Do Greenhouses Get too Hot?

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Greenhouses seem to be the way to go for a successful garden, but I have a lot of questions when it comes to starting my own. I live in a warm area, so I started wondering if greenhouses can get too hot.

Do greenhouses get too hot? Yes, greenhouses can get too hot. The ideal temperature for plants in a greenhouse is 80 degrees Fahrenheit. With the warming capabilities of greenhouses, temperatures in a greenhouse can easily soar over this ideal temp.

Now that you know greenhouses get too hot, it’s important to understand how that happens and look at ways to solve the problem. A combination of ventilation and a cooling system can help you grow healthy plants in no time.

Why do Greenhouses Get Too Hot?

You might be thinking to yourself, “Don’t I want my greenhouse to be hot?” Most people build greenhouses to ensure their plants get plenty of sunlight, but too much sunlight can actually be a bad thing.

A greenhouse works by keeping in a reasonable amount of heat and water to ensure a good level of warmth and humidity. It does this by slowing down the rate that heat goes out of the greenhouse. 

The extra heat that remains in the greenhouse helps warm up the ground. The warm ground warms up the air, which helps the plants inside grow well. 

When the temperature outside gets too warm, though, that can cause the temperature in the greenhouse to shoot even higher. Those high temperatures can ruin even the best greenhouse garden. 

A greenhouse that is slightly cooler than the ideal temperature is actually better than one that is too hot. So, keeping the temperature down is vital for your plants.

Here’s a look at the ideal temperatures for growing for some vegetables.

VegetableIdeal Night TemperatureIdeal Day Temperature
Cabbage50 – 60° F60 – 70° F 
Broccoli50 – 60° F60 – 70° F 
Celery55 – 65° F65 – 75° F 
Tomatoes60 – 65° F70 – 80° F 
Cucumber60 – 70° F70 – 90° F 

The temperature for growing plants can vary as well. Here’s a look at some of those temperatures.

PlantIdeal Temperature
Marigolds50° F
Roses64 – 77° F
Carnations 50 – 59° F
African Violets65 – 75° F
Gardenias65 – 70° F

Most regions in the United States see daytime temperatures reach these ranges for months during the warm, summer months. In your greenhouse, the temperatures will be even higher, which means your greenhouse is too hot.

Buy a thermometer to track the temperature in your greenhouse. Here’s a thermometer on Amazon that measures temperature and humidity. I’ll explain in a bit why measuring humidity is just as important as measuring the temperature.

Now, that you can keep an eye on when your greenhouse is too hot, it’s time to figure out how you can lower the temperature and keep your plants growing healthy and strong.

Ways to Cool Down Your Greenhouse

Fortunately, you have quite a few options when it comes to cooling down your greenhouse so it doesn’t get too hot. Here’s a look at different ways you can lower the temperature.

  • Open the greenhouse door 
  • Add fans
  • Remove some panels 
  • Use a shade cover
  • Use evaporative coolers
  • Plant large-leaved plants strategically
  • Use water tanks
  • Use misters or foggers
  • Dim your lights if you use grow lights
  • Use an air conditioning unit

Open the Greenhouse Door

Opening the greenhouse door allows for more circulation through your greenhouse. This will hopefully lower the temperature a few degrees. You can also use some fans to circulate the air even more.

Remove Some Panels

Removing panels from the side and roof of your greenhouse also increases air circulation by allowing heat to escape and bringing in outside air. 

Use a Shade Cover

Cover the top of your greenhouse and the side that gets the most sun at the height of the heat to cool your greenhouse down. 

You’ll need about 45 to 70-percent coverage depending on how much sun your greenhouse gets. Shade cloth comes in a number of different materials, including this aluminum one I found on Amazon.  

Don’t go overboard on the shade cloth. Too much shade can actually slow plant growth and make the plants stretch as they struggle to find the sun.

Use Evaporative Coolers

Evaporative coolers are a good energy efficient way to cool your greenhouse. They work by collecting and storing water on a pad inside. Warm air passes over the pad, cools down and is blown back out into the greenhouse as needed. 

Evaporative coolers can cool your greenhouse by 20 degrees when they’re running. Here’s an evaporative cooler you can try from Amazon

Plant Large-leaved Plants Strategically

Designing the layout of your greenhouse with shade in mind can help reduce the temperature. Plants with large leaves placed strategically help shade the soil and keep the soil cooler. Cooler soil means lower temperatures.

Fig trees, grapevines, crepe myrtle, wisteria on an arbor and rhododendrons are all good plants for providing shade. 

Use Water Tanks

Placing a large water tank or even smaller containers of water around the greenhouse also help lower the temperature. The water absorbs the heat. Here’s a 50 gallon water tank on Amazon

Use Misters or Foggers

A mister or fogger cools your greenhouse down by putting small water droplets into the air, which cool the temperature as they evaporate.

