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The sun's powerful rays can penetrate glass and other manmade materials, which can be harmful to the plants in your greenhouse. Without ventilation, shade or any other cooling methods, the plants will overheat, damaging their leaves and fruits while drying out the soil. So, how do you keep your greenhouse cool in summer?
Ways to keep your greenhouse cool
Whether they are constructed with glass or plastic, greenhouses can get very hot during the summer months. So, to give your plants the warmth they need but also protect them from the heat, it’s important to keep the greenhouse cool. Here are our best recommendations.
Why ventilation is important
Ventilation is one of the best and most natural ways of keeping your greenhouse cool in hot weather. It’s also important during the spring and autumn months when the sun is still powerful. With the right type of ventilation, you will be able to control the temperature in your greenhouse.
Ventilation promotes air circulation and is created through:
- The greenhouse door is either a single door or double door depending on the size and design of the greenhouse.
- Roof vents, which can run the length of the greenhouse and are opened either by hand or automatically.
- Side vents are your side windows, which are usually louvred.
Bigger greenhouses should have a ventilation capacity of one square metre of roof ventilation per five square metres of floor area. This will deliver a 100% air rotation in the greenhouse every two minutes. Ironically, the smaller the greenhouse the higher the ratio between glass and floor area. Ridge ventilation is preferred but not all greenhouses have this. That said, louvred side windows and the door create an airflow through the greenhouse.
In really hot weather, increase the airflow by using fans, pedestals or smaller fans on the shelf. You can also remove some of the glass panes, particularly during a heatwave. Another option is to install automatic vent openers with will open at certain times of the day, even if you’re not there. However, because they are built with a cylinder filled with wax, which expands in the heat, it may take them some time before they open up. So, you’ll need other cooling methods.
Using shade to protect from the sun
Shade is an important aspect of keeping greenhouses cool. Just creating airflow through ventilation isn’t going to be enough to protect the leaves of plants. However, too much shade will mean the plants don’t get enough light so, it’s a bit of a balancing act. Just a small amount of shade will help keep temperatures between 25ºC and 27ºC.
If you’re growing edible plants in your greenhouse, like tomatoes or courgettes, they are going to need plenty of light. Sun-loving plants, such as succulents, need next to no shade but will still thrive if there is a little.
Types of shading include:
- Internal blinds. These are fitted on the inside of the glass panes and can be automated.
- External blinds. Fitted to the outside, these blinds will need to be hardy enough to withstand weather conditions but they'll repel a significant portion of the sun's rays before the heat gets trapped inside your greenhouse.
- Netting or polyethylene mesh. Can be easily fixed to the inside of the greenhouse glass with clips. They can be fitted to the outside, too, and they are often cheaper than blinds.
- Paints that shade. Shading paints, like SunClear, Nixol and Varishade, are diluted with water and then painted onto the outside of the glass panes. Depending on how much shade you want, you can add layers and when no longer needed, it can be washed and/or brushed off.
Evaporative cooling – what is and why?
Evaporative cooling in greenhouses is using the heat in the air to evaporate water from wet surfaces and plants to create a humid atmosphere and cool the temperature. Its effectiveness can make a huge 10-20ºF difference compared to outside the greenhouse. However, evaporative cooling does need fresh air to be continually flowing or the air won’t become damp, losing the cool temperature.
Evaporative cooling systems, which are similar to air conditioning units, are available for greenhouses. They use cellulose pads that have been soaked with water and the air is pulled through the pads by a fan. A pump and valve system adds more water, if needed, while the excess water is collected and recycled.
Which size evaporative cooling system to use depends on the size of your greenhouse. You will need to determine cubic footage before dividing it by 375 for 6” pads, or 250 for 4” pads. The cubic footage also works out how many fans you will need.
Much like maintaining your outside plants and managing the watering of them, you need to do the same with your greenhouse plants. In addition, watering helps to keep the greenhouse and the plants cooler during the summer.
Installing a drip irrigation system in your greenhouse not only saves water but supplies a constant drip of water to your plants. These systems work well with evaporative cooling systems, too.
Adding mulch around your plants, whether they are in pots, trays or beds in the greenhouse, will help to preserve moisture. When watering, take into consideration the plant size and species, the structure, texture and compaction of your soil, and its position in the greenhouse.
As you would in the garden, water your greenhouse plants either early in the morning or evening when the temperature is a little cooler. Your plants will be able to absorb the water better before it evaporates.
Monitoring and controlling greenhouse temperature
Weather conditions and temperatures fluctuate throughout the summer so, it will impact temperatures in your greenhouse, too. Therefore, it's important to measure and control how hot or not your greenhouse is to ensure a consistent temperature all the time.
There are two key items that when used together, give you all the tools to monitor and control the temperature in your greenhouse. These are a thermometer and a hygrometer; the first monitors the temperature and the second monitors the level of humidity.
Remember that if it gets too hot and dry, plants will tend to wither and die. However, if it gets too cold and wet, they won’t grow well at all. Therefore, by balancing the temperature and humidity levels, you can create the best-growing conditions inside the greenhouse and achieve optimal growth.
It’s worth noting that our above tips can also be applied to polytunnels and garden frames, even conservatories if you have plants in them.