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A greenhouse is a great way to germinate seeds outdoors, nurture or harden off seedlings, and give mature plants a warm place to grow late in the season. However, it is possible that your greenhouse can get too warm on the hottest days of summer. So, how do you cool a greenhouse? You can cool a greenhouse with fans, swamp coolers, or heat pumps if you have electricity. If not, you can cool a greenhouse by being creative with extra shade, ventilation, or water. The methods you use will depend on your budget, whether you have electricity in your greenhouse, and how much work you want to put into keeping your greenhouse cool. Let’s take a closer look at some of the methods you can use. How To Cool A Greenhouse If your greenhouse gets too hot, it can prevent seed germination or stunt the growth of seedlings or young plants. High temperatures can also prevent proper pollination. Excessive heat can even kill your plants. If your greenhouse gets too hot, you will delay seeds from germinating, stunt the growth of seedlings, or even kill your plants! So, how do you cool down a greenhouse that is too hot? If you have electricity, there are a few ways. Let’s start with one that I am a big fan of (sorry, I like puns). Use Fans To Cool Your Greenhouse Simply put, a fan cools a greenhouse by circulating air. This method is also called forced-air ventilation. A fan is a simple but effective method of circulating air and cooling your greenhouse. You need electricity hooked up to your greenhouse though. A fan on one end of the greenhouse pulls cooler air from outside the greenhouse and pushes it into the greenhouse. At the same time, the warmer air inside the greenhouse is pushed out through vents or shutters on the other end of the greenhouse. One advantage of this method is that you can set up an automatic system where a thermostat is used to control the fans. When the air inside the greenhouse reaches a certain temperature, the fans will turn on and cool it down. However, the fans and shutters are an additional one-time expense when building the greenhouse. You also need to take into account the ongoing cost of electricity to run the fans. Of course, if you had enough solar panels, their energy output could run the fans on the hottest, sunniest days of the year. For more information on using fans to cool a greenhouse, check out this article on greenhouse environmental control from the University of Tennessee Extension. Use Swamp Coolers To Cool Your Greenhouse A swamp cooler uses the evaporation of water to cool air. The key to this method is the fact that water absorbs a large amount of heat in order to evaporate. A swamp cooler uses pads to bring a large volume of water in contact with the air (the more surface area on the pads, the more water can be brought into contact with the air). A swamp cooler takes warm air from outside of a greenhouse and converts it into cool air to push into the greenhouse. Although they are energy efficient, swamp coolers still require some electricity to power a fan and a pump. Swamp coolers will also need to be cleaned more often than fans, due to the potential for mold growth (due to the water required for the device to operate). For more information, check out this article on evaporative coolers from Wikipedia. Use Heat Pumps To Cool Your Greenhouse A heat pump transfers energy (heat) from a source to a sink (or heat reservoir). In a greenhouse, it takes heat energy out of warm air inside of a greenhouse, and transfers it outside of the greenhouse. It uses a refrigerant, which is a fluid that is evaporated and condensed (changed from liquid to vapor and back again) by changes in pressure. As with a swamp cooler, the concept of evaporation is a key part of what makes a heat pump work. However, instead of using water for evaporation and condensation, heat pumps use a refrigerant (such as CFCs, or chlorofluorocarbons). Heat pumps are efficient, but like fans and swamp coolers, they require electricity to run. The refrigerant may also leak or need to be recharged from time to time, just as with a refrigerator. For more information, check out this article on heat pumps from Wikipedia. How To Cool A Greenhouse Without Electricity Maybe you don’t have an electrical outlet in your greenhouse, or you don’t think you can afford the electricity to run fans, swamp coolers, or heat pumps to cool your greenhouse. In that case, you’ll want to explore some of the greenhouse cooling options that don’t require electricity. Provide Shade To Cool Your Greenhouse Providing shade to your greenhouse is one way to cool it down, especially when the sun beats down on days with no clouds to block the sun and no wind to provide ventilation. Here are some ways to shade your greenhouse. Shade Cloth Shade cloth is one option that provides protection from the sun to keep your greenhouse cool. Shade cloth is made of netting or other material and they allow partial sunlight to pass through them, depending on their density. Shade cloth has a mesh pattern that blocks out some of the sunlight to keep a greenhouse cooler. You can put a shade cloth outside of the greenhouse to protect the entire structure from sunlight, or inside the greenhouse to protect a specific section (such as plants that are still sensitive to light and heat). If you put the cloth outside, it will block the light before it enters the greenhouse and warms it up. Shade cloths are rated with percentages in terms of their density, which tell you how much sunlight they block out. For example, a 90% shade cloth has 90% density and blocks out 90% of light. So, only 10% of sunlight (100% - 90%) would get through this shade