Here’s a reasonably priced mister I found on Amazon

Here’s a fogger you could try, also from Amazon

It’s important to keep an eye on the humidity when you use a mister or fogger. Too much humidity in your greenhouse can lead to mold or mildew on your plants and unwanted pests that thrive in warm, moist environments. 

Dim Your Grow Lights: 

You shouldn’t need grow lights too much in warmer months, but if you’re using them, turn them down or even off during the day. You can turn them back on at night when it’s cooler. 

Here’s a set of grow lights with a timer on Amazon that would be helpful. This set also allows you to dim the lights. 

The type of cooling system you choose depends on how hot your greenhouse is and how much you’re willing to spend. You can also use a combination of solutions to get the best results. 

Proper Ventilation Helps With Temperature

The right ventilation is essential to a healthy greenhouse. In addition to providing carbon dioxide to plants, ventilation also lowers humidity and temperature. Without ventilation, you’ll have a greenhouse that’s too hot and full of moisture. No matter how good of a gardener you are, you and your plants won’t win in that situation.

There are two types of ventilation to choose from when it comes to ventilating your greenhouse: natural ventilation or mechanical ventilation. Here’s a quick look at the basics of both systems.

  • Natural ventilation: This system relies on nature and the concept of thermal buoyancy. This is when warm, moist air rises and cool air replaces the warm air. This is accomplished through vents in your greenhouse. The problem with natural ventilation is you don’t have as much control because you rely on the weather.
  • Mechanical ventilation: This system is just as it sounds, mechanical. It uses an exhaust fan to blow hot air out of the greenhouse while bringing in cooler air through shutters at the opposite end of the greenhouse. You have much more control over this system.

Now, that you know the basics, we can take a more in-depth look at what’s involved with each system as far as the equipment involved and placement.

Natural Ventilation

You create natural ventilation by installing roof and sidewall vents. Your vents should each have a combined area equal to 20-percent of the floor area of your greenhouse. Here are some examples of how those numbers work out.

Greenhouse size20% of square footageRoof Vents AreaSide Vent Area
10 ft X 30 ft= 300 ft20% of 300 = 60 ft60 sq ft60 sq ft
20 ft X 40 ft= 800 ft20% of 800 = 160 ft160 sq ft160 sq ft
25 ft X 35 ft= 875 ft20% of 875 = 175 ft175 sq ft175 sq ft

As the air inside the greenhouse gets warmer, it rises and goes out through the vents in the roof. As this air goes out, it creates a vacuum, which then pulls in cooler air through the sidewall vents. 

On cooler days, natural ventilation is very effective because of the larger temperature difference between outside and inside your greenhouse. On warmer days, where there’s not a big temperature difference, there’s almost no buoyancy effect.

A helpful tool for natural ventilation is a solar-powered vent opener. They don’t require any electricity or batteries and automatically open the vent when the paraffin wax inside expands from a rise in temperature. This saves you multiple trips outside on a hot day. 

Here’s a solar vent opener on Amazon that would work well. 

Large electric fans can also help circulate air with a natural ventilation system. A large fan like this from Amazon would really help circulate the air. 

Mechanical Ventilation

When using a mechanical ventilation system, you have to find a system that’s appropriate for the size of your greenhouse. A fan’s rating is based on the air volume it moves in cubic feet per minute. 

You generally want one that allows for one air change every minute. This sounds confusing, but if you just calculate the volume of your greenhouse, you’ll have the rating you need. Volume is length times width times height. This table has some examples.

Greenhouse DimensionsVolumeFan Cubic Rating
30 ft X 10 ft X 10 ft3,000 cubic feet3000 cubic ft per minute
20 ft X 17 ft X 10 ft3,400 cubic feet3,400 cubic ft per minute
40 ft X 20 ft X 10 ft8,000 cubic feet8,000 cubic ft per minute

Since heat rises, it’s best to place the exhaust fan at the top of the greenhouse and intake shutters lower and across the greenhouse. This creates a path of airflow across the greenhouse.

So, which type of ventilation is best for you when it comes to cooling off your greenhouse if it gets too hot? It depends on a few factors.

  • Budget/cost of system: Both systems are comparable as far as the initial investment. Long term, a natural ventilation system will be slightly more cost-effective.
  • Time investment: A natural ventilation system will require more of a time investment because you can’t just put it on a timer. 
  • Size: A mechanical system would be too much power and money for a very small greenhouse as well as a very large one, but work well for a medium-sized greenhouse.

Whatever you decide, keep in mind, maintaining the appropriate temperature AND humidity is essential to growing healthy plants. A greenhouse that is too hot is not a good greenhouse.

Eyerly Family

The Eyerly Family is a family of 8 that loves gardening. Over the past several years we have been applying what we learn about gardening to our own 16x16 raised back yard garden. Our garden is very prolific and we grow a wide variety of vegetables which we love to eat! Click here to learn more about the Eyerly Family.

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