Keeping a Greenhouse Cool and Watered in Summer

A greenhouse is brilliant in spring and autumn, when its warmth gives your crops a head start and a long, gentle finish. In high summer, though, the very thing that made it useful turns against you. On a bright July day an unmanaged greenhouse can climb past 40°C inside, and that heat — together with the rapid drying out that comes with it — is what most often wrecks a UK summer crop.

The good news is that both problems have simple, cheap fixes. Master ventilation, shading, damping down and watering and your greenhouse will keep cropping right through a heatwave. This guide walks through each in turn. For the full picture of growing under glass across the year, start with our guide to greenhouse growing.

Why greenhouses overheat (and the damage it does)

Glass lets sunlight in, the plants and surfaces inside absorb it and re-radiate it as heat, and the closed structure traps that warm air. On a sunny day the temperature inside rises far faster than outside, and without intervention it keeps climbing. Most summer crops — tomatoes, cucumbers, peppers, chillies, aubergines — are happiest somewhere around 21–27°C. Push much above 30°C and things start to go wrong.

The damage shows up in several ways:

• Blossom end rot. That sunken, leathery brown patch on the base of tomatoes and peppers is caused by calcium not reaching the fruit — and it is driven by erratic watering, which heat makes far worse as pots dry out between waterings.
• Poor fruit set. Tomato flowers stop setting reliably above about 30°C, and pollen can become sterile in extreme heat. You get flowers, then bare trusses with no fruit.
• Leaf scorch. Intense, trapped sun can bleach and crisp leaves, and scald the shoulders of ripening fruit.
• Red spider mite. This tiny sap-sucking pest thrives in hot, dry, still air — exactly the conditions an unventilated summer greenhouse provides. You'll see fine pale stippling on leaves, then faint webbing, then collapse. It is the classic sign of a greenhouse run too hot and dry.

Every one of those is preventable. The four sections below are your toolkit.

Ventilation: the first and most important step

Moving air out of the way of incoming heat is your single best tool, and it costs nothing. Aim to get as much hot air out — and cool air in — as you can.

Open everything, early. On any warm day, open the roof vents, any side vents and the door first thing in the morning, before the heat builds. Hot air rises and escapes through roof vents while cooler air is drawn in low down, creating a constant flow. A greenhouse with only roof vents and a door is working with one hand tied behind its back; side or louvre vents make a real difference.

Don't forget to close up again. UK nights even in summer can be cool, so close the door and vents in the evening to hold a little warmth for ripening — unless the night is genuinely sultry.

Automatic vent openers are the best few pounds you'll spend. These are simple cylinders filled with a wax that expands as it warms, pushing the vent open with no electricity needed, then closing it as things cool. Fit one to each roof vent (and louvre vents too if you have them) and your greenhouse manages its own temperature while you're at work or away. They typically start opening around 15–20°C and are fully open by the mid-20s.

Shading: taking the edge off the sun

Once daytime temperatures are reliably warm — usually from late May through to early September in most of the UK — ventilation alone often isn't enough on the brightest days. Shading cuts the intensity of the sun before it ever becomes heat inside. You're aiming to reduce, not eliminate, light: roughly 30–50% shade is plenty for fruiting crops.

You have three main options:

• Shade paint. A milky liquid (sold as Coolglass and similar) that you dilute and brush or sponge onto the outside of the glass. It's cheap, lasts the season, lets light through while diffusing the heat, and conveniently washes off rain by rain through autumn or with a wipe. Apply it in May; it's the lowest-effort choice for a fixed greenhouse.
• Shade netting. Knitted mesh that drapes over the outside or clips inside. It's reusable year after year and easy to take down on a dull week, though fitting it neatly takes a bit more fiddling than paint.
• Blinds. Roller or pinoleum (thin wooden slat) blinds fitted inside or out give you the most control — roll them down in a heatwave, up when it's grey. They're the priciest option but the most flexible.

Damping down: cooling and humidity in one job

Damping down means wetting the floor and staging inside the greenhouse — typically with a watering can or a hose on a fine rose — on hot mornings, and again at midday in a real heatwave. It's an old technique that does several useful things at once.

As the water evaporates it cools the air (the same way sweat cools you) and raises humidity. That humidity matters: it helps fruit set on tomatoes by stopping pollen from drying out, eases the moisture stress that leads to blossom end rot, and — crucially — makes life miserable for red spider mite, which can only get going in hot, dry conditions. A greenhouse that is damped down regularly rarely gets a bad mite infestation.

Wet hard surfaces — the path, brick or concrete floor, staging — rather than soaking the plants' foliage, which can encourage fungal problems. A solid floor holds and releases the moisture slowly; a bare-earth floor works well too. Damping down in the morning sets the greenhouse up for the day; a second go around noon is worth it when it's truly baking.

Watering: the daily summer job

Under glass, in summer, plants drink hard and compost dries out astonishingly fast. This is the task that catches new greenhouse growers out, because a pot or growbag that was fine at breakfast can be bone dry and wilting by mid-afternoon.

How often? Expect to water every day in summer, and twice a day on hot, bright spells for anything in pots or growbags. Plants in a greenhouse border, with their roots in a larger body of soil, are more forgiving and may manage every other day. Check first thing each morning and water before plants show stress — wilting is already a setback. A finger pushed into the compost tells you more than the surface, which dries first.

Water steadily and consistently. Erratic watering — letting plants dry right out, then drenching them — is the direct cause of blossom end rot and of split tomatoes and cucumbers. Aim for the same amount at the same time each day. Water the compost at the base of the plant, not the leaves, and water in the morning so foliage dries before night. For more detail on getting this right with tomatoes specifically, see our guide to watering and feeding tomatoes and the greenhouse tomatoes guide.

Automatic and self-watering systems take the daily pressure off and keep watering even — which your plants love anyway. A few options, cheapest first:

• Capillary matting. Lay this absorbent matting on the staging with one end trailing into a water reservoir or kept wet by hand. Stand pots on it and they wick up moisture from below as they need it — ideal for trays of seedlings, salad pots and smaller plants.
• Drip irrigation kits. A length of pipe with adjustable drippers, one per pot or growbag, fed from the mains through a tap timer or from a raised water butt. Set the timer to run for a few minutes morning (and evening in a heatwave) and your tomatoes water themselves. These are inexpensive and transform watering for a greenhouse full of pots.
• Self-watering planters and growbag reservoirs. Containers with a built-in water reservoir that the plant draws from, smoothing out the peaks and troughs that cause fruit problems.

Holiday cover. A week away in July is the classic way to come home to a dead greenhouse. Before you go: move pots off bright staging onto capillary matting with a full reservoir, set up a drip system on a timer, group containers together out of the most intense sun, mulch the surface of pots and borders to slow evaporation, and ask a neighbour to open the vents (or, better, fit automatic openers so they don't have to). Shading the glass first also buys you time. Our greenhouse growing guide covers setting the greenhouse up for the wider season, and a cloche or two outside can keep tender plants ticking over while you're gone.

A few related jobs

Keeping a greenhouse cool and watered sits alongside the rest of summer greenhouse care — feeding fruiting crops weekly, keeping on top of side-shoots on cordon tomatoes, and watching for pests. If you're planning what to have under glass and when, the planting calendar helps you line up sowing and harvest dates for the UK. And if you're still deciding how to use the space, the getting-started hub and the main greenhouse growing guide are the place to begin.

Get ventilation, shading, damping down and watering working together and the summer greenhouse stops being a worry and goes back to being the most productive corner of the garden — tomatoes, cucumbers and chillies cropping happily right through the hottest weeks of the year.