How to Keep a Greenhouse Cool

Last summer, I had several problems with my greenhouse overheating. I decided to be more prepared for this growing season, researching different ways to keep my greenhouse cool this summer. So, I thought it'd be useful to share what I learned.

The best ways to keep a greenhouse cool involve managing airflow, ventilation, and shade, as well as using cooling walls. However, how much cooling you need depends on a number of factors, like the types of plants you're growing, how hot your greenhouse gets, and your location & climate.

Quick overview of greenhouse cooling options

Vents and windows allow for cool air to enter the greenhouse, and fans can keep fresh air circulating through the greenhouse. Cooling walls use evaporating water to keep the structure cool, and shade prevents the greenhouse from warming too much.

You have many options when it comes to keeping your greenhouse cool. Primarily, these options fall into three different categories:

  • Shade
  • Vents and airflow
  • Cooling Walls

Shade can be offered through artificial or natural resources. For instance, if you place a greenhouse under or near trees, you’ve placed your greenhouse in a place where it will naturally get shade. Conversely, if you lack trees, you can use materials that are made for the purpose of providing plants shade.

Vents allow for cool air to enter your greenhouse, helping to maintain a cooler temperature. Airflow is just as important as vents, in that fans will help fresh, cool air enter into the greenhouse. Lastly, cooling walls use water that’s evaporating to cool the air that enters the greenhouse.

Temperature

So, first of all, why is it important for a greenhouse to stay cool? If a greenhouse gets too hot, you run the risk of doing serious damage to your plants. If plants get too hot, they may:

  • Wilt.
  • Dry out.
  • Become stressed out.
  • Stop producing leaves/produce.

Most plants tend to have an ideal temperature to grow and produce vegetables at. If you surpass this temperature, your plants won’t stand a chance at being as large or as healthy as they could be.

How hot is too hot? As a general rule, anything over 90 degrees Fahrenheit is too hot for most types of plants. For instance, tomato plants won’t produce fruit if they’re in environments warmer than 90 degrees.

Ideally, your greenhouse will thrive at temperatures between 80 and 85 degrees Fahrenheit. However, some plants can thrive in conditions cooler than that. For example, beet seeds germinate in cooler, moist soil. Before deciding on the best temperature for your specific greenhouse, it may be best to take what plants you plant to grow into account.

Now let’s talk about the many different ways you can keep your greenhouse cool. Some of these methods may require an initial investment and a little bit of work. Other methods, as you’ll soon learn, are free and require no initial work besides pre-planning where you place your greenhouse.

Shade

Shade can be created through three different ways:

  • Using a structure that already exists.
  • Using trees and vegetation.
  • Using artificial means, such as shade cloth.

Using already-existing structures

Before erecting your greenhouse, it’s important to assess how much shade you will need. If you’re going to require a fair amount of shade, consider erecting your greenhouse so that it shares a wall with an already-existing structure.

For example, you could site your greenhouse so that one of its walls shares a wall with the back of your house. Then, your house can shade your greenhouse during certain time of the day. Pay attention to what parts of your house receive shade at what times of the day.

Keep in mind that the sun will rise in the east in the morning, and set in the west in the evening. Think critically about what time of the day you’d like to offer shade to the plants in your greenhouse.

Using trees and vegetation

Another way to create shade for your greenhouse is to use trees and vegetation that already exists. By building your greenhouse near or under the leaves of trees, you’ll naturally create shade for the plants in your greenhouse.

If you plan to operate your greenhouse year-round, consider placing your greenhouse under or near deciduous trees. Deciduous trees have leaves that fall off in autumn and grow back in the spring.

The advantage behind deciduous trees is that during the hot, summer months, the tree will provide shade with its leaves. Then, when the cooler, winter months come, the trees’ leaves will fall off, providing your greenhouse with the sunlight it will need.

Be aware of erecting your greenhouse near coniferous trees, as coniferous trees (evergreens and firs) will not shed their leaves in the fall. If you build your greenhouse so that it receives shade from coniferous trees, expect to receive that shade throughout the entire year.

Shade cloth and artificial shade

The last option to create shade for your greenhouse is to use an artificial material designed to provide shade, with the best known material being shade cloth. Shade cloth is essentially a blanket that you use to cover your plants, which blocks a certain amount of the ultraviolet (UV) rays your plants are exposed to.

Shade cloth can be placed in your greenhouse a few feet above your plants, or it can be placed over the entirety of your greenhouse.

You can buy shade cloth at different percentages or grades. For instance, shade cloth that is labeled as 30 percent will block out 30 percent of UV rays. Conversely, shade cloth that is rated to 90 percent will block out 90 percent of the Sun’s UV rays.

