According to James Quinn, regional horticulture specialist for the University of Missouri’s East Central Region and Cole County Extension Center, high tunnel structures are unheated, plastic-covered structures that provide an intermediate level of environmental protection and control compared to open field conditions and heated greenhouses.
“The structure is typically made of aluminum or galvanized metal,” Quinn said. “However, PVC pipe or wood materials are also used. Wood is often incorporated into the structure at places such as end and sidewalls.”
The primary uses for high tunnel structures are vegetable and small fruit production, Quinn added.
These structures are used more than a greenhouse because the cost is lower and the level of management is somewhat less because the plants are (generally) grown directly in the soil.
Costs of constructing a high tunnel structure will vary with the type. “Some may be as little as $0.50 per square foot,” Quinn said. “For a more typical good quality, year-round model, $1.50 to $2.50 would be normal.”
Quinn added that the most typical size of a tunnel is 3,000 square feet.
The location should allow full sun, as little shading as possible. “It is good to be somewhat open around the tunnel so air movement is good,” Quinn said. “A tunnel can be placed on a grade. A 2 percent grade is fine, however more than 5 percent should receive some leveling.”
According to M. Elena Garcia, professor and extension fruit and nut specialist for the University of Arkansas’ Department of Horticulture, proper drainage along the outside of the tunnel may entail installation of French drains using relatively inexpensive perforated plastic drain pipe, set in gravel. “Also, building a berm and cutting a ditch can divert water away from the edge of the structure.”
Wind can also be a building factor to consider with these structures. “While the posts can be driven into the ground, on windy sites the posts are sometimes secured with concrete,” Quinn said. “A ‘windbreak’ is desirable on exposed sites.”
Maintenance of these structures includes keeping the height of vegetation around the tunnels short (e.g. mowed). “Sometimes the runoff from the roof can erode the soil, however a vegetative cover or gravel cover can control that,” Quinn said.
Maintenance also includes changing the plastic sheet covering about every 4 years. Rips in the plastic can be repaired with a clear tape (poly patch).
Efficiency of a structure will depend heavily on the crop’s specific needs (heating/cooling, humidity, etc.), season of production and variability of day-to-day weather, Garcia added. “Ventilation in the top is useful for bleeding off excess heat during hot periods along with higher side wall openings.”
For structures on a relatively level site, producers can buy kits or modify sidewalls with mechanical roll-up curtains. “Most of these use some type of crank system that can raise or lower curtains in a few seconds or minutes,” Garcia said. “Mechanical roll up/down curtains will save you much time especially during period when it is necessary to open and close the structure regularly.”
Producers interested in high tunnel structures should visit hightunnels.org or contact their NRCS office.
