Summer weather, however, can be unpredictable at times, and heat waves, prolonged hot spells, or hotter-than-usual conditions can raise the temperature inside your greenhouse beyond what you or the plants growing there want.

In this article, we’ll discuss ways to moderate your greenhouse’s temperature, from increasing airflow to adding shade.

How Greenhouses Get Hot

Because greenhouses are primarily constructed from transparent materials, they let in a lot of sunlight. This sunlight is absorbed by all the materials inside the greenhouse: the ground, pots, soil, shelves, and the plants themselves. Because the heat can enter more easily than it can leave, it is an effective way to store heat.

This is generally a wanted effect. A greenhouse can stay much warmer than the outside air, which is beneficial during colder weather and damp, rainy days. A greenhouse will also become drier throughout the day as the heat evaporates moisture, which can stress plant foliage and roots if left unmanaged.

Keeping it Cool

To moderate the temperature buildup inside the greenhouse, there are three main approaches:

• Stop the sunlight from getting in
• Help the heat get out
• Increase the humidity

A combination of multiple approaches is usually best.

Blocking the Light

Plants need sunlight to grow; however, some light can still be blocked from entering through shading.

Deciduous trees are an excellent way to add shade to a greenhouse while ensuring plenty of sunlight in spring and fall. If you have space along the western side of a greenhouse, this is a cost-effective way to moderate temperatures.

Additional methods include physical barriers and shade paint.

Blinds

Blinds can be internal or external. During shadier days, they can be opened as needed, making them very flexible. There is also a lot of customization for shading percentages to match the sun conditions of your garden.

Blinds can be expensive, and exterior blinds are more effective than interior blinds. They are, however, more expensive still. If you have a permanent greenhouse structure, this may be a cost-effective option in the long term.

Shade Cloth/Netting

This can be installed on the interior or exterior, and, similar to blinds, exterior installation will be more effective at blocking heat. Even something as open as bird netting can offer up to 10% shading.

Plastic-based nettings and shade cloths can degrade quickly — often in two to three years — so they will need regular replacement. There are also natural options available with fibres like burlap and jute that can be composted once they begin to degrade.

For a short-term solution, you can repurpose old flat sheets as an exterior shade cloth. Note that these are often comparable to 80% shade cloth, though this will vary depending on the weight and weave of the fabric.

Shade Paint

There are a variety of shade paint products, and most can be diluted to suit your shading needs. Thinner applications can be used in spring, and then applied more generously in the summer. Many are water-based and are easy to clean.

Shade paint is best suited for glass greenhouses with metal frames, as the paint can be difficult to remove from timber and from plastic covers, although there are products for polymer covers. Most shade paint will need to be washed off in the fall and then reapplied in the spring. 

Generally, shade paint is very cost-effective, although if you have lots of cloudy summer days, it can slow plant growth, as it can’t be rolled up like blinds or shade cloth.

Getting the Heat Out

Preventing heat buildup is essential. If your mornings are not too cold, you can ‘supercool’ the greenhouse by opening all the doors and windows to match the external temperature.

Leaving windows, doors, and any louvres open during the day — and night, if temperatures remain high enough — can maintain a general level of consistency through most of the day.

Plenty of high points for hot air to escape and lower points for the cooler air to enter will create a continual air exchange throughout the day.

Automatic Vents

There are varying levels of automation for greenhouse venting. Manual opening and closing is simple, but not always convenient. Some automatic options have wax cylinders that expand and push open vents, and solar-powered options attached to temperature sensors are also available for greater customisation and speed — wax cylinder options can be slow to open.

Airflow Aids

Many areas either have slow summer winds that don’t draw air out of a greenhouse or are very hot, so additional help is needed.

Fans are an excellent way to increase airflow and air exchange in a greenhouse, and there are many solar options available. A fan at floor level near the door can draw in fresh, cooler air, and exhaust fans near the ridgeline can suck hot air out. Some fans can be installed in the external wall and come with temperature sensors, so they will run automatically.

If needed, be prepared to remove window panes during hot weather events or, for plastic greenhouses, roll up the sides to moderate temperatures.

Increasing the Humidity

Evaporative cooling, or swamp cooling, is an effective way to cool areas down. Because it requires energy to turn water into a gas, the local air temperature drops and the humidity rises, producing a localised cooling effect.

To make this happen in a greenhouse, you can ‘dampen down’ the floor in the morning. Simply water the floor (or parts of it if the greenhouse is quite large) until it is wet, and throughout the day, this water will evaporate and cool the air in the greenhouse. Because the air is more humid, plants will be less susceptible to heat stress as they lose less water through transpiration.

The effect is most felt when hard surfaces, such as concrete pavers, are dampened. During the height of summer, this may need to be repeated two to three times a day.

Wrapping Up

Keeping a greenhouse cooler is straightforward once you have the right combination of techniques in place. Starting simple — opening vents and dampening the floor — costs nothing and can make an immediate difference. From there, shade cloth, fans, and automatic vents can be added gradually as your needs and budget allow. A combination of approaches will always be more effective than any single method alone, and your plants will thank you for it.