Providing adequate shade and ventilation in your greenhouse is essential. During the height of summer most of the plants – with very few exceptions – will need some protection from the full strength of the sun, while good ventilation is also a key factor in avoiding many of the disease problems of growing under glass.
In the Shade
Even in a British summer, the sunshine through the glass can raise the temperature within the greenhouse to unacceptable levels and many commonly cultivated plants are particularly susceptible to being scorched. African violets, begonias, ferns, fuchsias, gloxinias, orchids, poinsettias or streptocarpus all fall into this category, as do most seedlings, new cuttings and freshly potted plants.
The greenhouse owner can be faced with a major problem, but fortunately there are many ways to deal with it – ranging from the simple and cheap to more sophisticated and expensive approaches. The easiest way to provide shade within the greenhouse is simply to paint the windows. Traditionally gardeners used whitewash, often mixed with a little decorator’s size to make it stick better and today there are many proprietary brands of paint available from garden centres and hardware shops which are specifically made for the purpose.
It is important not to use too thick a layer – the idea is to provide shade without keeping out too much light. The biggest drawback with this approach is that once applied, it is difficult to alter the placing of it to allow for either changes in the weather or the needs of different plants.
Other more flexible methods include the use of fine net curtains, plastic or fabric roller blinds and slatted wooden blind and for the ultimate in control, there are automatic systems to consider. Adjusting the blinds to provide the best growing conditions in response to the changing light and temperature, they are ideal if you have to be away for long periods of the day.
Good ventilation is another help in keeping the temperature down during the summer and also plays an important role in the prevention of disease. Some designs of greenhouse – especially the cheaper versions – can be a little sparing in the provision of ventilators, principally to keep the cost down. It is worth bearing this in mind when planning a purchase, since there can never really be too many ventilators – you do not have to use them all, but having them there does give you tremendous flexibility, should the need arise.
Careful use of ventilation allows the atmosphere within the greenhouse to stay healthy, maintaining a gentle flow of air through the building to stop things going stale and also avoiding many of the problems of condensation and moulds. The trick is to manage things so that there is some air movement, but not actual draughts – so using the side ventilators when the temperature outside is cool is best avoided. Again there are automatic systems available to make life much easier for the gardener who needs to be out at work during the day, which open and close the vents hydraulically while an extractor fan attached to a thermostat makes a simple DIY alternative.
The greenhouse is a closed environment and needs careful management, particularly of temperature and air conditions, to provide the best surroundings for growth. A surprising amount of damage can occur from overheating and scorching in a very short time, particularly to tender plants and where the air flow is poor, fungal diseases such as botrytis and mildew can thrive with disastrous effect. Balancing the demands of ventilation while avoiding chilling the plants and keeping everything warm enough, but not too hot, can sometimes seem a bit of a tight-rope to walk. It is one thing to talk about maintaining a fairly even temperature and a gentle air-flow, but it not always just so easy to achieve. Every greenhouse is different, so it may take a while to develop the right “feel” for yours, but it is certainly worth the effort; in the long run, developing the knack brings major benefits.