EXPERT Tips… pottering in THE GREENHOUSE Mick Lavelle, gardening expert and senior lecturer in horticulture at Writtle University College, offers seasonal advice

Greenhouses are deservedly popular with gardeners, offering the chance to grow tender plants and extend the growing season into the cooler months. Almost all plants like to grow in reasonably warm conditions, with most growing best between about 16–24°C (60–76°F). A greenhouse traps daylight, which then heats the air and raises the temperature inside. The real trick is to ensure that the temperature inside doesn’t get too hot in the day or too cold at night.

the heat is rising

By June, the long, sunny days can increase temperatures within a greenhouse very rapidly, which can result in a surprising (and occasionally devastating) amount of damage from overheating and scorching to your plants in a very short period.

Shading the greenhouse will greatly reduce overheating. The simplest, cheapest and (arguably) most effective way for gardeners to do this, is to apply shade paint to the outside of the glass. There are numerous glass shading paints available, some of which claim to ‘react’ to sunlight. These stay clear (letting in full light) when wet but when dry, they ‘turn’ white (thereby reflecting the sun). Of course, if it is dull and dry, their shading effect remains, even though overheating is not a problem then.

air it

Shading works most effectively when combined with good ventilation and humidity. The latter is easily increased by spraying a hose on hard surfaces, such as paths or under benches on hot, dry days. Roof vents help ventilate excess heat but need to occupy the equivalent of 15 per cent of the floor space to be fully effective. If you are not there during the day to open vents, then there are systems available that will open and close them with a thermostatic control – although many gardeners just open the vents in the early morning and close them again at dusk.