How hot is too hot in a greenhouse?
The temperatures at which growth slows, pollination fails, ripening stops and damage begins are all different. Which one you have hit tells you how urgently to act.
- Difficulty
- beginner
- Time
- 12 min
- Read
- 4 min
Short answer
Most greenhouse crops are happiest in the low to mid twenties. Above about thirty, pollination starts failing and tomato ripening stalls. Above the mid thirties, growth stops and leaf damage begins within hours. A closed greenhouse in British summer sun can pass all of these before mid-morning, which is why vents need opening early rather than when it feels warm.
People ask this expecting one number, and the useful answer is a short ladder, because the plant fails in stages and each stage has a different consequence. Knowing which threshold you have crossed is the difference between a slightly disappointing crop and a greenhouse full of scorched plants.
Step by step
- Know the comfortable range.Broadly the low to mid twenties Celsius suits tomatoes, cucumbers, peppers and aubergines, which are the crops most British greenhouses hold in summer. In that range growth and fruit set are both good and nothing needs intervening.
- Recognise the first threshold: fruit set fails.As temperatures climb past roughly thirty degrees, and particularly if they stay high overnight, tomato and pepper pollen becomes less viable and flowers drop without setting fruit. This is the first real loss and it happens well before anything looks damaged.
- Recognise the second: ripening stalls.Around the same point, tomatoes stop producing the red pigment and fruit sits green or ripens blotchy. Again, the plant looks perfectly healthy. This has its own guide, because it is so commonly misdiagnosed.
- Recognise the third: growth stops.Into the mid thirties, photosynthesis effectively shuts down. The plant closes its stomata to conserve water, stops taking up carbon dioxide and simply idles. Nothing gains, and water use stays high.
- Recognise the fourth: direct damage.Higher still and you get scorched leaves, collapsed seedlings and cooked roots in small pots, within hours rather than days. Seedlings and anything in a small container are killed long before an established tomato is.
- Measure where it matters.A max-min thermometer at plant height, in the shade, not on the sunny side of the glass, is the tool. A thermometer in direct sun reads the sun, and one under the ridge reads far higher than the plants do. The overnight minimum matters as much as the daytime maximum for fruit set.
- Act on the forecast rather than the thermometer.A closed greenhouse gains heat far faster than it loses it, and by the time it is uncomfortable inside it is already past several thresholds. Open the vents and the door early on any morning that is forecast warm, before the sun is on the glass.
- Understand what your interventions are worth.Ventilation is the largest single lever. External shading is the second. Damping down the floor cools by evaporation and raises humidity, which helps pollen too. Moving small pots out into shade removes the most vulnerable plants entirely. Misting the plants themselves does very little.
- Watch the night temperature as well.Tomatoes and peppers set poorly if the night temperature stays high, because the plant never gets a recovery period. Leaving the vents open overnight in a hot spell is normal practice in summer, not a risk.
Tips
- A cheap max-min thermometer is the single most useful thing in a greenhouse, because it tells you what happened while you were at work — which is when the damage occurs.
- Small pots and seed trays are the first casualties by a wide margin. A tray of seedlings can be lost in an afternoon in conditions an established tomato shrugs off.
- A polytunnel behaves differently from a glasshouse — less thermal mass, faster to heat, and shading and ventilation work differently. It has its own guide.
- Automatic vent openers cost little, need no power and open before you get home. They are the highest-value thing most greenhouse owners can add.
- Humidity matters for pollination: very dry air stops pollen releasing properly, which is part of why damping down helps fruit set as well as temperature.
Common mistakes
- Waiting until the greenhouse feels hot before opening it — By then it has usually passed the fruit-set and ripening thresholds. On a sunny morning a closed greenhouse climbs extremely fast — vents open early, not on demand.
- Judging temperature from a thermometer in the sun — It reads the radiation striking it, not the air the plants are in, and can be many degrees out. Put it at plant height in the shade.
- Closing up at night in summer to keep it warm — High night temperatures work against fruit set in tomatoes and peppers. In a hot spell, leave the vents open overnight.
Questions people ask
What is the maximum temperature a greenhouse should reach?
There is no single figure, because the crop fails in stages. Practically: aim to keep it in the twenties, treat the low thirties as the point where fruit set and ripening are being lost, and treat the mid thirties and above as actively damaging, particularly to seedlings and small pots.
Is it too hot for my seedlings?
Almost certainly sooner than for anything else. Seedlings and small pots have tiny root volumes that heat and dry fast, and they can be lost in a single afternoon. Move them out into shade rather than trying to keep the whole greenhouse cool enough for them.
Should I leave the greenhouse door open at night?
In a hot spell, yes. The overnight minimum matters for fruit set, and a greenhouse that does not cool down overnight puts the plants under continuous load. Security and wind are the practical constraints, not the plants.