How hot is too hot?
A common sight I see when visiting gardens is Greenhouses and Polytunnels with their doors sitting closed in good weather. There is a common held myth that you should keep your protected growing structure in a way that there’s ‘fierce heat in there today’, this of course is completely wrong. Of course a certain amount of heat is desirable for many of the tender plants we grow in these protected structures, but excessive amounts will lead to stunted growth, fruit ripening issues, pest and disease issues and even plant fatality.
Many of us keep a greenhouse for one (or a few) major crops, such as to grow a few tomatoes and chillies. Plants like these will benefit from being kept above 6C (as they don’t like the cold) and maintained at a nice temperature of 25-30C. A common sight with tomatoes in the summer months is ‘leaf curl’ this is where leaves curl up tightly on the plant, this is specifically as a results of the plant experiencing the afore mentioned ‘fierce heat’ and the cooler temps at night. This extreme of heat activates a reaction in the plant where it curls up its foliage to retain moisture and slow down its rate of transpiration – as the plant is losing more water than it can uptake.
An essential bit of kit for any greenhouse of polytunnel owner is a Thermometer, in particular a Max/Min Thermometer which records the highest and lowest temperature reached each day. As a result of having one of these you can work out if you need to leave the doors open, and by how much. This idea of opening doors (and even vents) can be an issue especially with ‘home made’ structures – many forget about including enough doors and vents when they’re up-cycling. A handy rule of thumb for a DIY glasshouse is that the ventilation should be around 1/6 of the total floor space.
For those with a glasshouse a worthwhile investment is the automated roof vent openers, these are available in most garden centres and are easily attached to existing aluminium frame structures, and work via a valve which contains gas which expands and contracts as a result of temperature. This can take the hassle out of remembering to open and close these roof vents.
Ventilation is also vital for Photosynthesis, as when plants are growing well they use up large quantities of carbon dioxide to produce oxygen, and if there’s little fresh air entering the structure then the plants growth will stop as they’ll run out of the carbon dioxide. And finally let’s not forget about pollinators – the wee lads we need to visit our tomatoes to pollinate the flowers can’t get in if the doors and windows are closed (a common problem with Conservatory grown fruit is lack of pollination – as a result of the windows and doors being closed during the day). All these visiting insects also help to control many greenhouse aphid problems and the fresh air is a blessing in reducing disease pressure.
So a good rule of thumb is to open the door(s) of the polytunnel and glasshouse in the morning, leave them all open during the day and then close them about 5pm(ish), this will help to retain some heat for the evening and will trap some insects inside the structure, giving a bit of focussed pollination.
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Gareth Austin is a broadcaster and lecturer in Horticulture. Join Gareth on Twitter @GardenerGareth or connect via Facebook to enjoy his regular adventures in the world of Horticulture.
