Growing exotic plants can be really thrilling. Whether you’re a commercial producer of flowers, fruits or vegetables, or just a hobbyist gardener, you’ll always want to get the maximum yield.
Plants like azalea, daisy, calendula, radish, primrose and sweet pea all require slightly cool temperatures at night to grow the best. This means that you require a cooling system in place to help bring the temperature down to the required levels.
This is where a greenhouse proves to be very useful.
The main feature of a greenhouse is that its conditions can be controlled so that the plants grown inside have the optimum temperature for their growth. It is also called a glasshouse, or with enough heating, a ‘hothouse’. It is important to understand that ‘optimum temperature’ means either heating or cooling the temperature of the greenhouse as required.
Why Cooling is Required in a Greenhouse
A greenhouse will need heat during winter, which means it will be geared towards retaining heat. But if you’re going to cultivate your crop year-round, summer temperatures might be too high to be productive. That’s when cooling the greenhouse becomes a necessity. There are many different ways to regulate the temperature of a greenhouse, and automated systems abound – but increase operating costs significantly.
Why Cool a Greenhouse Without Electricity?
Temperature reduction can be done relatively easily with the help of electricity, but energy conservation is the name of the game. If you take away the dependency on electricity when you want to cool your greenhouse, not only will you manage to reduce operating costs, but you will also conserve natural resources.
In this post, we’ve listed various methods of keeping your greenhouse cool while eliminating dependence on electricity. Of course, some methods might require electricity for a short while, or use electricity from sources other than the mains. Let’s get down to brass tacks, shall we?
Steps to cool the greenhouse without electricity
Venting the Greenhouse
This seems so obvious, it almost seems like a joke. However, it is the most economical way to cool your greenhouse. You see, hot air rises – you’ll know this if you’ve ever seen a hot-air balloon – and if you allow the hot air at the top of the greenhouse to escape, that’s half your job done. The other half involves the replacement of the hot air, so the sides need to be open to let fresh air in.
There are a few things to consider for this method: one, the total area that needs to be vented should be around forty percent of the area of the greenhouse. Divide it equally between the vents in the roof, and the open space around the sides. A hand-cranked ‘wall’ that can roll up works really well for this.
Some greenhouses might require a ‘skirt’ at ground level to prevent extremely cool air from near the ground entering the greenhouse and cooling it too quickly or too much. If the roof vents are shutters, they need to open beyond the horizontal to allow the hot air to flow upward as well as outward as easily as possible.
The opening and shutting of the various vents can be done manually, which works well in places where labor is affordable. There are also solar-powered roof vent openers available in some areas. These mechanisms contain wax that expands when heated, thus pushing the vent open, and pulls it shut when the temperature drops and the wax contracts.
The placement of vents can also make a difference in the efficiency of cooling. Constructing vents that are low down on the side of the greenhouse that will receive the wind (the ‘intake vents’, so to speak), and vents constructed high up on the side downwind from the intake vents (the ‘exhaust vents’) seem to offer the maximum benefit.
The fresh air injection into the greenhouse is important, as the plants need a steady supply of carbon dioxide for photosynthesis to keep happening.
There are two sub-types to this. The easier-to-explain one is wetting the greenhouse itself, on the outside. This works if your greenhouse is made of glass or acrylic sheets, but it won’t work so well if you’re using polythene sheets, especially on the roof, as they might sag and hold the water instead of having it runoff.
Evaporative cooling on the inside can be done in various different ways: watering the entire greenhouse is what one has to do in one situation. Laying pathways of stone or brick will help increase the effectiveness of this method. Another way of approaching it is to make walls of coir so that the water doesn’t evaporate very quickly, or basins that can be filled with water to keep the evaporation constant.
This will have to be coupled with the first method, venting, for maximum effectiveness, as some plants might not respond very well to an extra-humid environment. The excess humidity will also encourage the growth of fungus, which can be harmful to the plants.
An interesting side note is that plants themselves keep cool through evaporative cooling, as do humans. Although it behaves like the process of perspiration, it is called ‘transpiration’ instead. They exude water through their leaves and let it evaporate. Therefore, it is essential to keep the soil hydrated enough that the plants don’t start withering due to lack of water to keep themselves cool.
A corollary of this is to keep your greenhouse full of plants. The more plants in the greenhouse, the more the transpiration, and the better the cooling for your greenhouse.
This is another way of making sure the humidity in the greenhouse is high and thus keeping it cool through both evaporative cooling and transpiration. Yes, the misting system will need a pump to pressurize the pipes, but it can be powered by solar-charged batteries, and it won’t need to perform the function all the time – just whenever necessary. As with the previous method, it will need to be coupled with venting for maximum effectiveness.
Using Solar Energy to Power Cooling Fans
Yes, we know we said ‘no electricity’, but it is perfectly okay to use an alternative source of energy. If the greenhouse is getting too hot in summer, it stands to reason that there is an abundance of sunlight, which can be used to power fans to circulate air in the greenhouse, bringing with it all the benefits of fresh air, along with lowering the temperature.
You can deploy the fans either as exhaust fans, to remove the air from the greenhouse and let air in from intake vents, or as circulating fans, to set up a breeze in the greenhouse and thus keep it cool. Remember to try and follow the same placement as regular vents with the fans.
The rule of thumb is, don’t fight prevailing winds. Place the ‘intake’ fans low down on the side of the greenhouse that receives the wind, and the ‘exhaust’ fans high up on the downwind side of the structure.
Shade Cloth and Shading
This is exactly what it sounds like; you can use cloth to offer shade to your greenhouse. Shade cloth can be used both inside the greenhouse or outside, but the effectiveness is much more (to the tune of 40 percent better) when used outside the greenhouse.
Take care not to let the shade cloth get in contact with the greenhouse itself, else it will conduct the heat inside anyway. Maintain a minimum distance of 6 inches between the shade cloth and the structure. Shade cloth needs to be used with care since blocking too much sunlight might stunt the plants’ growth. It should also allow pollination agents to move through it.
There are different types of shade cloth available, like woven or knitted material. The most effective is the reflective type. As of today, the most popular reflective shade cloth is the aluminet shade cloth which helps to reflect the harmful UV rays to a greater extent. Dark-colored shade cloth provides better cooling to the plants than light-colored shade cloth. Monitoring the greenhouse temperature and deploying or pulling away the shade cloth to regulate the temperature will be an ongoing process.
Keep your insect screens clean
Insect screens perform a valuable role in keeping pests off your precious greenhouse plants, but they gather dust (not to mention insect victims) all too easily, and their ability to let air through is thus reduced by a fair margin. Keeping them clean will ensure good airflow, which, as we’ve explained before, contributes to cooling.
Use deciduous trees as cover
Construct your greenhouse under deciduous tree cover, if possible. When they shed their leaves in autumn, you’ll have quite the task clearing the greenhouse roof of discarded leaves, but that also means that through most of autumn and winter you’ll have added light in the greenhouse, which will help keep the temperature up.
Come spring and summer, though, the leaves of the deciduous trees will offer shade and function like natural shade cloth, providing cover and lowering the temperature of the greenhouse during the hottest times of the year.
Use the prevailing winds
This is another of those obvious-sounding tips, but it is really very effective. Do some research about the area before you construct your greenhouse. If you can align your greenhouse to take advantage of the prevailing winds in the area, you will have automatic cooling, with no dependence on even solar-powered fans for long periods of time.
As with many things, it might not be possible to keep the temperature of a greenhouse down without dependence on electricity supplied from the mains, but every little bit done to reduce the dependency on that goes towards building a sustainable business and is simultaneously kinder to the environment.