By eleven o'clock on the first warm Saturday in May, a closed 8x6 greenhouse can sit at 35°C while the lawn outside stays in the high teens. The glass that saved your seedlings in March has turned against you. Tomato flowers stop setting, seedlings wilt by lunchtime, and the whole structure becomes a problem you have to solve before the afternoon sun finishes the job.
Keeping a greenhouse cool comes down to three moves done in the right order: let the heat out, keep some of it from coming in, and add moisture to the air. This guide gives you the numbers that decide each one, a heat-day routine you can run in ten minutes, and the one ventilation rule most greenhouses quietly fail.
How hot is too hot?
Aim to keep a growing greenhouse below 25 to 27°C. The RHS puts the damage line at roughly 27°C: above it, plant tissue starts to suffer and growth stalls. For fruiting crops the threshold is sharper. Tomatoes need daytime temperatures below about 32°C and nights below 21°C to set fruit, and pollen viability starts dropping from around 30°C, so the flowers open and then fall without forming anything.
Here is what the common greenhouse crops will tolerate before they stop working for you.
| Crop | Happy range (day) | Trouble begins | First sign |
|---|---|---|---|
| Tomatoes | 21 to 27°C | Above 30°C | Flowers drop without setting |
| Cucumbers | 21 to 28°C | Above 32°C | Bitter fruit, wilting at midday |
| Peppers and chillies | 21 to 28°C | Above 32°C | Flower drop |
| Seedlings (most) | 15 to 22°C | Above 25°C | Leggy, scorched, dried out by noon |
Buy a maximum and minimum thermometer before you buy anything else. It costs a few pounds and tells you the one thing guesswork cannot: how hot the greenhouse actually got while you were at work.
Ventilation: the 20% rule most greenhouses fail
This move does most of the work, and it catches cheap greenhouses out. The RHS guidance is specific: your vent area should equal about 20% of the floor area, which is one square metre of roof vent for every five square metres of floor. At that ratio the greenhouse changes its entire volume of air roughly once every two minutes on a breezy day. Most mass-market greenhouses ship with a single small roof vent that comes nowhere near 20%, which is why they cook.
Roof vents matter more than side vents. Warm air rises and leaves through the ridge while cooler air is drawn in low down, the same chimney effect that pulls smoke up a flue. Side louvres help feed that low intake, so the best result comes from having both: heat out at the top, fresh air in at the bottom. On a small greenhouse the door does a lot of this work, which is why the RHS notes it partly makes up for a shortage of roof vents.
If you are out all day from April onwards, fit an automatic vent opener. A wax-piston opener needs no power: the wax expands as the air warms, starts lifting the vent at around 16°C, and has it fully open by about 25°C. It takes fifteen to twenty minutes to react, so it will not catch a sudden spike, but it means the vent is already open before you would have got home to do it yourself.
Shading: paint, netting or blinds
Ventilation removes heat that has arrived. Shading stops some of it arriving, and on a south-facing greenhouse in July that difference earns you several degrees. The three options trade cost against control.
| Method | Cost | Control | Best for |
|---|---|---|---|
| Shade paint (wash) | Low | None once on, until you wash it off | Hot fixed seasons; brush on in late spring, off in autumn |
| Shade netting / mesh | Low to medium | Drape on and off by hand | Changeable British summers; cheaper than blinds |
| Internal blinds | Higher | Adjust daily | Growers who are home to manage them |
Shade paint works hardest for anyone out all week: you accept slightly less light all summer in exchange for never having to think about it. Skip heavy shading altogether if your greenhouse is north-facing or already loses afternoon light to a fence or tree, because then the heat was never your main problem and you need every photon you can get.
Damping down: the free trick that buys you a few degrees
On a hot afternoon, wet the floor. Splashing water across paths and any hard standing, called damping down, raises the humidity and pulls the temperature down as the water evaporates. The RHS suggests doing it at least three times a day in sunny weather. It costs nothing but a watering can and two minutes, and it does double duty: the damp air also makes life harder for red spider mite, which thrives in the hot dry conditions an unwatered greenhouse creates.
The body test is simpler than any schedule. If you walk in and the air feels dry and still and warm on your face, the floor is dry and the vents are not coping. Wet the path, open everything, and come back in twenty minutes.
Your heat-day routine
When the forecast says warm and bright, the whole job takes about ten minutes split across the day.
- Before work, open the roof vent and the door fully. Do it early, ahead of the climb, rather than after the greenhouse has already heated.
- At midday, if you are home, damp down the floor, check the max-min thermometer, and drape shade netting once the reading nears 27°C.
