Many farmers are extending their seasons into three or even four seasons using low tunnels, high tunnels or hoop houses. Food crop producers have multiple choices for low tunnels and high tunnels, also known as hoop houses.
But as Penn State Extension Horticulturist Steve Bogash points out, high tunnels are of two basic types – three-season and four-season. "The four-season tunnels generally have bows that are 4 feet apart vs. just over 7 feet for the three-season types.
"You can get by with a Quonset type for a three-season, as long as there'll be no snow load. For a true four-season type, you need to have a peak as in a Gothic-arch type, he adds.
These are generalizations as DeCloet places their bows six feet apart, but uses an oval shape (actually more like two facing U's) for the bow cross section for greater strength. Haygrove has long made three-season houses, but has entered the four-season market.
Low-down on low tunnels
Low tunnels are polyethylene plastic row covers supported by wire hoops, according to the National Sustainable Agricultural Information Service. Since polyethylene is too heavy to serve as a row cover, wire hoops are used to support it, usually 14 to 18 inches above the ground (or just tall enough to cover the crop) and as wide as the planting bed.
Low tunnels aren't permeable to air or water and may require more labor than a traditional row cover, but they often have more benefits. Slitted row covers have pre-cut slits that provide a way for excessive hot air to escape. At night, the slits are closed to reduce heat loss and maintain higher temperatures inside the tunnel.
Punched row covers have small holes to ventilate hot air. These punched covers generally trap more heat than slitted covers and are best for northern areas.
Hoops for low tunnels often are made from 10-gauge galvanized wire and cut 65 to 75 inches long. Each end is then placed 6 to 12 inches deep in the ground on each side of a row or bed, forming a hoop. These hoops then are placed 2 to 5 feet apart from one another until the end of the row or bed.
High tunnels, are arched or hoop-shaped frames covered with plastic that are high enough to stand in or drive a tractor through, depending on the needs of your operation. They're completely solar heated and require no electricity for ventilation when constructed properly. Crops in hoop houses are grown in the ground or in raised beds, and high tunnels often use drip irrigation systems.
For more details, check out: Sustainable Season Extension: Considerations for Design for more details.