Keep in mind that, depending on your climate and location, you may need to remove the shade cloth in the morning and evening hours, and then place it back on your greenhouse in the afternoon to keep your greenhouse at an ideal temperature.

Venting and airflow

It’s no surprise that one of the easiest ways to keep your greenhouse cool is through using vents and windows. Vents and windows allow cool, fresh air to enter your greenhouse while simultaneously allowing the hot air in your greenhouse to exit.

Typically, you’d open the vents in the morning to allow for airflow, and then close the vents in the evening to keep the greenhouse warm. However, if you live in a particularly warm and dry climate, you may not need to close the vents in the summer months.

Timers and sensors

One thing to consider regarding vents is whether to include manual vents that are opened by hand, or electrical vents that open by themselves on a timer or sensor. For obvious reasons, vents that are electric and on a timer will be more expensive.

However, they’ll also afford you the luxury of not needing to remember if you opened the vents in the morning. If you travel a lot and aren’t always home to routinely open your vents, it may be best to consider electric vents on a timer.

Timed openers will open your vents and windows, whereas sensor openers will open your vents and windows when your greenhouse reaches a certain temperature.

Fans

Perhaps one of the easiest ways to ensure that your greenhouse stays cool is to use a fan to keep air consistently moving throughout the greenhouse.

Depending on the size of your greenhouse, you may need more than one fan to keep your greenhouse cool. One of the most popular ways to utilize fans in your greenhouse is to use two fans, with one pulling in cool air and the other pushing out hot air.

To do this, place one fan (positioned to pull air from the outside) at the entrance of your greenhouse. Placing this near the ground will maximize the amount of cool air that gets pulled in.

Then, place another fan near a high vent or window (positioned to blow warm air out of the greenhouse). This will ensure that your greenhouse has the best possible airflow.

Evaporating walls (i.e., cooling walls)

Another fantastic, innovative way to keep your greenhouse cool is through the use of evaporative cooling walls. Evaporative cooling can cool your greenhouse by as much as 10 to 20 degrees Fahrenheit by evaporating water off of wet surfaces and using that evaporative water to cool the air.

It’s important to note that the efficiency of an evaporating wall depends on the humidity of the area you live in: evaporative cooling works best in dry and arid climates.

Evaporating walls work by keeping a breathable surface (like cardboard) continuously wet. Then, fans are used to pull air through that wet surface and into the greenhouse.

Because the breathable surface is wet, the hot air that gets pulled through gets cooled down, bringing cooler, wetter air into your greenhouse. Evaporative walls and cooling mechanisms can be bought through stores, or you can build them on your own.

Other things to consider

A great way to heat a greenhouse is to use materials that absorb heat during the day and release heat during the night. Materials that absorb heat include things like water, cement and rocks.

Another thing to consider is the color of the pots you use in your greenhouse.

If your plants are growing in solid black, plastic pots, the greenhouse will warm much quicker due to sunlight warming those black surfaces. To prevent this, use white, clear, or cloth pots. This certainly won’t keep the greenhouse from overheating, but it will help.

One last thing to consider is the importance of utilizing thermometers to monitor the temperature of your greenhouse.

Many different thermometers exist that record the highest temperatures and lowest temperatures for the last 24 hours. This is helpful if you need to know how warm your greenhouse is getting during the heat of the day, or how cool your greenhouse gets at night.

That's a quick overview for keeping a greenhouse cool during the hot summer months. Again, depending on what you're growing, where you've sited your greenhouse, and your climate & USDA zone, you'll want to consider factors specific to your situation.

Related questions:

How do I keep a greenhouse cool in the desert?

One of the most common ways to keep a greenhouse cool in the desert is by utilizing evaporative cooling walls. While shade and vents can help, by themselves, they might not provide sufficient cooling--that's where evaporative cooling (aka swamp coolers) are helpful, given the low humidity prevalent in deserts.

What temperature is too hot for a greenhouse?

Typically, anything above 90 degrees Fahrenheit is too hot for a greenhouse. However, the maximum temperature will depend on what types of plants you're growing. Cacti & most succulents can tolerate higher temperatures, while greens and lettuces do better with lower temperatures.

What temperature should a greenhouse be kept at?

For most scenarios, the ideal temperature for a greenhouse is going to be between 80 and 85 degrees Fahrenheit. However, the optimal greenhouse temperature depends on the kinds of plants you're growing: greens like it cooler, while cacti like it hotter.

About the author

Greg Volente

Greg Volente holds a Naturalist Certificate from the Morton Arboretum, and worked for The Nature Conservancy leading environmental education programs and doing natural areas restoration. Besides gardening, he's an avid wildflower enthusiast, and loves botanizing, hiking, and backpacking.