cloth. The higher the density of the shade cloth, the more sunlight you block out, and the cooler the greenhouse will stay. You can often find shade cloths rated in increments of 10%. For example, 30%, 40%, 50%, 60%, 70%, 80%, and 90% shade cloth densities are all available. They can also come in different colors, from black to green to red to white. The most intense sunlight comes from south or west sides of a greenhouse. To minimize heat from the sun on the hottest days, put a shade cloth on south or west facing sides of the greenhouse. Don’t forget to remove the shade covers on overcast days, so that your plants get enough light in those cases. Remember that if a shade cloth is larger than 100 square feet (a square that is 10 feet per side), then you might need extra support. This will prevent the shade cloth from sagging in the middle. Blinds You can also install blinds to keep sunlight out of a greenhouse, either on one side or on multiple sides. Exterior blinds are more effective, since they block the light before it enters the greenhouse to warm it up. However, if you only want to cool down the greenhouse slightly on sunny days, then interior blinds can work. You can even install automated blinds, but they will cost much more than manual ones. Foliage Instead of using shade cloth or blinds, you can take a more natural approach to cooling your greenhouse. The foliage of plants or trees can provide the shade you need to keep your greenhouse much cooler on sunny days. One option is to plant a tree with thick foliage to hang over the greenhouse. Just be sure that it does not block out too much light, or you will soon be trimming it or cutting it down! You can also plant grapes or other vining plants with large leaves to provide shade. Simply let them climb up one side of the greenhouse and down the other, and they will provide shade Grapes have large leaves that can provide lots of shade when the vines grow in thick. Grow grapes or other vining plants with large leaves up one side of the greenhouse and down the other. Again, growing the grapes outside of the greenhouse will keep it cooler than growing the grapes inside. You can even build a pergola over the greenhouse and allow grapes to climb up and over to provide shade. For more information, check out my article on pergolas. Use Vents To Cool Your Greenhouse Another option to cool your greenhouse involves ventilation by natural air currents. That means that either wind blows to cool down the greenhouse, or the warm air inside naturally moves outside and is replaced by cooler air. You can build a greenhouse with either roof vents or side vents to allow warm air to move out of the building when it gets too hot. Greenhouse vents can be installed to open automatically when it gets hot. You can make these vents manual, or you can automate them, but remember that there is extra time and cost involved with automation. Remember that the greenhouse door itself could act as a kind of vent, either manually or automatically. For more information on using vents to cool your greenhouse, check out this article on greenhouse ventilation from the University of Massachusetts at Amherst. Use Water To Cool Your Greenhouse Water is another way to cool your greenhouse, and this method is often overlooked. As we discussed earlier, evaporation requires lots of heat (energy). When water evaporates, it cools the air, just like sweat cools your body as it dries. There are a few different ways you can implement this method – in fact, you can use all of them if you wish! Wet Down The Hard Surfaces In Your Greenhouse If you wet the floor and walls of your greenhouse, the resulting evaporation will cool down the greenhouse. If you wet the ground inside your greenhouse, the evaporation will cool it down. Just watch the humidity level! Just remember that when water evaporates, it has to go somewhere, and it ends up in the air. Your greenhouse can end up very humid if you use this method. Keep A Water Tank In Your Greenhouse Keeping a water tank in your greenhouse can serve multiple purposes. First of all, it can be used as a convenient source of water for your plants. Keeping a water tank in your greenhouse is another way to allow cooling by evaporation. More importantly, a water tank helps to cool down the greenhouse when water on the surface evaporates. The larger the surface area of the water, the more evaporation will occur. So, use a wider tank if possible, to increase the amount of water in contact with the air. Use A Misting System In Your Greenhouse One more option is to install misting hoses in your greenhouse to keep it wet. Not only will this water your plants slowly and consistently, but it will cool things down when the water evaporates. For more information, check out this article on greenhouse cooling from the University of Georgia Extension. Build Your Greenhouse Partially Underground There is one more method to mention, and this one is also overlooked. If you build your greenhouse so that part of it is underground, then it will stay cooler, since more of the building is in contact with the earth. This might not be practical if you have already built a greenhouse. However, if you are thinking of building an additional greenhouse or replacing your old one, this is an option to consider. Conclusion By now, you have a much better idea of how to cool a greenhouse, whether you have electricity available or not. You also know how to use shade and water to help cool your greenhouse without electricity. I hope you found this article helpful – if so, please share it with someone who can use the information. If you have any questions about how to cool a greenhouse, please leave a comment below.