The Basics of High Tunnels for Crop Production

Reader Contribution by Deb J. Holley

If you haven’t worked with a high tunnel (also called a “hoop” tunnel), fall is a great time to become acquainted with how these micro-environments allow you to grow fresh and delicious fall, winter, and early spring produce, even in geographical areas not usually warm enough to accommodate such produce. I’ve always enjoyed growing produce in these off seasons — cooler temperatures in which to work, no weeds, and, with no pests to ravage your crops, no pesticides or residues on your food!

As a rule of thumb, high tunnels are thought to boost your local growing capability one zone warmer on the USDA Plant Hardiness Zone Map. For example, if you’re normally in Zone 5, the use of a high tunnel can increase your growing ability to Zone 6. Many gardeners use high tunnels for “season extension”, meaning that the tunnel can give crops a month or more of viable growing temperatures compared to being planted outside. However, if row cover is used in the tunnel in colder regions, lettuces and greens can normally survive the entire winter. They can even endure an occasional nighttime temperature of 20 degrees below zero while continuing to produce into the spring.

What exactly is a “high” tunnel? These structures usually are constructed of steel or PVC plastic pipe frames, covered with UV-resistant clear film, and are tall enough to enter. They have one or more doors so that the tunnel can be accessed all year, even in winter. Note that there are also low tunnels of various types, which are only three or four feet high. The term “hoop tunnel” actually can refer to either of these types of structures.

The term “high tunnel” is sometimes used interchangeably with “greenhouse” in the gardening industry, but for purposes of this discussion, we’ll use the term “high tunnels” to mean those film-covered structures which have no artificial means of heating or cooling. The inside temperature is usually slightly warmer relative to the outside temperature during the nighttime, is somewhat warmer on cloudy days, and is much warmer on sunny days, particularly as the days grow longer in the late winter and spring. In greenhouses, the temperature is usually controlled to remain constant all year long, mandating the use of heaters, fans, and good ventilation systems. Plants are typically planted in trays or pots on tables or benches in greenhouses; in high tunnels, crops are usually planted directly in the soil.

High tunnels come in a variety of sizes. Our family’s experience has shown that a tunnel only 12’ x 28’ (containing 336 square feet) provides a family of four with more than enough fresh salad ingredients all winter long, with additional room for other cold-weather vegetables. Tunnel-growing requires work, experience, and patience to produce the best growing results, so it’s best to select a small tunnel to begin your experience in seasonal gardening. You can make your own using instructions found on the web, or you can purchase a tunnel from any number of commercial sources. As with purchasing any product, be sure to read and compare all of the features and prices of several tunnels.

Choose a flat area of ground with full sun exposure on which to erect your tunnel. The closer the location is to your house, the more likely you are to run out and avail yourself of the tunnel’s bounty. A nearby water source is also important. A handy hydrant and garden hose will keep the tunnel watered from mid-spring to late fall, if needed. (The tunnel usually doesn’t need to be watered during the winter if your locale receives enough moisture in the form of rain or snow.) The close proximity of your garden tools and equipment also makes using a hoop tunnel easier.

You will need to set the bases of the structural pipes several feet deep in the ground and undertake squaring, leveling, and other carpentry skills, per the instructions from the manufacturer. Because of this, the tunnel will take some time and knowledge to erect. The two end structures of the tunnel often require additional wooden support framing to be made, as well as the door to enter. Additional anchoring is always desirable, since just about every area of the country can experience high winds. Our family’s tunnels have been subjected to 80-mile-an-hour winds and survived intact, thanks to solid anchoring!

This is only an overview of the basics of high tunnels and their use in crop production. There are many more points to know in erecting and using a high tunnel, some of which I’ll cover in coming blogs.

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