Temperatures outdoors in late November in New Mexico may seem too cold to plant salad greens.

But inside hoop houses, also called high tunnels, the plants are protected from the elements and produce a marketable crop, according to a news release.

For the third year in a row, a team led by New Mexico State University agronomist Steve Guldan is studying the viability of hoop houses to stretch the vegetable growing season.

A dozen hoop houses are split between the university's Leyendecker Plant Science Research Center south of Las Cruces and the Sustainable Agriculture Science Center near Alcalde.

In addition, six more are at individual cooperators' sites in New Mexico, Arizona and Colorado.

The crops involved are both cold hardy: Trout's Back lettuce and Bloomsdale spinach.

The research involves two replications of three different hoop house designs at each of the university's centers.

The low-end design is a single layer of plastic covering the frame.

The medium-end design involves a single layer covered by a second layer. A fan flows air between the layers to provide insulation.

The high-end design adds several 55-gallon drums painted black that capture heat energy during the day and release it at night.

The goal of the project is to determine the best growing practices and structures for winter vegetable production.

The idea is the produce could be sold to restaurants, year-round farmers markets, community supported agriculture and even schools.

The project is being funded by a $194,000 grant from the U.S. Department of Agriculture's sustinable Agriculture Research and Education program.