Should Grow Tent Fans be Left on 24/7? (Answered)


If you’re a new grow tent owner, hopefully you’re enjoying the perks of growing plants in a controlled environment. But you might wonder if you should always keep a fan blowing in your grow tent. While it requires enough ventilation to mimic the natural breeze plants receive outdoors, is a constantly-running fan good or bad for your plants?

Fans should always be kept on in a grow tent if you have your grow lights on or if your tent flaps are kept closed, so that your plants get enough air they need to be healthy. You need to increase or decrease the fan speed to accommodate different stages of plant growth and ensure the correct humidity levels.

In this article, we’ll explore what you need to know about using fans in a grow tent and how to adjust them depending on your plants’ growth stage.

Two grow tent fans set side by side inside of a grow tent above some pepper plants.
For my grow tent, I use VIVOSUN fans. Photo by Spicy Trio

How Long to Run Fans for Different Plant Growth Stages

Your plants’ ventilation requirements vary depending on the plants you are growing, their growth stage, and the tent conditions. For example, if you need to decrease the humidity of your tent, you should increase how much you’re using your fan until the humidity levels are ideal.

Let’s look at the different plant stages and see how much air circulation they require.

Sprouting Stage 

Seeds will germinate and sprout within days or weeks, depending on the plants you’re growing in your grow tent. Seeds require lots of ventilation to grow and prevent what’s known as damping off disease. This disease occurs when fungus or mold attacks the vulnerable seeds, as the University of Minnesota Extension reports.

To ensure your seeds get enough ventilation and to keep humidity low so that fungus and mold can’t strike, keep your fan on at a low speed throughout the day to mimic a gentle breeze. Maintain a balanced humidity that’s around 50% to 60%.

It’s essential to check the humidity levels regularly in your grow tent, not just during the sprouting stage but during all plant growth stages, to adjust how much ventilation you’re providing them.

You can do this by purchasing a hygrometer that measures humidity. We recommend the Vivosun Digital Indoor Hygrometer, available from Amazon.com. It provides high and low readings for temperature and humidity while working most effectively when placed close to your plants, which makes it ideal for use in grow tents.

Seedling Stage

When plants reach the seedling stage, they need lots of air circulation in the tent as moisture or dampness makes them vulnerable to mold and disease. However, this should be a gentle breeze that isn’t too drying for the seeds, bearing in mind that they need a large amount of humidity.

Aim for between 60% and 70% humidity so seedlings will get the moisture they need for roots to grow.

Vegetative Stage

This stage of plant growth falls between germination and flowering. As Indiana University reports, plants in this stage are photosynthesizing and gathering resources that they will need to flower and reproduce.

Your grow tent fan should provide the plants with moderate airflow day and night. Humidity levels can decrease a bit, so aim for around 50% or 60% after the first three weeks that plants are in this state. This requirement is because the plants are growing, so they don’t require as much humidity as they prepare for flowering. 

Jalape_o_Pepper_Lines (4)
Jalape_o_Pepper_Lines (4)

It’s important to note that while plants require air that’s rich in carbon dioxide (CO2) to complete photosynthesis, the small space of a grow tent will increase the amount of CO2 they receive, so you want to remove stale air and bring in fresh, oxygen-rich air to provide the optimal balance. 

Budding Stage 

During the budding stage, not to be confused with the flowering stage, plants will prepare to flower. To get plants ready to bloom, they must receive phosphorus, an element that plays the most crucial role in stimulating strong buds and developing plants’ flowers and fruits, as SFGate reports.

Interestingly, how well plants can draw phosphorus relies on the correct moisture of the soil. When the water in the soil is low, phosphorus mobility decreases, as the Journal of Plant Nutrition reports. This is why you should avoid using your grow tent fan at high speed for the entire day as this will reduce the moisture that plants require during their budding stage.

An oscillating fan is ideal for your grow tent as it ensures good air circulation without blasting your plants directly and drying them out. Stick to humidity levels of approximately 55%.

Flowering Stage 

During the flowering growth stage, plants need constant air circulation. This circulation will ensure that they are not negatively affected by humidity or heat.

