How to Cool an Overheated Greenhouse

Greenhouses are an incredible place to promote healthy plant growth, grow organically and extend your growing season well beyond summer. However, it takes regular monitoring and a human touch for your garden to thrive, even in a greenhouse. If Summertime temperatures overheat your greenhouse, there are solutions.

It is too warm inside your greenhouse? When it's too warm inside a greenhouse, plants can become stressed. Most plants grow best in temperatures ranging from 59°–86°F. When temperatures are above 90°F and the temperatures are sustained for long periods, plant growth is slowed, and some plants begin to show signs of stress. Delicate plants can wilt, sweat, drop leaves and droop. Photosynthesis decreases when temperatures get too hot. Too much heat–especially during the height of summer is one of the most common problems for greenhouse growers.

The ideal growing temperature varies from plant to plant. For example, succulents and alpine plants have different heat sensitivity than vegetables.

 Heat-Tolerant Vegetables: 
  • Arugula
  • Beans
  • Cabbage
  • Chili peppers
  • Cucumbers
  • Eggplant
  • Okra
  • Sweet potatoes
  • Tomatoes
  • Watermelon
Heat Sensitive Vegetables:
  • Arugula (rocket)
  • Beets
  • Broccoli
  • Brussels sprouts
  • Collards
  • Carrots
  • Cauliflower
  • Kale
  • Lettuce
  • Onions
  • Pak choi (Bok choy)
  • Peas

11 Options to Keep your Greenhouse Cooler

Combine two or more of these options to yield a better outcome.

  1. Water Frequently and Consistently
  2. Install roof vents with automatic openers
  3. Shade cloth
  4. Passive Ventilation
  5. Opening windows
  6. Exhaust fan
  7. Interior circulating fans
  8. Misting plants and floors
  9. Monitoring Temperature and Humidity
  10. Portable Air Conditioners
  11. Evaporative Air Coolers


1. Water Frequently and Consistently

The most important task to care of plants in the heat is watering. Consistent watering is crucial for your plants to flourish, for without water, the heat will stress the plants. As excess irrigation water evaporates, there can be a cooling effect. Set reminders for yourself, or install an automatic water system with a timer.
Whenever you water your plants, they’ll release moisture through their leaves and, in turn, lower the plants inside temperature. This process called transpiration helps keep your garden from wilting and is the simplest way to keep your greenhouse cool on sweltering summer days.


2. Roof Vents with Automatic Openers

As warm air rises the air inside a greenhouse becomes trapped at the ceiling. Vents on the greenhouse roof provide an escape route for the warmer air. The size and number of roof vents needed is a function of your local climate and greenhouse size.


Here are some guidelines:

  • Size and count of roof vents should be between 2% and 2.5% of the cubic feet inside your greenhouse. Example: A greenhouse with about 1,000 cubic feet inside (say a 10’Wx16’Wx6’-6”H greenhouse) should have roof vent openings equal to about 25 square feet.
  • Manual operation of the roof vents can be avoided. An automatic opener is recommended. Bayliss Autovent openers provide hands-free operation without batteries or electricity.
  • Screens on roof vents to keep pests such as flying insects, birds, snakes or rodents (critters) out of the greenhouse. Yes, snakes! In parts of the USA snakes are found in trees including Ratsnakes and Ribbonsnakes/Gartersnakes.

3. Shade Cloth

Greenhouse shade cloth is one of the simplest and most cost-effective ways to

reduce heat and direct sunlight. Shade cloth is used to protect plants from getting burned by intense sunlight and as a way to help keep a greenhouse cooler during the hot summer months. Select greenhouse shade cloth fabric that is UV stabilized, rot and mildew resistant and abrasion resistant.

Here’s an easy guide to selecting the proper density for shade cloth – recent adjusted for trends in climate change in North America:

Density Percentage Zone/Crops
50 – 55% Northern Climates (north of 40 degrees latitude): Most vegetables
50 – 55% Southern Climates (south of 40 degrees latitude): Lettuce, Flowers, Tomatoes, mixed crops, and most Vegetables
60 – 75% Ornamentals/Succulents
70 – 80% Hot Climates (over 100%): Light sensitive plants, such as orchids

 
Assuming your location has full sun, shade cloth on the southern, eastern and western facing sides or roof may be a necessity. Eastern sides depend on the sun intensity in the mornings. Shade cloth on northern sides is rarely needed – except for light reflected into a greenhouse – say for orchids. For most greenhouses, this means that you may only need shade cloth for one side of the greenhouse. This will include one side of the roof and one side wall.

It’s easy to install shade cloth on the roof or side wall of your greenhouse if your shade cloth includes grommets on the outside perimeter. However, if you need a temporary solution, try hanging your shade cloth directly over your garden beds or attaching it from a wire inside the greenhouse. That way, the shade cloth can be removed on cloudier days.

