Structures such as row covers, low and high tunnels, cold frames and greenhouses are ways that urban farmers can extend their growing season and increase yields and profits. Urban farmers must, however, adhere to all municipal building and construction codes when designing and building structures, and should consider the permanency of their land tenure. Hoop houses and high tunnels, for instance, are much more portable than permanent greenhouses.
Types of Season Extenders
The following information is from the National Sustainable Agriculture Information Service publication, “Season Extension Techniques for Market Gardeners,” which provides information about cultural and other season extension techniques as well as sources of equipment, supplies, and other information, available for download or purchase at https://attra.ncat.org/attra-pub/summaries/summary.php?pub=366.
Floating Row Covers
Floating row covers are made of spun-bonded polyester and polypropylene fabric that is permeable to sunlight, water, and air, and provides a microclimate similar to the interior of a greenhouse (giving 2 to 8°F of frost protection). They are typically placed directly over crops without support. The exception is when covering crops with tender growing points, such as tomatoes, in which case low tunnels should be used. Floating row covers come in light, medium, and heavy weights, with sizes ranging from widths of 3 to 60 feet and lengths of 20 to 2,550 feet.
Low tunnels are heavy-weight floating row covers made of clear or white polyethylene that are supported by wire hoops. These offer many of the same advantages of floating row covers, but are not permeable to air or water and are more labor-intensive.
Cold frames are low structures use to protect crops in cold weather. They traditionally rely on solar heat, though more recently developed models can include heating systems. Though cold frame work well in protecting crops and can be more durable than row covers or low tunnels, construction costs of cold frames are high compared to plasticulture systems, which provide many of the same benefits.
Also called hoop houses, high tunnels are typically arched or hoop-shaped frames covered with clear plastic and high enough to stand in or drive a tractor through. High tunnels are usually solar heated, and do not require additional energy sources. Compared to greenhouses, high tunnels are relatively inexpensive, ranging in price from $1.50 to $3.00 per square foot.
Penn State University offers information and several resources for purchasing, building, and using high tunnels at http://extension.psu.edu/plants/plasticulture/technologies.
For urban farmers growing in raised beds, the Samuel Roberts Noble Foundation provides simple instructions for building a small hoop house on an existing raised bed for less than $90 at http://www.noble.org/ag/horticulture/minihoophouse/.
Greenhouses are similar to high tunnels, but are traditionally more durable and permanent, and use either glass or greenhouse-grade plastic to protect crops and capture heat. Greenhouses also typically use a heating source in addition to solar energy, as well as automated heating and cooling systems. These systems allow farmers to plant a greater variety of crops throughout the year.