Plant health in a breeze

Features - Production

Experts explain how horizontal air flow provides temperature uniformity, higher levels of carbon dioxide and fewer disease problems in a greenhouse.

April 26, 2016

Photo courtesy of Peter Ling

Air circulation throughout a greenhouse helps growers consistently produce hardy plants ready for retail, and one way to achieve consistent movement is through horizontal air flow (HAF).

“A horizontal air flow system is necessary if you have a heat source that is located in one area of the greenhouse,” says Peter Ling, professor of agricultural engineering at Ohio State University. “You would need it in order to bring that heat throughout the rest of your greenhouse.”

In describing HAF, Tom Dudek, an extension educator for Michigan State University, says the main function of the consistent movement lies in preventing temperature fluctuation.

“You’re utilizing the principle that air, [which] moves in a horizontal pattern in a building, is needed in order [to prevent] hot and cold spots,” he says. “It gives you a more uniform temperature in the greenhouse.”

The second reason for providing consistent circulation, according to Ling, is to prevent negative foliar and fungal strains from developing within the greenhouse.

“From the disease prevention perspective, high humidity can cause condensation that can keep leaves wet, which can promote fungal diseases,” he says. “It’s a good idea to move that high humidity out of the plant canopy and spread it around the greenhouse. Then you don’t need a vent for dehumidification.”

Ling says another benefit of HAF is an increase in carbon dioxide (CO2) levels, which aids in the vitality of each plant.

“The plants need CO2 for photosynthesis,” he says. “And there’s only a very thin boundary layer of CO2 surrounding the surface of each plant. But the CO2 in that boundary layer can get depleted very quickly if there’s no air movement. You want to average out the CO2 content within the greenhouse to resupply what the plants need.”

Dudek says that he has seen the advantages of HAF for both vegetable and flower greenhouses.

“[HAF] would benefit anybody that would grow any type of plant material,” he says. “I don’t think that there’s a crop that wouldn’t benefit from it.”

Horizontal air flow fans hang from the trusses of a greenhouse to improve carbon dioxide circulation and reduce humidity.
Photo: Laura Watilo Blake

Designing a functional pattern

One challenge to HAF is the physical barriers present within a greenhouse, says Tim Malinich, a horticulture extension educator at Ohio State University.

“If air isn’t moving and flowing because something is physically blocking it, then you have an inefficiency with your HAF,” he says.

Malinich says the remedy to this problem is simple: When designing the air flow pattern, keep in mind barriers that you can’t move and don’t overcrowd the hanging baskets.

Ling advises using a perforated plastic tube system to move air through hanging baskets. For the general design of HAF throughout your greenhouse, make sure each fan has enough clearance behind it in order to maintain intake efficiency.

“If you have the fan too close to the side wall, it makes the air flow very inefficient,” he says. “You need to have your system designed so that your air will circulate inside the greenhouse. For example, if you have two sets of horizontal air flow fans, then you would have one set moving the air down one aisle and then the other blowing in the opposite direction down the next, creating a circular pattern to the movement.”

He says a simple way to test if there’s enough air flow throughout the greenhouse is by using a tissue.

“There’s a rule of thumb. It’s basically that you need to have enough air movement that the leaves move just a little bit,” says Ling. “One way to test this before you have plants in there is to take a tissue and hold it up and see if it flutters just a little bit in each part of your greenhouse.”

Photo courtesy of Peter Ling

Installing the system

Dudek says setting up HAF in a greenhouse only requires a few calculations and the time to install each needed fan.

“Total fan capacity should be about two times the floor area,” he says. “So for example, a 30-foot by 100-foot house, 6,000 cubic feet per minute (cfm) is what you’re looking at. Then all you have to figure out is how many fans it’s going to take to cover that, and each fan is rated by how many cfm it can move. So if you plan to use fans that have a 1,675 cfm capacity each, divide that into the 6,000 cfm’s needed and that [equals] roughly four fans.”

Dudek also emphasizes the need to maintain the HAF system once installed.

“The brackets may move, the chains may need adjustments periodically and just like your fans at home, they will collect dust over time that will need cleaning off,” he says. “But it’s not a hard system to understand or install, and any grower can benefit from it.”


According to the University of Connecticut HAF factsheet published in 2013 by John Bartok Jr., standard HAF fans cost between $100 and $150, plus installation. Bartok says to check with your local U.S. Department of Agriculture’s Natural Resources Conservation Service office to see if there is grant money available to offset the cost of installation. He further explains that a grower’s operating cost will depend upon the local cost of electricity.