Q How can I grow crops in my very hot greenhouse?
Vikki Summers, Lancashire
A BOB SAYS Tomatoes will seldom find it too hot or bright if given enough water, so grow them in the greenhouse border or in larger containers with trays underneath them. They prefer it brighter and less humid than cucumbers and melons, which both like diffuse light and higher humidity. Thus I suggest arranging the plants so the tomatoes get all the sun, with the others in their shade.
You probably also need to increase the ventilation. This helps the plants get enough CO2 and keeps the greenhouse cooler. A fan, solar-powered ideally, would also increase air movement and alleviate hot spots.
Hanging semi-clear plastic sheet as a ‘tent’ for the cucurbits would give them both diffuse light and higher humidity in one part of the greenhouse. Shade cloth works well, but it’s fiddly to put up, and you have to make sure that you don’t obstruct any vents. White paint is very easy to put on – just not so easy to get off afterwards! A CHRISTINE SAYS Most plants have a limit at which they stop photosynthesising. This is related to high temperatures and varies with each plant. Plants may survive very high temperatures but then not crop very well. Adding shading ensures the greenhouse does not get too hot. There are paint-on materials that can be applied to the outside of a greenhouse but these do not give great flexibility if the weather then becomes more overcast for a long period. Various forms of exterior blinds are available. These are easily manipulated but expensive and you may need someone to install them for you. You could also try horticultural fleece, using wires to suspend it from the roof. This is fiddly to put up but easy to adjust, drawing it across the span of the roof as required.