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7 April 2016

Ventilating and Shading Your Greenhouse

In my previous article I covered the reasons for ventilating and shading a greenhouse, while in this one I outline the practicalities of doing it.

There are usually two places in a greenhouse in which air can come in and out, though larger greenhouses may have more. An average-sized domestic greenhouse will frequently have an automatic opener for one of the roof panes, which will operate when the temperature reaches a certain figure. Combining this with opening the door allows some airflow through the greenhouse.

Larger greenhouses will have the door or doors, opening roof vents, which sometimes run the length of the ridge, and which may be operated manually or automatically, and finally side vents, which are often louvred. With larger glasshouses, one square meter of ridge ventilation for each five square metres (20%) of floor area will give one complete change of air within the greenhouses every two minutes.

Smaller greenhouses have a higher glass to floor area ratio, and should really have an even higher percentage of ridge ventilation. Unfortunately, amateur greenhouses don't usually have the extra ventilation and are therefore vulnerable to overheating. Some small greenhouses may have side ventilation in the form of louvres, but this is less effective than roof ventilation. Extra shading is commonly needed.

It is useful to have a maximum-minimum thermometer in your greenhouse so that you can monitor the temperature. Don't have the sun shining directly onto the mercury in the thermometer as this will give a false result.

Look out for signs that shading and ventilation are required, such as sun-flag (partial collapse), leaf scorch, desiccation of tender young plants and shoots

Open all doors and vents on sunny days, and leave them open at night if the temperature remains high. In changeable weather, vents and doors often have to be left partially open to limit sudden increases in temperature.

Fit automatic vent openers to ensure roof vents open even when you are not around. However, remember that since they work by the expansion of wax in a cylinder to open the vents, it takes a little time for them to react. Provide plenty of alternative ventilation (i.e. doors and side vents) to prevent damaging temperatures occurring before the openers respond.

For additional ventilation during heat waves you can temporarily remove some panes of glass from your greenhouse.

In addition to providing ventilation, it is often necessary to use shading in a greenhouse from mid-spring until early autumn to protect your plants, though the amount will depend on what you are growing.

The downside of shading is that it reduces the light plants receive, and as plant growth depends on light, only the minimum amount of shading should be used to keep temperatures below about 25-27C (77-81F). Otherwise, allow as much light in as possible, especially when growing edible plants such as tomatoes. Sun-loving plants such as succulents don't need shading although you may find your greenhouse too hot without it.

There are various ways of shading your greenhouse, the cheapest of which is using shading paints. These are diluted in water and painted onto the outside of the glass in spring. As the season progresses, thicker applications can be applied, then in early autumn it can be washed and brushed off. Shade paints are less suitable for use on greenhouses glazed with acrylic or polycarbonate because even thorough cleansing may fail to remove all visible traces of the shading paints.

External blinds are the most expensive option as they give shade and also provide the maximum cooling effect by preventing the sun's rays from passing through the glass. In cloudy weather they can be easily drawn up again to allow maximum light on to plants.

Internal blinds don't work as well as external blinds since sunlight is allowed to pass through the glass, generating heat. However, they are probably more easily automated than external blinds in order to provide shade when it is most required. There are a wide variety of materials available in a range of degrees of shading and with varying permeability to allow air exchange.

Polyethylene mesh or netting (shade netting) is a cheaper option, and is usually placed inside the glasshouse and fixed with clips. The plastic is likely to biodegrade over a number of years but is not expensive to replace.

Keeping a good level of humidity during bright sunny weather is a further important factor in guarding against heat damage within a greenhouse. This is easily achieved by thoroughly damping down the floor of your greenhouse with water, though it may need doing several times a day.

All being well, using a combination of the above measures should keep your greenhouse plants happy!
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