How do you choose a site for an urban high tunnel?

Choosing and properly preparing a site to install a high tunnel are among the most important factors in how well the high tunnel will perform for years to come. Most urban farms have a limited amount of space and workable soil. Although intensive high tunnel growing methods will make the most use of limited space, the lack of land base on urban farms can make it hard to find an ideal place to install a tunnel. A level (less than 5% grade), well-drained, fertile soil is preferred for high tunnel production. Soil can be prepared after the high tunnel is installed, but starting with deep, well-drained, fertile soil will make the process easier during the first year.

If the site is not well drained, it’s important to elevate the area of the tunnel slightly from the surrounding ground. This can be done with small excavating equipment or by hand if the area of the high tunnel is small enough. Soil can be taken from outside the perimeter of the high tunnel and placed where the interior of the high tunnel will be. This will create a small swale along the sides of the tunnel and a slightly raised plateau in the area that the high tunnel will cover.

Another consideration for installing a high tunnel on an urban farm is shading. High tunnels are usually unheated structures that rely on the sun and solar heat gain to raise daily temperatures and allow for growing crops for an extended season. For that reason, it is imperative that a high tunnel is not shaded by other structures, buildings, or trees for the majority of the day. This is especially important if fall, winter, and spring growing will be a focus for your farm. A good rule of thumb is that if, at a minimum, the area is completely unshaded from 9:00 a.m. until 3:00 p.m. year-round, the site will probably work well. If there is a question as to the level of shading at a certain site, it may be a good idea to have a solar site evaluation. Solar panel installers can conduct these evaluations using a Solar Pathfinder tool to determine the shading of a particular area.

Orientation—the direction the length of the tunnel runs—is another important factor to consider. Most high tunnels that are installed to grow summer crops are oriented north-to-south to limit shading from bed to bed inside the high tunnel, and to allow for airflow from a predominately west wind when the sides of the high tunnel are rolled up. North of 40° latitude, an east-to-west orientation is preferred for cool-season growing because it exposes a larger area of the high tunnel (the long southern-facing side) to the sun. This south-facing side of the high tunnel then acts as a big solar collector and allows the high tunnel to gain heat more quickly.

To learn more, consult the ATTRA publication High Tunnels in Urban Agriculture. This publication covers the basics of siting and constructing a high tunnel, as well as some of the policy and zoning challenges urban growers face when planning to erect a tunnel. It also discusses high tunnel management, including soil fertility, irrigation, and disease and pest control. Finally, it includes resources on intensive crop production and other uses for high tunnels.

Source: ATTRA