High tunnels provide an option for growers where temperatures limit cropsWritten by Natasha Wheeler
“The key thing about a high tunnel is that it is not a greenhouse,” states Dan Drost, Extension specialist at Utah State University (USU). “There are different zoning regulations for greenhouses relative to removable, temporary structures like a tunnel.”
In terms of zoning, tunnels are also structures without power sources such as cooling fans or heaters.
“We heat the thing with sun coming through it and cool it by opening doors and ventilating the sides. If we put fans in it, it becomes a greenhouse again,” explains Drost, noting that greenhouses can be subject to different rules and taxes.
A high tunnel is also different than a low tunnel, which only covers a single plant or series of plants. However, low tunnels can be used inside high tunnels for increased temperature control.
“We can grow a wide variety of crops inside a high tunnel, and we want to build or purchase one that allows us to use our existing equipment,” recommends Drost.
The tunnels in Logan, Utah at USU, for example, are built to accommodate a small tractor that can be driven through from end to end to manage the soil inside.
“How does the tunnel work?” he asks. “Sunlight brings in shortwave radiation to warm up the soil. Once we have plants in there, they warm up, as well as absorb some of that shortwave radiation.”
At night, as the shortwave radiation is converted to long-wave radiation, it rises in the tunnel, keeping the air warm inside.
“Some of it passes out, and that’s why it cools over time, but a lot of that heat gets reflected back into the structure,” he explains.
To grow crops in a high tunnel, it is important to know about the temperature environment that it is located in to be able to strategize which crops are grown and with what timelines.
“We want good thermometers and good records, and we can also use general climatic data,” Drost suggests.
Temperatures inside the tunnel can vary quite a bit from the outside temperatures based on sunlight, wind and other factors.
Temperatures around the edges of the tunnel can also differ from those further toward the middle, and buffer zones may be necessary for growing crops.
“Effectively, the heat of the tunnel melts some snow during the daytime, the snow tends to run down the sides of the tunnel, and it subs back into the structure as a cold, dense, wet environment. It takes a lot of sunlight to warm that soil back up enough to make these plants capable of growing,” he describes.
One way to combat this is to dig two fit ditches along the edges of the tunnel and fill them with plastic-wrapped foam board to create insulation. The USU tunnels use this technique along the long edges of their tunnels with non-growing buffer zones on the short ends where the doors are located, allowing people and equipment to move in and out.
“The main thing we are trying to do is manage temperatures, and there are a lot of ways for us to do that,” Drost continues.
The plastic sides of the structures can be tied up to let cool air in on a hot day, and some designs include vents that can be opened in the ceiling.
“Ultimately, we’re looking at growth and trying to prevent cold injury,” he says.
Understanding optimum temperature ranges for crops being grown within the tunnel is a key to this objective. Drost warns growers that plants with similar heat preferences should be grown together to increase success.
“In the summer, we pull all of the plastic off,” he adds. “In a very short time, we can go from indoor production to outdoor production.”
Using a simple meat thermometer, growers can use a probe to measure soil temperature to determine when covers should be removed for the season.
“We usually suggest measuring between noon and 2 p.m. because that’s when temperatures are going to be about mid-range,” he notes.
Across the United States, a wide variety of crops are grown in high tunnels, with over half of tunnel acres devoted to tomatoes.
“In Europe, they’re growing fruit trees. They are also starting to grow cherries and peaches under plastic in the Northwest, because one rain event can ruin a cherry crop,” Drost says.
Raspberries, blackberries and strawberries are often successful in tunnels, although they may limit other crop varieties that can be grown in the same structure.
“There is a little interest in squash. In the winter, cool weather crops like Chinese cabbage, lettuce, spinach, bok choy, kale and mustards are really good options because they can take the cold temperatures,” he comments.
Melons have also been grown in tunnels, although they present a challenge because bees in some areas do not prefer to be under plastic.
Basil, on the other hand, can compliment many other crops quite well, according to Drost.
Cool season crops such as lettuce, broccoli, carrots, beets, onions, rhubarb and asparagus have been successful for some producers, as well as warm season crops such as peppers, eggplants, summer squash, cucumbers beans and sweet potatoes.
“There are all kinds of designs and cost structures that go with tunnels,” he adds, adding that the Natural Resources Conservation Service and other programs often offer incentives for interested growers.
To learn more about using high tunnels and making decisions about what to grow inside, Drost directs growers to tunnels.usu.edu.