In spring a greenhouse is your best friend; in high summer it is almost your enemy. On a clear July day, when it is 28 degrees outside, the temperature inside a closed box of glass or film quickly climbs to 45 degrees and more. For a Tomato 'Berner Rose' or a slicing cucumber that is not warmth they enjoy; it is stress that drops flowers, splits fruit and forces the plant into a midday standstill.
The good news: you do not need expensive technology, just an understanding of three things that work together, namely venting, shading and moisture. Anyone who masters these three levers from late May to early September harvests twice as long and twice as healthily from the same little house. This article shows you how to carry your greenhouse through the hot weeks from CW 22 to CW 35.
Auf einen Blick
- From CW 22 every degree counts: at outside temperatures above 25 degrees it can rise past 45 degrees under glass without venting. That is the threshold at which tomatoes drop their flowers.
- The chimney effect is your strongest ally: a roof vent up top plus a door or side vent down low draw the heat out by themselves, with no electricity at all.
- Open the vents early in the morning, by 8 at the latest, not once it is already hot. An automatic vent opener takes that decision off your hands.
- Shading lowers the peak temperature by 5 to 10 degrees. A shade net, shade paint or a climbing plant on the south side is often enough.
- Wipe the floor damp in the morning: evaporative cooling lowers the temperature and calms spider mites, which love dry heat.
Why a greenhouse turns into an oven in summer
Glass and film let short-wave sunlight in but hold back the long-wave heat radiation given off by the warmed soil. That is exactly the greenhouse effect: welcome in spring, but in high summer it works against you. The trapped air heats up, and because it cannot escape, the heat builds.
It becomes critical for most fruiting vegetables beyond 30 degrees. From about 35 degrees tomatoes largely shut down photosynthesis and, under lasting heat, drop their flowers because the pollen turns infertile; cucumbers let their leaves hang and produce bitter fruit. If the hot air also stands still, the little house feels like a sauna, and fungal diseases love exactly that climate.
The chimney effect: free and effective
The most important physical trick is called the chimney effect. Warm air is lighter than cool air and rises; if you create an opening up at the roof, it escapes there. At the same time cooler air has to flow in below, otherwise a partial vacuum forms and the draught breaks off. That is why the iron rule holds: always open top and bottom at the same time.
In practice that means: roof vent wide open, plus the door or a side vent near the ground. A gentle, steady flow forms that draws cool air in at floor level and pushes the hot air out at the ridge. It does not just cool; it also exchanges the humid, stale air and denies fungal spores their footing. A house with only one roof vent and a closed door barely ventilates; with an open door as well it airs itself almost on its own.
Do not fling the door wide open when it is windy outside. A strong gust snaps tall tomatoes and dries out the soil. In wind, a door gap of 20 to 30 cm plus open roof vents is plenty.
When to vent: early, not late
The most common mistake is to vent only once it is already hot; then you chase the temperature all day long. Better: open the vents in the morning as soon as the sun hits the house, so on clear days by 7 to 8 at the latest. That way you keep the heat from ever building.
The evening timing matters just as much. On the cool nights from CW 22 to mid-June you are better off closing the roof vents partway so that heat-loving cucumbers stay warm; on the warm nights of July and August they can stay open and pre-cool the house for the next day.
If you work during the day and cannot keep looking in on the greenhouse, an automatic vent opener is the best investment under 30 euros. It needs no electricity: inside the cylinder sits a special wax that expands when warm and lifts the vent via a piston; when it cools, it closes again. Most models open from about 18 to 20 degrees and stand fully open at 25 degrees, so your little house stays in the green zone even without you.
Shading: the second lever
On the hottest days venting alone is not enough. When the sun beats straight down on the glass, you have to keep part of the radiation out in the first place. Shading lowers the peak temperature by 5 to 10 degrees depending on the method and protects sensitive leaves from scorch.
- Shade netGreen or white net with 30 to 50 percent shading, stretched over the roof on the outside. Outside works better than inside because the heat never gets through the glass, and on dull days you roll it back easily.
- Shade paintA chalky wash you brush onto the outside of the glass in June. It lasts all summer, slowly fades in the rain and washes off in September. Cheap for large areas.
- Living shadeA climbing plant such as nasturtium or a runner bean on the south side gives natural shade and draws in pollinators. Charming, but less controllable than a net.
A fine point: do not shade the whole house completely and permanently, because tomatoes and peppers need light for sugar and aroma. The aim is to break the midday peak, not to darken the house. A net with 30 to 40 percent is usually the best compromise for fruiting vegetables; only for lettuces and young plants may it be denser.
