High tunnel construction tips
Producers should take care to choose an appropriate site for a high tunnel, according to University of Missouri Extension horticulture specialist David Trinklein.
Trinklein said since there are no fans or motors, high tunnels don't need to be near an electric source. However, the labor-intensive high tunnels require frequent access and constant monitoring, so are best suited for a location near the producer's residence.
Critical considerations include availability of sunlight, drainage and prevailing wind direction. Producers should also evaluate the type of soil and its history of production. There might be herbicide residue if corn or other grain crops have been raised on the intended soil in the past decade, he said.
The rule of thumb on the physical orientation of greenhouses does not necessarily hold true for high tunnels, Trinklein said. Since high tunnels are passively cooled, in Missouri they should be oriented northwest to southeast because of prevailing winds.
Trinklein said there are two styles of high tunnels. The most popular is a Quonset design with a single layer of plastic fit over bows or ribs. The second is a Gothic arch style that has a pointed apex and is more similar in appearance to a greenhouse.
High tunnels are typically 20 to 30 feet wide and up to 96 feet long. Most are nine to 12 feet tall and have 39- to 60-inch sidewalls. Bows are spaced four to six feet apart. Trinklein said bows should be constructed from galvanized steel pipe of 50/55 strength rating. High tunnels with corner posts set in concrete will be more likely to withstand strong winds and heavy snow accumulation.
Some high tunnels in Missouri are placed on tracks to allow portability for producers raising a variety of crops.
Larger high tunnels lose heat more slowly, provide a greater light intensity and are less prone to excessive humidity. Trinklein recommends large end doors that can be moved for ventilation and access.
The plastic covering for high tunnels should be replaced every three years, and Trinklein said growers should use high-quality plastic.
"Cheaper is not always better," he said.
It is critical to have adequate manpower and little wind when installing the plastic.
"Speed is of the essence," he said.
Shade cloth also is needed for protection from summer heat.
For the beginning high tunnel producer, he recommends using a kit.
Wood for baseboards should be resistant to decay. Trinklein recommends wood that has been pressure-treated with a nontoxic preservative. He warns growers not to use coal tar creosote or pentachlorophenol-treated wood, which can be toxic to plants.
For more information on high tunnels, go to www.extension.missouri.edu and enter "high tunnels" in the search box.
Information about high tunnel research is also available at www.hightunnels.org, a joint effort of MU Extension, Kansas State Research and Extension, and University of Nebraska Cooperative Extension.