14:56 - 06 April 2011
A greenhouse by its very nature can become very hot in summer and very cold in winter. Getting the temperature at just the right level for healthy plant growth can be a bit of a juggling act, but with a little planning, some technology and improvisation it’s not as difficult as you might think.
Good ventilation in a greenhouse is essential to keep the temperature as consistent as possible without too many extreme highs and extreme lows. Just opening the door or windows is not usually enough. You need a good airflow, which is best achieved by a system that can draw in cool air low down in the greenhouse via floor level louvre vents or similar and can allow the hot air out at the top of the glasshouse. As a general rule a greenhouse requires an area equivalent to approximately a third of the floor area for ventilation. Large side windows that can be opened and shut daily are ideal. If you want to keep insects or larger creatures out, install a fly screen or hang fine netting over the openings, but remember that if you need pollinators in your greenhouse you need to provide access and exit points for them.
Automatic ventilation that opens when the weather is hot and closes as the temperature cools helps the busy (and absent) gardener keep temperatures under control. If your budget doesn’t stretch to automated systems they you need to open the greenhouse windows in the morning and close them up again before the temperature drops in the evening. In midsummer when the nights are balmy and hot you can leave them open all night but do make sure you keep an eye on things as some creatures such as rodents and even cats can find the greenhouse warmth irresistible. Leaving a door or window open can be an open invitation.
But it’s not just ventilation that helps moderate the internal greenhouse temperatures; you also need to consider shading the glasshouse from above. This might seem a bit daft when the whole point of a greenhouse is to harness solar energy to warm the space inside, but unless you want an oven to bake your plants, by midsummer you will need to shade the glass or start to move plants outside. The cheapest way to shade your greenhouse is to apply an external shading paint that blocks the heat of the sun but still allows the vital light through. This can be very effective, but it needs polishing off for winter and it can look unsightly. At the other extreme you can install automatic or remote controlled external greenhouse blinds that can be raised or lowered according to the weather, these can be operated on separate controls so that you can have half the glass house shaded while the plants in the other half enjoy the full heat of the sun. External blinds are considerably more effective than internal ones as they stop the sun’s rays from reaching the glass, they are however, exposed to the weather. Internal blinds are another option and can be fitted inside the glasshouse to reduce the glare of the sun. You can also create your own homemade versions of exterior and interior shading using a variety of materials.
The other major method of cooling things down in the greenhouse is referred to as damping down. The paths and floor of the greenhouse are dampened in the evening of hot days and the evaporation of the water helps to cool the greenhouse and increase the humidity inside. You can do this using a watering can or a garden hose or if you are absent for any length of time you can organise an automatic irrigation system to do it for you.