Keep the fan in your grow tent active when night falls for one hour, then slow it down. At the fruit and flowering stage, you want to ensure that daytime temperatures in the grow tent are between 76℉ and 78℉ (24℃-25℃) and night-time temperatures are 70℉ (21℃).

Humidity during the flowering stage, which can last eight weeks, should be kept at around 50%.

Ripening Stage

The ripening stage is when the plants have ripened and matured, whether they bear flowers or fruit. You want to keep your grow tent well ventilated during this time to help ripen the plants’ fruits. Hypoxia, or a severe lack of oxygen, can delay the ripening of some fruit.

Ensure you keep the fan in your grow tent constantly running to provide good air circulation. During this phase, you should aim for approximately 40% humidity in your grow tent to encourage the ripening of plants. This level will ensure that plants’ air isn’t too dry or wet, which can cause harvesting issues.

Keep your fan blowing steadily but gently all around the plants in your grow tent. This circulation will prevent humidity from remaining on your plant’s leaves and causing health issues such as fungal disease.

Photo of two LED grow lights hanging above pepper plants with two black oscillating fans hanging off to the left
Here is my grow tent set up, two LED grow lights with two oscillating fans.

How Powerful Should Your Grow Tent Fan Be?

When choosing a fan for your grow tent, you need to consider the tent size to calculate how many cubic feet per minute (CFM) your fan should have.

Your grow tent fan should be powerful enough to generate the correct CFM for your tent size. You can calculate the CFM it requires by multiplying the length, height, and width of your grow tent to get its measurement in cubic feet. Thus a 5′x5′x5′ would require a 125 CFM fan.

To be safe, you should calculate your fan’s CFM to be slightly larger than this number to ensure that it will provide enough ventilation.

For our grow tent fan we use two VIVOSUN AeroWAve A6 Clip-on fans

How to Set Up Enough Ventilation in Your Grow Tent

You can’t rely solely on using a fan in your grow tent. Your air exchange system should consist of a fan to circulate air and an extractor, or exhaust, fan so that you can remove stale air from the grow tent. This exchange will draw hot, humid air out of it. Extractor fans are combined with ducting so that you can effectively pull the air away from the tent.

Ducting is usually made out of aluminum, as it’s more affordable when setting up your grow tent, but ensure that your ducting is short so that you reduce how far the exhaust air has to travel.

Why Ventilation is Important for Your Grow Tent

As we’ve seen in this article, ventilation in your grow tent is essential. It ensures that you don’t expose your plants to too much humidity. There are other important reasons why you should ventilate your grow tent.

  • Ventilation ensures photosynthesis. Live Science reports that plants turn light, water, and CO2 into oxygen and sugars (or food) during photosynthesis. Plants will run out of CO2 because they have to continue using it for effective photosynthesis. So, to ensure this process works well, you need a constant fresh supply of air.
  • Ventilation prevents too much heat from your grow lights. Since those grow lights you’ve installed in your tent can become hot, they can cause high temperatures for your plants which aren’t conducive to their growth. You need to ensure you maintain a cooler interior with ventilation systems.
  • Ventilation prevents stagnant air. When the air doesn’t circulate, this not only results in higher humidity levels, but humid air attracts mold, mildew, and insects. These can harm your plants and stunt their growth.
  • Ventilation strengthens plants. When plants are grown outdoors, their exposure to the wind helps them to become stronger. It encourages their stems to be strong enough to bear seeds and fruits. When you grow plants indoors, such as in a grow tent or room, there is no natural breeze, so you need to provide constant ventilation to mimic the outdoor conditions.

Closing Thoughts

If you want to grow plants in a grow tent, you’ll need to ensure that you provide enough ventilation, or you may risk stunting your plant’s growth. As we’ve seen, plants’ ventilation requirements depend on their growth stages, but generally, it’s essential to maintain a constant flow of air in your grow tent with a fan.

Supriya

Hi! I'm Supriya. I'm a home cook, bulldog mom, spicy food lover, and founder of The Spicy Trio. I have been a home cook for about 15 years and have been growing plants for the past six years.

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