Evaporative cooling and humidity
There is a third lever many people overlook: water that evaporates draws heat from the air, the same way our body cools itself by sweating. In the greenhouse you use this by wiping the floor and paths damp in the morning or wetting them with the watering can. In short: wet paths in the morning, a cooler house at noon.
There is a second benefit. Spider mites, the tiny sap-suckers that blanket whole cucumber stands in high summer, love dry, hot air; a moist climate in the morning slows them noticeably. But do not overdo it: constant wetness overnight in a closed house encourages fungi such as late blight. So the rule is: damp in the morning, dry in the evening, airy at night.
Put a tub of water in the greenhouse. During the day it steadily evaporates a little water and buffers the humidity; at night the mass of water releases stored warmth again and dampens the swings between day and night.
Watering in a hot greenhouse
At 40 degrees under glass an enormous amount of water evaporates, and the roots work at full tilt. Water thoroughly once rather than shallowly three times; that sends the roots deep and makes them more heat-tolerant. The best moment is early morning, so the plant is ready for the day and the leaves dry off by evening.
Always water at the root, never over the foliage: wet leaves in full sun act like little burning glasses and encourage fungi. A mulch layer of grass clippings or straw keeps moisture in the soil and dampens the soil temperature. How to water sparingly yet effectively in the heat is covered in detail in the article Watering wisely in summer.
Solanum lycopersicum
- Standort
- Full sun, but midday shade above 35 degrees
- Saison
- Harvest CW 28 to CW 42
- Wasserbedarf
- Evenly, in the morning at the root, never over the foliage
- Frosthärte
- Frost-sensitive, do not plant out below 10 degrees
Cucumis sativus
- Standort
- Warm and sheltered, tolerates more humidity than the tomato
- Saison
- Harvest CW 26 to CW 39
- Wasserbedarf
- Plenty and regular, tolerates no drought
- Frosthärte
- Very heat-hungry, cold below 12 degrees slows it
Your routine for a hot day
On a forecast 30-degree day a fixed routine pays off, so your little house stays in the green zone without constant thought.
Open early
Before 8, all roof vents wide open, door and side vents too. It is still cool, but you get ahead of the heat.Wet the floor
Wipe or water the paths and beds damp. The evaporation cools through the morning and keeps spider mites in check.Check the shading
Is the shade net stretched or the shade paint fresh? With over 30 degrees forecast, fix it now rather than at midday.Check at noon
Glance at the temperature. If the air stands still despite open vents, a simple fan keeps it moving.Decide in the evening
On warm nights leave the vents open. On cool nights early in the season half-close the roof vents so the cucumbers stay warm.
The seasonal roadmap from CW 22 to CW 35
Not every week in summer is the same. Roughly, you can orient yourself around three phases.
CW 22 to CW 25 (late May to mid-June): the days turn hot while the nights often stay cool. Vent consistently by day, close the vents in the evening, prepare the shading. Now is the right moment to fit the automatic vent opener.
CW 26 to CW 31 (late June to early August): high summer, the full programme: open early, shade, dampen the floor in the morning. In the hottest weeks the vents may often stay open around the clock. Watch especially for spider mites and blossom-end rot on tomatoes, both consequences of heat and uneven watering.
CW 32 to CW 35 (mid-August to early September): the peak heat eases, the nights turn cooler again. Reduce the shading so the last fruit still gets light to ripen, and wash off the shade paint in late August. Now you also plan the autumn planting; how to make the most of your house across the year is set out in the greenhouse year plan for planting.
The frost-free season ends inside the greenhouse too. An unheated house protects against the outside temperature by only a few degrees in September and October. The first cold night can arrive as early as late September. That is no drama, just your cue to bring heat-loving potted plants in on time and to pick the last green tomatoes to ripen indoors.
When it stays too hot anyway
If open vents and a net are not enough, for example in small film greenhouses without roof vents, a few tricks help: a pedestal fan keeps the air moving, a light tarpaulin on the south side reflects radiation, and in an emergency you fling the door wide open and lay wet cloths over sensitive young plants. Houses with no opening at all you run on hot days only with the front open, otherwise you cook your plants inside.
A greenhouse in summer does not run itself, but it is no science either. Anyone who opens the vents early in the morning, breaks the midday sun with a net and keeps the floor moist already has the three most important levers in hand. The rest is observation: wilting plants tell you more reliably than any thermometer that your climate is not right yet.
Häufige Fragen
How hot may it get in the greenhouse at most?
Should I leave the greenhouse door open at night?
Is a single roof vent enough for ventilation?
Which is better: shade net or shade paint?
Why do my greenhouse tomatoes set no fruit despite plenty of flowers?
Spotted a mistake?
