The U.S. Department of Agriculture (USDA) has released a new, interactive version of its Plant Hardiness Zone Map (PHZM) for gardeners, horticulturists and researchers, offering more accurate and granular information than the 1990 version it replaces.
The new map divides the U.S. into 13 zones according to the average annual extreme minimum temperature. Each zone represents a difference of 10° F, and is further divided into A and B zones representing 5° F differentials.
In addition to offering nationwide, regional and state maps, the online utility also enables users to locate their plant hardiness zone by ZIP code.
Compared to the 1990 map, many zone boundaries have changed, with most areas measuring 5° F warmer than before.
According to the USDA, the changes are primarily due to when data was collected: the new map is based upon measurements during the 30-year period from 1976-2005, whereas the older version used data from 1974-1986.
The USDA cautions gardeners whose hardiness zones may have changed that there many other factors that determine whether a specific plant will thrive in a particular location, including wind, soil type, soil moisture, humidity, pollution, snow and winter sunshine.
The way plants are placed in the landscape and how they are planted — along with their size and health — can also influence a plant’s chances of survival. In addition, site topography can create microclimates with conditions that vary from that of the hardiness zone indicated for the surrounding area.
Still, the new map is a welcome improvement and a good resource to consult when considering new plantings.