If you have a small greenhouse or polytunnel in your garden or allotment plot and you find that your plants, especially some quick growing tomatoes in the heat, have curly leaves don’t be afraid. The curling is almost entirely down to the fact that the night temperature is much lower than the daytime temperature. In the greenhouse in direct sunshine at around 2pm the highest I have measured was 45 Celsius with vents and door open; there was no wind whatsoever at that time and after midday is the hottest time of the day. And during the night the greenhouse does get nearly as cold as the outdoor temperatures believe me, which still can be around 10 Celsius.
The plants in your greenhouse make a surplus of food during the warm daytime and the cold nights slow down their biological activities therefore they cannot absorb and use all that food quickly. This results in a start growing and stop growing pattern which will lead to curly leaves. It is not such a big problem with outdoor tomatoes as the temperature fluctuation is not that great. Controlling the temperatures in the greenhouse is not always easy but you can throw a shading net over it or paint it with shading paint. You can also increase humidity in the greenhouse by leaving a bucket of water inside and spraying water in the air in really hot days. Fungal diseases love humid environments so make sure that you use this method only on really hot days and make sure that your plants are strong and healthy. Also if you can do shut the greenhouse for the night and then open it again in the morning, this will keep the temperature fluctuation at a minimum.
