Glasshouses can quickly overheat in hot summer weather. Open doors and vents and use shading such as blinds, paint or netting to keep temperatures down
Preventing overheating
Sultry summer days can cause glasshouse temperatures to rocket. While opening vents and doors helps to release some of the heat, this can often be insufficient. Glasshouses, conservatories, cold frames and even cloches therefore need to be shaded, either with shade paint, netting or blinds (left). Exterior blinds are best as, unlike interior blinds, they prevent heat penetrating the glass and can be rolled up in dull weather. Shade paint is cheaper, but has to stay in place for the entire season, as it would be too time consuming to remove it temporarily. Netting is effective but not attractive.
Maximising light
Unfortunately, shading does limit the amount of light that plants receive. As plant growth depends on light, only the minimum amount of shading should be used to keep temperatures below about 25-27°C (77-80°F). Otherwise, allow as much light in as possible, particularly when growing edible plants such as tomatoes or cucumbers. There is less need to shade succulents and other sun-loving plants.
By September, shading should be reduced gradually, and removed as soon as ventilation alone is sufficient to prevent overheating.
Leigh Hunt