Why a Greenhouse Needs Shading
The same glazing that makes a greenhouse work in April becomes the problem in July. On a clear summer day an unshaded house can run fifteen degrees above the air outside, which is past what most crops tolerate and well past what seedlings survive.
Greenhouse shading cuts the light entering before it becomes heat, which is more effective than trying to remove heat afterwards with ventilation alone. It also diffuses direct sun, so leaves close to the glass are not scorched.
The Three Approaches
- Shade netting is fixed over the outside or inside of the glazing and stays up for the season. Different mesh densities cut different proportions of light, typically between 30 and 70 per cent.
- Roller blinds can be raised and lowered as the weather changes, which suits a British summer better than anything fixed. They cost more and need mounting.
- Shade paint is brushed or sprayed onto the outside of the glazing in early summer and washed off in autumn. It is the cheapest option and the least adjustable.
External shading works better than internal, because it stops the light before it enters. Internal shading still helps, but the heat is already inside the glass by then.
How Much to Cut
This depends entirely on what you grow. Tomatoes, cucumbers and peppers tolerate high light and need only modest shading through the hottest weeks. Leafy crops, herbs and anything being propagated need considerably more.
A maximum and minimum thermometer tells you what the house actually reaches before you buy anything. Guessing at shading density without knowing the peak is how growers end up with too much and slow growth all summer.
Shading Is Not a Substitute for Airflow
Cutting light reduces heat input, but air still has to move or humidity builds and disease follows. Shading and ventilation work together rather than as alternatives, and a shaded house with closed vents will still overheat.
An automatic vent opener handles the daily cycle without attention, which matters most during the weeks when shading is up.
Taking It Down
Remove shading in autumn. Light is the limiting factor from October onwards, and netting left up through winter costs growth at exactly the time plants can least afford it.
Wash the glazing at the same time. Shade paint residue and algae both cut transmission, and the cleaning range covers both jobs.
