A greenhouse can feel pleasantly warm when you step inside, but on a bright spring or summer day it can become too hot for plants surprisingly quickly. Knowing how to ventilate a greenhouse is one of the simplest ways to avoid scorched leaves, poor pollination, fungal problems and weak growth. The aim is not to keep it warm at all times. It is to keep fresh air moving while maintaining conditions your plants can cope with.
UK weather makes this a balancing act. A chilly morning can turn into a hot afternoon, particularly in a small glazed greenhouse or polytunnel. The right setup depends on its size, where it stands, what you are growing and how often you are at home to adjust vents and doors.
Why greenhouse ventilation matters
Ventilation replaces stale, humid air with fresher outside air. This helps control temperature, reduces condensation and gives plants a steadier supply of carbon dioxide for healthy growth. It also makes a real difference when plants are flowering. Tomatoes, chillies, peppers and aubergines are more likely to pollinate well when warm air is circulating gently around them.
Without enough ventilation, humidity builds up after watering and overnight. Damp leaves and still air create the conditions that fungal diseases enjoy, including botrytis, mildew and damping off in young seedlings. Excessive heat causes a different problem: plants may stop growing, drop flowers or develop pale, scorched patches.
A little airflow is useful even in cooler months, although it needs to be managed carefully. A greenhouse that is completely sealed for days can stay damp enough to cause trouble, especially if it is crowded with overwintering plants.
How to ventilate a greenhouse effectively
The most effective approach is to create a route for air to enter low down and leave higher up. Warm air naturally rises, so roof vents allow heat to escape while open doors, louvre vents or low-level openings draw cooler air in. This is called natural ventilation, and it works well for most home greenhouses.
Use roof vents and side vents together
Roof vents should be your first priority because heat collects at the highest point of the greenhouse. As a general guide, aim for roof vent area equal to around 10 per cent of the floor area where possible. A small greenhouse may only have one roof vent, but larger structures benefit from several vents spread across the roofline.
Side louvre vents add lower-level airflow and are especially useful when the door cannot be left open. They are a sensible feature for gardeners who are out during the day, as they can be fitted with automatic openers. In a compact lean-to greenhouse, a roof vent combined with a louvre vent can make a noticeable difference without taking up growing space.
For the best airflow, open roof and lower vents together. Opening only a roof vent releases some heat but may not draw in enough fresh air. Opening only a door can leave hot, humid air trapped near the roof.
Open the door on warm days
On mild and warm days, opening the greenhouse door provides a large, immediate source of air movement. It is particularly useful for walk-in greenhouses and polytunnels, where the doorway is often the biggest opening available.
There is a trade-off. An open door can invite in pests, catch the wind and make the greenhouse colder very quickly if the weather changes. If your greenhouse is exposed, secure the door properly so it cannot slam in gusts. A mesh door or insect netting can be worthwhile for protecting plants while still allowing air to pass through.
In summer, it is often sensible to open doors and vents early, before the greenhouse has heated up. Waiting until the air feels uncomfortably hot means plants may already have spent hours under stress.
Add automatic vent openers for everyday convenience
Automatic roof-vent openers are a practical upgrade for gardeners who work away from home or cannot check the greenhouse several times a day. They use a wax-filled cylinder that expands as the temperature rises, lifting the vent without electricity or batteries. As the greenhouse cools, the vent closes again.
They are not a replacement for good ventilation design. An automatic opener cannot compensate for too few vents, a poorly positioned greenhouse or a door that is never opened in a heatwave. However, it offers useful protection against sudden sunny spells and removes some of the daily guesswork.
Check the opener is compatible with your vent and can carry its weight. It should be cleaned and checked at the start of each growing season, as stiff hinges and dirty mechanisms can prevent it opening fully.
Use a fan when natural airflow is not enough
Fans are most useful in still, enclosed spaces or in larger greenhouses packed with plants. A small circulation fan does not necessarily lower the temperature, but it moves air around foliage, helps leaves dry after watering and reduces stagnant corners. An extractor fan actively pushes hot air out, so it is a stronger option for larger greenhouses where summer overheating is a regular issue.
For a modest garden greenhouse, natural ventilation plus shading is usually better value than fitting powered extraction. A fan becomes more worthwhile if you grow throughout the year, raise lots of seedlings or have limited vent space. Any electrical equipment must be suitable for outdoor or damp conditions and connected safely.
