Does Your Greenhouse Need a Fan?


Providing the plants growing in a greenhouse with proper ventilation is key to their health and productivity. A greenhouse cannot function correctly without some type of ventilation system. This system can be manual, in the form of vents, or electric, with the use of a fan.

Regardless of its size, a greenhouse needs to have proper ventilation. A greenhouse does not need to have a fan if it has adequate vents that allow hot air to escape and cool, fresh air enter. Fans promote air circulation inside the greenhouse, which helps plants to grow optimally.

Most people get a greenhouse with the idea of keeping plants warm and protected from the frost, but do not consider the converse – keeping plants cool. Without ventilation in a greenhouse, plants will die from too much heat. There are a number of ways to ensure that your greenhouse is well ventilated, including but not limited to using a fan.

The Importance of Greenhouse Ventilation

Learning to use a greenhouse is a process of trial and error. It helps to understand the reasoning behind why ventilation is so crucial to success with growing inside a greenhouse.

  • Temperature regulation. Gardeners that are new to using a greenhouse are always astounded at how quickly it can get scorching hot inside. To keep the temperature within the optimal range for the plants you are growing, there needs to be some way for the hot air to escape. While most plants enjoy the extra warmth, a greenhouse provides, too much heat will cause them to die.
  • Plants need fresh air to photosynthesize. Apart from water and nutrients, plants require a constant supply of carbon dioxide from the air to enable them to produce the sugars they need to survive. A vented greenhouse allows fresh air in to circulate around all the plants. Without ventilation, the plants will struggle to photosynthesize.
  • Improves pollination success. Many plant species rely on wind to spread their pollen, like tomatoes, eggplants, and peppers. Inside a greenhouse, plants are completely sheltered from the wind, so they may not produce fruits because their flowers are not getting pollinated.
  • Prevent pests and diseases. Insect pests, like whitefly, are comfortable when there is no wind. Without good air circulation in your greenhouse, the space will attract all the pests in your garden. Fungal diseases, like powdery mildew, also thrive in warm, humid conditions with little airflow.
  • Helps plants to grow strong stems and roots. Plants that grow outdoors have to content with the wind and so develop robust stems and root systems to anchor themselves to the ground. Simulating the gentle force of the wind on a plant inside a greenhouse will encourage plants to grow sturdier stems and roots, resulting in healthier, more productive plants.

Here’s a quick video showing how to install vents in a greenhouse so you can see what’s involved:

Does a Small Greenhouse Need Ventilation?

It is easy to assume that only big greenhouses need ventilation – is it really necessary to put a fan into your cupboard-sized greenhouse? The answer is always yes.

Even inside a tiny greenhouse, like a DIY one made from hula hoops and a clear plastic sheet, temperatures can get really high – too high for your plants to survive. Even the smallest greenhouse requires vents for a good cross-breeze.

Vents alone may provide sufficient ventilation for a very small greenhouse, but the addition of a fan will greatly improve air circulation.

The larger the size of the greenhouse, the more air vents it will need, as there is a greater volume of air that needs to circulate. Without a fan in a medium to large-sized greenhouse, you will need a huge number of air vents near the roof and the base of the structure.

Manual versus Electric Ventilation

Manual ventilation refers to vents at the top and the base of the greenhouse walls that you have to open in the morning and close in the evening. The vents create a cross-breeze due to natural convection.

The air inside the greenhouse warms up as the sun rises. The warm air expands and floats to the top of the greenhouse, where it can escape via a vent. This creates a low air pressure near the floor of the greenhouse, which causes fresh, cool air to be sucked in via the lower vents, supplying the plants with carbon dioxide-rich air.

Electric ventilation refers to the use of exhaust fans and circulation fans to pump hot air out and circulate fresh air around the greenhouse. These can be powered by electricity from your mains supply, or they can be solar-powered.

Electric ventilation is advantageous because you do not have to remember to open and close the vents each day. It is easy to connect the exhaust fan to a timer that will automatically start and stop the fan at a set time. This allows you to go away for a weekend without having to recruit someone to take care of your greenhouse.

Here’s a video with a smart way to vent a greenhouse with a little fan:

Types of Greenhouse Fans

There is a wide range of fans on the market, ranging from low-tech to high-tech. The basic types are:

  • Exhaust or extraction fans. These are mounted into the wall of the greenhouse and suck hot, humid air out, similar to the fans found in bathrooms and above the stove.
  • Circulation fans. This type of fan is for moving air around within the greenhouse.
  • Hanging fans. These are great for moving the layer of hot air that gets trapped at the top of a greenhouse, allowing for a better cross-breeze.
  • Standing fans. Fans that stand on the floor of the greenhouse are a simple solution for ensuring good air circulation.
  • Climate control fans. More high-tech fans have a thermostat that controls the speed of the fan, according to the temperature inside the greenhouse. They are more expensive but are well worth the investment.

Ventilating a Greenhouse Without a Fan

It is perfectly okay to choose the natural, low-tech way to ventilate your greenhouse. You may be looking to minimize the cost of setting up your greenhouse or are put off by the idea of adding to your electrical bill.

If you are planning to ventilate your greenhouse without using fans, then the design of the greenhouse becomes more important. The design must incorporate a greater number of vents to ensure adequate cross-ventilation.

Designs that place vents on opposite sides of the greenhouse, near the roof and the floor, are optimal. This will provide a steady breeze throughout the greenhouse.

Advantages & Disadvantages of Using a Fan in the Greenhouse

If you are still in two minds about whether or not to use a fan in your greenhouse, let us look at some of the advantages and disadvantages:

Advantages

  • Fans guarantee airflow inside the greenhouse.
  • There is no need to remember to open and close vents every day, giving you more freedom.
  • Greater pollination success for wind-pollinated or self-pollinating plants.
  • Extra air pressure on plants, especially seedlings, causes them to grow stronger stems and root systems.
  • Lower risk of pests and diseases taking hold in the greenhouse.

Disadvantages

  • Fans are an added expense – to purchase, install and run.
  • They add another maintenance element to the running of the greenhouse, especially if they are solar-powered.
  • Fans require a greenhouse to be built near a mains power supply, which is not always possible.

Conclusion

Ventilation is one of the fundamental factors to consider when building a greenhouse. Regardless of the size of the greenhouse, adequate ventilation is always necessary.

Proper ventilation allows you to regulate the temperature and humidity inside the greenhouse. Having good airflow promotes healthy plants by ensuring a constant supply of carbon dioxide-rich air, which plants need to photosynthesize.

Other benefits of a well-ventilated greenhouse are a lower risk of pests and diseases and improved rates of pollination.

It is not necessary to use a fan inside your greenhouse, provided that there are enough vents on opposite sides of the greenhouse, near the top and the bottom. This is critical for natural convection to create a cross-breeze through the greenhouse.

Robert Sampson

I'm Robert Sampson and I live in Colorado where I spend a lot of time in the backyard with my family either grilling, playing games and sports, or working on a project to make our backyard a better place to be.

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