USDA Plant Hardiness Zone Map

The USDA (United States Department Of Agriculture) has updated their longstanding Plant Hardiness Zone Map (aka garden zones) to better reflect the average minimum temperatures across the U.S. For the first time, the map is interactive and you can zoom in on your state and see your local zone in detail.

Most likely, your hardiness zone has changed slightly, but it doesn’t mean you should panic and replace your plants. What you’ve been growing will most likely continue to grow, but you might be able to introduce new plants that are adapted to slightly warmer zones.

Most hardiness zones are now categorized as slightly warmer than on the previous map, an update that won’t come as a surprise to most gardeners. I live in Zone 6 (now Zone 6b) and for years I’ve been growing a number of Zone 7 perennials which shouldn’t be winter hardy in my area, yet their bulbs survive every winter intact.

According to the USDA, “Zones in this edition of the USDA Plant Hardiness Zone Map (PHZM) are based on 1976–2005 weather data… Each zone represents the mean extreme minimum temperature for an area, calculated from the lowest daily minimum temperature recorded for each of the years 1976–2005. This does not represent the coldest it has ever been or ever will be in an area, but it simply is the average of lowest winter temperatures for a given location for this time period.”

The previous Plant Hardiness Zone Map, published in 1990, was drawn from weather data for 1974 through 1986, a scant 12-year window. The new, longer period of data (30 years, 1976-2005) was selected by a group of horticultural, botanical and climatological experts who determined that the longer data period more accurately reflected year-to-year weather variation.

New Hardiness Zones

Two new zones, 12 and 13, were introduced in the 2012 revision. These frost-free zones appear on the Hawaii and Puerto Rico maps and are meant to better define conditions for tropical and semi-tropical plants which many gardeners use as indoor or summer patio plants. Zones 12 and 13 will provide a way to communicate the differences in cold sensitivity of tropical plants and may help gardeners decide when to bring these plants indoors as the season in their zone cools.

Do warmer hardiness zones indicate climate change?

Many are drawing the conclusion that the warmer hardiness zones are equivalent to the USDA officially recognizing climate change. The Department Of Agriculture is quick to point out on their website that “Climate changes are usually based on trends in overall average temperatures recorded over 50-100 years. Because the USDA PHZM represents 30-year averages of what are essentially extreme weather events (the coldest temperature of the year), changes in zones are not reliable evidence of whether there has been global warming.

The new (map) is generally one half-zone warmer than the previous (map) throughout much of the United States, as a result of a more recent averaging period (1974–1986 vs. 1976–2005). However, some of the changes in the zones are the results of… more sophisticated mapping methods…which has greatly improved accuracy”.