- Through the afternoon, damp down again on the hottest days, when tomato flowers face the greatest risk.
- In the evening, close the vents only once the air has cooled, usually well after six. Close early and you trap the afternoon's heat overnight.
- Climbs past 38°C by early afternoon
- Well above the 27°C line where most crops suffer
- Tomato flowers stop setting, seedlings scorch
- Vents and door open early, before the climb
- Floor damped down at midday and afternoon
- Stays near 26°C, vents closed only once cool after six
If you cannot be there for the midday steps, the automatic opener and a coat of shade paint cover the two you will miss. That combination is what keeps a greenhouse survivable for people who work weekdays.
How the greenhouse itself helps or hurts
Some of this is down to how the greenhouse was built, before you touch a vent. A structure with cross-ventilation designed in, an opening at high level and another low down on the opposite side, vents itself far better than one with a single token roof flap. Our NORDIC timber greenhouses include a rear opening window directly opposite the door as standard, so the through-draught runs the length of the house without you propping anything open. For growers who are out all day, a wax-piston SmartVent auto-opener is available as an optional upgrade.
From Waldenhaus: the NORDIC wooden greenhouse range is built with cross-ventilation as standard, a rear opening window opposite the door, and screw-fixed 4mm twin-wall polycarbonate. Five sizes, 8x6 ft to 8x20 ft. The advice above works for any greenhouse you own; this is simply how we have set ours up to handle a British summer.
Greenhouse cooling questions
What temperature is too hot for a greenhouse?
Above about 27°C most crops start to suffer, which is the RHS damage line. Fruiting plants are fussier: tomato flowers stop setting and pollen loses viability from around 30°C, so a greenhouse that regularly passes 30°C in summer will drop blossom rather than fruit.
Do I really need an automatic vent opener?
If you are out on warm weekdays, yes. A wax-piston opener needs no power, starts lifting the vent at around 16°C and is fully open by about 25°C, so it does the morning job you would otherwise miss. If you are home to open up early every day, manual venting plus damping down covers it.
Does shade paint cut too much light?
It trades a little light for several degrees of heat, which is the right trade on a south-facing greenhouse in high summer and the wrong one on a shaded or north-facing site. Brush it on in late spring, wash it off in autumn when the light is worth more than the cooling.
What to do this week
Hang a maximum and minimum thermometer today and read it tomorrow evening: that single number tells you whether you have a problem at all. If it shows over 27°C, open the roof vent and door first thing each morning, damp the floor down on bright afternoons, and add shade netting or a wash of shade paint before the next hot spell. Get those right and the same glass that overheats in May will be keeping tomatoes setting fruit well into September. For the next job in the calendar, see our guide to growing tomatoes in a UK greenhouse, and the RHS ventilation and shading notes for the full horticultural detail.
In a larger walk-in such as the SteelRoot steel-arch greenhouse, ridge and side venting together move more hot air than a single roof vent ever can.
What to do as the thermometer climbs: a heat decision ladder
A max-min thermometer at plant height turns guesswork into a clear sequence. The RHS treats roughly 25 to 27°C as the comfortable ceiling for most greenhouse crops, with plant damage building once readings push past about 27°C. Use the table below as a ladder: act at each rung before the next one arrives, because every degree you hold back early is a degree you do not have to fight at midday.
| Reading at plant height | What is happening | What to do now |
|---|---|---|
| Up to 25°C | Comfort zone for tomatoes, peppers and cucumbers. | Vents and door cracked open for airflow. Water in the morning. No intervention needed. |
| 25 to 27°C | The RHS comfort ceiling. Growth still fine, but the margin is thin. | Open every vent and the door fully for a through-draught. Confirm shading is already on the glass, not applied in a panic at this point. |
| Around 27°C and above | The point where the RHS says plant damage can start. Heat stress builds. | Damp down the floor and benches: the RHS advises at least three times a day, and the evaporation helps cool the surrounding air. Water the root zone, never the foliage in strong sun. |
| Low 30s°C | Once daytime highs reach the low 30s°C, tomato pollen turns sticky and non-viable, so flowers drop without setting fruit. Peppers can shed flowers and cucumbers wilt. | Damp down again. Move seed trays, cuttings and young plants to a shaded bench, cold frame or sheltered spot outside. Run a fan if you have power. |
| Above 35°C | Most crops are now losing the day, whatever else you do. | Accept some loss. Keep roots soaked, keep damping down, and check vulnerable plants again in two hours rather than chasing the reading. |
The verdict: get shading on the glass before the season turns hot and open vents the moment you reach about 27°C, because once readings climb into the low 30s°C you are managing damage instead of preventing it.
