Greenhouse shading ideas

Tim Rumball suggests ways to avoid your greenhouse overheating

- Tim Rumball

THE protected environment of a greenhouse, which is so important for nurturing plants in winter and spring, can be a problem as the weather warms. Bright sunshine bearing on clear glass is magnified, and inside the greenhouse temperatures rocket on a hot, bright day. Trays of seedlings can be scorched, and even mature plants can wilt as roots fail to keep up the much-needed supply of moisture to the furiously transpiring greenery above ground.

In general, sunlight is not a problem – plants thrive on it – but the heat it generates can be. Unfortunately, to keep the heat down, it’s usually necessary to shade out the sun. So the principle is to introduce as little shade as possible to maintain plant growth, but enough to keep the temperature below about 27ºC (81ºF). This might sound easy, but the temperature range in my 6x6ft greenhouse back in late April was a low of 8.8ºC at night, with a daytime high of 34.9C (which nicely illustrates the value of a good max/min thermometer).

Get ventilated

Good ventilation is the first line of defense. A steady flow of air from outside reduces the temperature inside and keeps plant fungal infections at bay. The RHS recommends that ventilation openings in a greenhouse should represent 20% of the floor area – a greenhouse with 5sq m of floor area needs 1sq m of ventilation from windows or louvers.

If your greenhouse has a window or louver, consider fitting an adjustable automatic opener that kicks in when the temperature rises and closes as it drops. Small greenhouses, which are often poorly equipped with vents, are more vulnerable than big ones to overheating. Leave the door open on hot, sunny days where possible to increase airflow.

Shading options