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How To Keep Your Greenhouse Cool.


Although we often concentrate on keeping a greenhouse warm enough for our plants, during the summer months we can be faced with soaring temperatures which will do far more harm than good. So it’s important that you monitor the temperature in your greenhouse and prevent it from overheating – sometimes your plants will remain wilted even when you water them or just not produce any fruit if they have been damaged by temperatures being too hot.

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Monitor The Temperature
Before you start to worry that your greenhouse is too hot, you should monitor the temperature to see if it is too hot for the types of plants which you are growing. The easiest way to do this is to have a max/min thermometer within your greenhouse. To monitor the average temperature ideally place the thermometer out of direct sunlight and in the middle of your greenhouse. This type of thermometer will show you both the highest temperature reached as well as the lowest, whilst also show the current temperature within your greenhouse. If you decide that your greenhouse is getting too hot then it’s time to start looking at ways to keep your greenhouse cool.
There are basically three factors to consider when keeping your greenhouse at a reasonable temperature throughout the summer months – these are ventilation, shading and humidity. By having the correct amount of each of these you will be able to keep your greenhouse cool.

The first is root vents in your greenhouse. Most greenhouses are supplied complete with a number of roof vents – if you’re looking to purchase a new greenhouse remember that you can’t have too much ventilation so always order extra roof vents when possible. Ideally roof vents should be placed on the opposite side of the greenhouse to the prevailing winds – this prevents roof vents being damaged by strong winds forcing them open.
The second is automating the roof vents. Although you can open and close them manually, you can make it easier by fitting automatic openers. These will open the roof vents when temperatures soar, and close the window when temperatures drop. Very easy to fit they work by a wax within a cylinder expanding when warm – so opening the window. These cylinders are replaceable, so you can have years of service from the same automatic opener – making them both efficient and cost effective.
The third thing to consider is installing louvre windows in your greenhouse. These are an excellent way of adding more ventilation to an existing greenhouse – it is often incredibly difficult to increase the number of roof vents within an existing greenhouse, whereas louvre kits are available that will suit most types of greenhouses.
Circulation fans
are the fourth option in your arsenal to keep your greenhouse cool. These fans don’t reduce the temperature, however by moving the air around they ensure that there are no hot spots within your greenhouse. They are also useful when used in combination with open roof vents or louvre windows as they will help to draw air in from outside the greenhouse.
If you want to expel hot air from your greenhouse then consider adding an extractor fan. These will remove the air from your greenhouse whilst replacing it with cooler air from outdoors. When selecting an extractor fan look at how much air it will expel per hour – for example if your fan will extract 400 cubic metres of air per hour than the air within an 8’ x 12’ greenhouse will be replaced every 3½ minutes. The best place to fit an extractor fan is in the roof of the greenhouse opposite the door as this is likely to be the place where the greatest amount of hot, stale air will collect.

Whilst your aim of ventilating your greenhouse adequately may have need to keep temperatures down, good air circulation within your greenhouse will ensure the air is clean and excess moisture is removed helping to reduce pest problems. Whilst the air is circulating well throughout your greenhouse pollen will be lifted from plant to plant so helping in pollination.

The first option to consider when shading your greenhouse is to use a paint on shading. These are easy to apply and the weather sensitive ones will provide up to 7 months of controlled shading – when it’s dry it turns white reflecting the rays of the sun to protect from heat and glare. When wet or damp it becomes clearer, restoring light transmission.
Another way of shading your greenhouse is to use shading materials to block out some of the sun, a primary source of heat. Usually you can use shading materials from between June to August when there is plenty of sun for your plants. These shading materials will be 40% more effective when installed on the outside of your greenhouse, whereas if you install the shading inside although it will prevent your plants from being scorched, the heat from the sun will be trapped within the greenhouse so your greenhouse can still overheat.

Another way of cooling your greenhouse is the simple principle of water evaporation. By hosing down your greenhouse floor and having the roof vents open, the temperature within your greenhouse will drop quickly.

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