4. Passive Ventilation

Openings low in a greenhouse knee wall/base wall are passive ventilation. Passive ventilation requires no batteries or electricity. With openings low in the greenhouse and close to the ground, hot air flows out of any vents higher in the greenhouse, say on the roof, while cooler air is brought in through the openings low in the greenhouse base wall. The effect is to lower the greenhouse temperature inside.
The size of the opening should be about 1 square foot for every 6 linier feet of greenhouse length. The openings should be screened to keep critters out. A door or cover for the opening is recommended so the opening can be secured in the in the winter.

5. Opening Windows

Managing the heat in your greenhouse is reliant on adequate airflow. Passive ventilation with a window helps to bring in cooler air inside, while making the greenhouse more comfortable to work in are two benefits of having opening windows on a greenhouse. Wind pressures forces air into a window one part of the greenhouse and then out through a window or roof vents. Wind passing over the roof vents on a greenhouse can also create a “vacuum” effect and draw fresh air into the windows. Windows should be able to operate in the moist environment of a greenhouse, and also provide protection from critters entering the greenhouse.

 

Jalousie Windows are one option for greenhouse windows. These louvered windows are easy to operate can be mounted in doors or walls to provide extra ventilation. These are great for adding cross ventilation or as vents for greenhouses built on masonry knee walls.

 

Ventilation windows are a great way to bring in a fresh breeze from outside and allow the hot air to escape, giving your plants a break from a hot and stuffy enclosed space. The number of windows you should install depends on the size of the greenhouse, your local climate, and the plants you’re hoping to grow.

6. Automatic Exhaust Fans

Use Exhaust Fans to force hot air out your greenhouse to pull in cooler, outside air. Exhaust fans are typically installed in one or both of the gable ends. Exhaust fans automatically eliminates heat, moisture, odor, and dust. Exhaust fans are typically controlled with a thermostat.
For approximate CFM requirement to size an exhaust fan, divide total greenhouse cubic foot volume by 1.5. For larger greenhouses, consider two or more fans.

7. Interior Circulating Fans

Mixing up the air inside a greenhouse will help normalize temperature inside. If you just finished watering plants, the circulating fan wicks up excess moisture and cools the air inside. Working with your opening windows, a fan also benefits by providing plants wind tension, so they grow up strong. Pest, mildew and mold will be reduced due to high humidity and lack of airflow. Pollination levels will improve by mimicking the outdoor breeze that many plants rely on to propagate.
A good-quality fan is your best friend. However, while a fan works to lower your inside temperature, it will work most effectively alongside ventilation windows and watering.

8. Misting Plants, Benches and Floors Cools the Greenhouse

Use misting solutions to add moisture to the air, plants and any other surface in your greenhouse. This low-cost method will keep your greenhouse cooler. Wetting the greenhouse floor with a hose or an automated misting system will have a similar effect. The cooling effect is intensified when an interior circulating fan is also on.

Misting increases the humidity inside your greenhouse. As the water evaporates, the inside moisture levels rise, the air cools which allows your plants to cope on hot days. Automating the misting with brass misting heads and an automatic timer. Run your misters often as you need. Just be mindful that if you choose to do this regularly, you will need a circulating fan to avoid mold and mildew.

9. Monitor Temperature and Humidity

This low-cost investment provides growers with information to know when to action needs to be taken. These tools offer the temperature and humidity inside your greenhouse. Some solutions just be a display in the greenhouse, others provide a remote display in your kitchen and even on your mobile device. No matter which solution you select, monitoring will provide knowledge on when you may need to take action, or not.

10. Portable Air Conditioners

Portable Air conditions for ‘garage use’ are perfect for most greenhouses. Although costly to purchase and operate, the hope is you only use a Portable Air Conditioner on days when other solutions are insufficient to cool the greenhouse. There are many brands and suppliers, with solutions at most home goods stores.

11. Evaporative Air Coolers

Evaporative Air systems typically require less maintenance than air conditioners, which can produce a considerable savings over the course of a year. While evaporative coolers do use water to cool naturally, they do not produce any mist, fog or spray water. Evaporative coolers are one of the more expensive solutions to cool a greenhouse, but they are effective in non-humid climates. The larger coolers typically sit outside the greenhouse with a vent leading into the greenhouse.

Why Sturdi-Built Greenhouse?

• Wind loads over 100 MPH
• Snow loads over 200 PSF
Strong framing of clear all-heart redwood
• Low-cost temperature control with our unique Thermal Option™
Optional Polycarbonate roof panels that provide 100% protection against UV rays and 10% shading
Optional Tempered Glass roof panels that are 4-times stronger than standard glass
Multiple models and designs to choose from to adapt to wind, snow and hail
• Most models can be beyond 100ft long and up to 16 feet wide
• Pleasing to the eye Sturdi-Built Greenhouse look terrific in any yard
Made in the USA
• Low Maintenance

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