Greenhouse: ventilation and shading
Plants grown in glasshouses, conservatories, plastic greenhouses, polytunnels and garden frames, especially in the summer months, all require adequate shading and ventilation.To get more news about
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Greenhouses are vulnerable to overheating from spring until autumn. Without protection from heat, few plants are likely to survive unharmed when subjected to prolonged high levels of heat and dry atmosphere within a greenhouse or conservatory.
However, with sufficient air circulation, humidity and shading, many plants will tolerate high summer greenhouse or conservatory temperatures in the same way that they survive in the tropics and subtropics where many greenhouse and conservatory plants originate.
The aim of gardeners is to prevent leaf temperature rising to levels at which tissue damage occurs. By September, shading should be reduced gradually, and removed as soon as ventilation alone can control overheating. Blinds and netting used for shading can be deployed on cold winter nights to limit heat losses.Leaf temperature is mainly controlled by movement of water through the plant and out through the leaf surfaces in the form of transpiration – this process has a cooling effect on the leaves.
Plants which are dry at the roots or drying out are therefore more at risk than plants which have adequate moisture at the roots.
Air movement is the second important factor, air movement over the leaf surfaces also has a cooling effect.With larger glasshouses, one square meter of ridge ventilation for each five square metres (20%) of floor area provides the ventilating capacity to give one complete change of air within the greenhouses every two minutes.
Smaller greenhouses have a higher glass to floor area ratio, and should ideally have an even higher percentage of ridge ventilation. Unfortunately, this is seldom provided and amateur greenhouses are very vulnerable to overheating. Extra shading is commonly needed which can cut down on light falling on the plants to a degree which reduces growth and cropping. Thankfully, the door of small greenhouses offers an alternative means of ventilation which partially compensates for lack of roof ventilation. Side ventilation, commonly by louvres, is less effective than roof vents.
Opening vents and doors helps to release some of the heat but it is often insufficient and therefore shading is usually required from mid-spring until early autumn.
Unfortunately, shading limits the light plants receive. As plant growth depends on light, only the minimum amount of shading should be used to keep temperatures below about 25-27ºC (77-81ºF). Otherwise, allow as much light in as possible, particularly when growing edible plants such as tomatoes. There is often no need to shade sun-loving plants such as succulents although the greenhouse is more pleasant to be in when shade is provided. .
