How to control greenhouse temperature

What Greenhouse Temperature Control Actually Involves

Greenhouse temperature control is the practice of managing heat at both ends of the scale: limiting overheating in the warmer months and retaining enough warmth to protect plants when temperatures fall. The tools split broadly into ventilation and shading for cooling, and insulation and heating for warmth. Monitoring sits underneath both: without knowing what the temperature is actually doing, the other decisions are guesswork.

Most gardeners encounter the overheating problem first, often before they have had the chance to set anything up properly.

How to Cool a Greenhouse When Temperatures Rise

Ventilation first

Hot air rises. Roof vents give that rising air somewhere to go, drawing cooler air in lower down and creating a through-flow that takes the edge off peak temperatures. The Royal Horticultural Society advises that roof vent area equivalent to roughly one fifth of the floor space provides approximately one complete air change every two minutes. Smaller greenhouses heat up faster because they have a higher glass-to-floor ratio, so they benefit from as much roof ventilation as possible.

The practical difficulty is that most gardeners are not standing next to the greenhouse on a warm Tuesday morning. An auto vent opener responds directly to rising temperature and lifts the roof vent without any input from you. It works through a wax-filled cylinder that expands as it heats up, so no wiring is needed. One limitation worth knowing: the wax takes a few minutes to react, so temperatures can spike briefly before the vent is fully open.

Adding a louvre vent lower down reduces how much heat builds before the opener responds, and gives you genuine cross-ventilation rather than a single escape point at the top.

Greenhouse ventilation accessories

Shading

Ventilation alone is not always enough on a very bright day. Direct sun through glass generates radiant heat that moving air cannot fully counteract.

Internal blinds reduce the amount of solar radiation reaching plants and moderate peak temperatures. External shading is more effective in principle, but harder to manage around vent operation and more vulnerable to weather damage. For most hobby greenhouse owners, internal blinds are the more practical option.

Shading becomes particularly relevant for lettuces, herbs, and other crops that bolt or suffer leaf scorch under intense summer light.

Shade paint applied directly to the outside of the glass is a lower-cost alternative. It reduces solar gain before light enters the structure and can be diluted to adjust coverage as the season intensifies. Most shade paints wash off in autumn, which is worth confirming on the product label before buying.

Greenhouse shading blinds

Damping down

Wetting the floor and staging cools the air through evaporation and raises humidity at the same time, which slows water loss through plant leaves. Early morning is the best time to do it. A second damp down at midday helps on the hottest days.

It is low-tech and effective. The limitation is straightforward: you need to be there. On a hot weekday when you are away, damping down cannot help.

Keeping plants adequately watered on hot days matters for a related reason. Plants release moisture through their leaves, and that process draws heat away from the leaf surface in the same way we cool down when we sweat. A plant that has run short of water loses this cooling effect, which is why well-watered plants in a hot greenhouse cope noticeably better than stressed ones.

If leaves are wilting first thing in the morning before temperatures have built, dry roots are often the cause rather than the air temperature itself.

Keeping the Heat In When Temperatures Drop

Most tender plants need a minimum of around 5°C to avoid frost injury, though the threshold varies considerably by species. The key risk period runs from October through to April, with late spring frosts often catching gardeners off guard after plants have been moved back in.

A thermostatically controlled electric heater, set to trigger at a low target temperature such as 3°C or 5°C, uses considerably less energy than heating to a higher level and avoids unnecessary warmth on milder nights. Bubble wrap fixed to the inside of the glass reduces heat loss noticeably without blocking too much light. It is particularly effective on the north-facing end of the greenhouse, where little useful light comes through in winter anyway.

The two main electric options are tube heaters and fan heaters. Tube heaters run at low wattage along the base of the greenhouse wall and are well-suited to background frost protection: they use less energy and run quietly. Fan heaters move air around the space as they heat it, which reduces cold corners and gives more even temperature distribution.

For a greenhouse used mainly to keep tender plants frost-free through winter, a tube heater is typically sufficient. For one that is heated more regularly or contains plants with varying requirements, a fan heater gives better overall control.

Grouping plants together towards the centre of the space rather than against the glass helps too. The glass is always the coldest surface, and plants positioned directly against it are the first to suffer on a frosty night.

Greenhouse heating accessories

How to Set Up Basic Temperature Control in a New Greenhouse

The order in which you add things matters as much as what you add. This sequence works for most hobby greenhouses in the UK.

1. Fit a minimum-maximum thermometer before anything else. You need a baseline reading before any other decision makes sense. Place it at plant height in the centre of the greenhouse, away from the glass and away from any heater.

2. Add an auto vent opener to the roof vent. This handles the most common risk: daytime overheating when you are not present. It requires no wiring and most models take under an hour to fit.

3. Add low-level airflow. A louvre vent on the wall opposite the prevailing wind creates cross-ventilation. If you do not want a second vent, opening the door achieves something similar on hot days when you are home.

4. Fit shading blinds once you have seen where the hottest light falls. Observe the greenhouse through June before committing to a shading arrangement. South and west-facing glass tends to drive the most heat in UK conditions.

5. Add a heater only if you grow plants that cannot tolerate frost. Set it to come on at 3°C to 5°C, not to maintain a comfortable working temperature. A heater running at that threshold uses considerably less energy than one set to 10°C.

Full Halls Greenhouses accessories range

Common Mistakes to Avoid

Relying on a single roof vent

One vent at the top lets hot air out but does not draw cool air in effectively. Without low-level airflow, the temperature at plant height stays much higher than the reading suggests.

Fitting blinds that block the vent opening

Blinds that run all the way across the roof restrict airflow at the same time as they shade the glass. Position them below the vent opening so air can still escape freely.

Setting the heater too high

A heater running at 10°C or above creates unnecessary overnight humidity and costs significantly more to run. Most frost protection only requires 3°C to 5°C, and setting it higher does not improve plant health.

Only checking the thermometer during the day

The overnight minimum is the reading that tells you whether frost damage is likely. Daytime highs tend to look reassuring. It is the early morning low that catches people out.

Monitoring: The Step Most Gardeners Skip

A minimum-maximum thermometer records the highest and lowest temperatures since you last checked it. The overnight low is often a more useful reading than the daytime figure. Many plants can tolerate heat above their ideal range for a few hours, but a night temperature below their minimum does lasting damage that shows up days later and is frequently misdiagnosed as overwatering or disease.

Thermometer placement matters more than most gardeners realise. The RHS advises placing a min-max thermometer at plant height in the centre of the greenhouse, away from the heater itself, since thermostat dials located near the heat source can read several degrees higher than the temperature the plants are actually experiencing.

Digital models with wireless displays show the internal temperature without requiring a trip outside. For anyone growing crops that need close management, or who travels regularly, a model that sends temperature alerts to a phone removes a significant source of uncertainty.

Greenhouse care and maintenance guide

Greenhouse Temperature FAQs