to take the temperature down a bit further."
Ben Vanheems on Monday 12 July 2021
"Great discussion....unfortunately I've been trying most of these ideas...Shade cloth, fan, windows and doors open...will be installing a roof vent this week as soon as I figure out how. I built this last year myself (9x7 interior space, 11 ft high roof). I live in PNW - and its gotten as hot as 125 in there! (yes, it fried some cactii too). It so surprised me that I had to HEAT it...and now I have to COOl it. LOL the dark side of having a greenhouse that no one tells you. Hope we all can figure it out. :D "
Diana on Monday 19 July 2021
"Hi Diana. It's incredible the range of temperature that a greenhouse can achieve, particularly somewhere with 'proper' winters and heatwave summers (I was alarmed to hear about the recent record highs in the PNW). I hope you manage to keep your greenhouse a bit cooler for the rest of summer - it's worth aiming for."
Ben Vanheems on Tuesday 20 July 2021
"I Have a 10' x 16' Greenhouse That's In a Partially Shaded Area In North Central Florida. I Haven't Used It Yet, But Noticed That On A Slightly Warm Day This Winter The Thermometer Was Reading 120 Degrees With The Sun On It For a Couple Of Hours. All 4 Windows Were Open. I Was Wondering If I Used a Lawn Sprinkler To Wet Down The Outside Of The Greenhouse On a Timer a Few Times a Day Would Help. What Are Your Thoughts. Joe. S."
Joe S. on Monday 14 February 2022
"Hi Joe. That may help but it seems like a terrible waste of water. The best thing to do would be to apply greenhouse shading (ideally on the outside) to stop some of the light getting in in the first place. Then just make sure to keep everything open for good airflow."
Ben Vanheems on Monday 28 March 2022
"Hello, I'd like some advice about having an attached greenhouse /conservatory to our house. We live in Central Texas. The winters can have a solid week of below freezing temps and the summer's have temps well above 100 degrees F. We haven't even started the build yet. I was thinking that we should put the greenhouse attachment on the south side of the small house we plan to build to protect from the North wind and have shade cloth and/or denser light filtering material on the roof and West facing sides. I want this greenhouse to be like an extension or livingroom in the house. The problem is heating and cooling without the benefit of conventional insulation. Is there a way to make this a living space as well as a place that plants can survive and grow? I would love some input on how to make this plan a reality. Thank you for your time."
Kay on Thursday 13 October 2022
"Hi Kay. I know that a lot of passive houses use glass-fronts to the sun-facing side to trap in the sun's warmth during the winter month. They then have brise soleils to shade the glass from the high summer sun. But I'm not sure of the best technique for what you are describing. Certainly shading the roof would make sense for your hot summers. Perhaps you could plant deciduous shrubs/trees in front of the conservatory/greenhouse - so that they cast shade during the hot summer, but then in winter, once the leaves have fallen, more light can get through? Another option would be external blinds to shade the greenhouse in the summer."
Ben Vanheems on Monday 17 October 2022

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