25/05/2026
Keeping a Greenhouse Cool: Today has recorded the hottest temperature on record for May in the UK. Whilst the main purpose of a greenhouse is to trap the heat in, on days like today they get too hot and this can be damaging for our greenhouse plants.
There are many ways of keeping a greenhouse cool but I always go for the quick, easy and cheap ways!
Most of us already have Greenhouse venting systems that automatically opens a window when the temperature rises. One thing to note about these is that the cylinders do leak after a while and stop working properly. When this happens, you should be able to buy a new cylinder rather than replace the whole mechanism.
As this heatwave is predicted to last only a few days, today I removed a window directly opposite the door to allow air to move ‘through’ the greenhouse to prevent hot air stagnating. It’s easy to do and easy to reinstall the window when the days get cooler. Clearly you should wear gloves when handling glass and be very careful not to break the window when removing it, storing it and replacing it. What I should really do is get something that I tell myself every year I will buy and, to date, haven’t …….. that is a louvre vent window opener ……. then I wouldn’t have to remove any glass! Look them up – and when I eventually get mine, I’ll send you a pic!
Last year we had prolonged heat and I used Greenhouse Shading ‘Paint’ to lower the temperature in the greenhouse. You simply mix it with water and paint it onto the glass on the outside of the greenhouse where it dries. It forms a white, chalky coating on the glass preventing some of the shortwave radiation from the Sun entering the greenhouse (it is this shortwave radiation that converts to heat once inside the greenhouse). Some of this ‘paint’ will come off when it rains but it does last quite a long time and often, I’ve eventually had to remove it with a cloth, brush or even broom for the higher parts.
Finally, in the past I have used an old sheet pegged to the frame of the greenhouse to prevent some of the shortwave radiation entering the greenhouse. Again, this is a very temporary solution and only possible when there is little wind.