University of Maryland Extension

Season extension

Neith Little, Extension Agent, Urban Agriculture

Urban Ag home | Table of contents

Growers producing food for the local market use a variety of methods to extend the growing season beyond what is traditionally possible in the local climate.

Season extension methods can be as low-tech as starting transplants indoors and laying inexpensive row cover cloth over small metal hoops to create “low tunnels.”

High tunnels or hoop houses are mid-range options. As opposed to greenhouses, high tunnels are usually made of less expensive materials with flexible plastic coverings, in-ground or raised beds (as opposed to bench-top production), and passive heating and cooling such as row covers and roll-up sides. Some local zoning boards consider high tunnels as temporary structures (check your local rules first!). The relatively low construction and maintenance costs and the potential (with significant work!) to move a high tunnel to a new site if necessary have made high tunnels popular with urban farmers. Penn State Extension has particularly good resources on high tunnel construction and management. And the USDA-NRCS has offered incentive grants to help farmers build high tunnels to extend the growing season.

As a side-note, low tunnels are also a valuable pest-exclusion technique. They can be used to exclude pests that have a known, brief population surge every year or to give young crops a head start to outgrow pest pressure.

Greenhouses with permanent plastic or glass walls, supplemental lighting, and heating systems, are much more expensive and are mostly used at a small scale for transplant production and at a large scale by plant nurseries to produce potted plants for sale to public customers or to landscapers and garden centers.

Indoor production could also be considered a form of season extension. There are also hybrids of the various methods, with some aquaponic urban farmers setting up their systems in high tunnels and ground-based urban farmers using double-envelope systems of low-tunnels within high tunnels to maximize the temperature differential between what the crop experiences and the outside temperature.

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