Control heat with shading as well as airflow
Ventilation is far more effective when direct sun is reduced. Greenhouse shading does not make a poor ventilation setup good, but it can stop temperatures reaching damaging levels in the first place.
External shade netting is often the most effective option because it blocks sunlight before it passes through the glazing. Internal blinds and shade netting are easier to fit in some greenhouses but allow heat to enter first. Shade paint is an affordable seasonal solution for glass, though it needs washing off when light levels drop in autumn.
For many UK gardens, 30 to 40 per cent shade is enough for summer vegetables and young plants. Heavier shading can be useful for more delicate crops, but too much shade may reduce growth and fruiting. In a polytunnel, choose shading that can be rolled back or removed as the weather changes.
Ventilation through the seasons
Spring
Spring weather is changeable, so start by opening roof vents or a door slightly on sunny mornings. Close them before a cold evening, particularly if tender plants are inside. On bright days, a greenhouse can overheat even when the outdoor temperature feels cool.
Summer
This is when full ventilation matters most. Open roof vents, side vents and doors early in the day, then use shading during prolonged hot spells. Water plants at the base rather than spraying foliage, and avoid overfilling the greenhouse. Plants need space between pots for air to move.
If temperatures are very high, dampening paths or staging can briefly increase humidity and make the air feel cooler, but do not soak the structure late in the day. Prolonged dampness encourages disease.
Autumn and winter
Openings should be smaller and shorter in colder months, but do not keep the greenhouse sealed indefinitely. On a dry, relatively mild day, open a vent or door for a short period to clear humid air. This is particularly helpful after watering or when there is visible condensation.
Avoid leaving vents open in strong winds, heavy rain or frost. If you are overwintering plants, prioritise a dry atmosphere over a warm but damp one.
Positioning and layout make ventilation easier
Where you place a greenhouse affects how well it performs. A sheltered but not enclosed location is usually best. Trees, fences and sheds can block cooling breezes, while a completely exposed position may make doors and vents difficult to use safely in windy weather.
Inside, avoid creating a solid wall of foliage. Leave small gaps between pots, keep staging tidy and remove yellowing leaves promptly. Tall plants such as tomatoes should not block the only louvre vent or doorway. Water early in the day where possible, giving surfaces time to dry before evening.
When choosing from a range of greenhouses or polytunnels, look beyond the footprint. Consider the number and position of vents, whether a louvre vent can be added, the width of the door and whether automatic openers are available. A slightly larger model with better airflow can be better value than a tightly packed smaller one.
Common ventilation mistakes to avoid
The most common mistake is relying on one opening. A single roof vent, opened late, cannot shift enough hot air from a greenhouse full of plants. Another is treating the temperature as the only concern. If glass is dripping with condensation in cool weather, humidity needs attention too.
It is also easy to overcorrect. Leaving every opening wide open during a cold, windy spell can chill tender plants and damage lightweight covers. Check conditions rather than following a fixed rule. A thermometer with maximum and minimum readings is a low-cost tool that shows how hot or cold the greenhouse becomes when you are not there.
Finally, do not assume ventilation will fix every issue. Plants can still overheat if they are too close to glazing, under-watered or exposed to intense afternoon sun. Move sensitive seedlings away from the hottest spots and use shade where needed.
Frequently asked questions
What temperature should I ventilate a greenhouse at?
For many common greenhouse crops, begin opening vents when temperatures reach around 20 to 25°C. Tomatoes and peppers enjoy warmth, but prolonged temperatures above 30°C can affect flowering and fruit set. Seedlings and leafy plants often need ventilation sooner.
Should I leave my greenhouse door open at night?
Usually, no. In the UK, temperatures can fall quickly after sunset, even after a warm day. Close the door in the evening, while leaving a roof vent slightly open only if the night is mild and humidity is high.
Can I ventilate a small greenhouse without a fan?
Yes. A roof vent, door opening and sensible shading are often enough for a small greenhouse. If it has no built-in vents, open the door regularly and avoid placing it in full afternoon sun without shade. A larger model with proper vents may be the more practical long-term choice.
A well-ventilated greenhouse should feel fresh rather than stifling, with leaves that dry quickly after watering and plants that have room to grow. Start with vents and doors, add shade when the sun is strong, and adjust your routine as the seasons change. Those small daily checks can protect months of growing.