Earlier this year, the U.S. Department of Agriculture (USDA) finally listened to what landscape architects, researchers, and gardeners have been saying for some time: plant hardiness zones are retreating north across the country. While the USDA argues that its new plant hardiness map, the first update since 1990, doesn’t show a clear connection with climate change, some experts disagree.
An article by The Washington Post quotes David Wolfe, a professor of plant and soil ecology at Cornell, who argues that the USDA is being “too cautious in laying off the climate change connection.” He said: “at a time when the ‘normal’ climate has become a moving target, this revision of the hardiness zone map gives us a clear picture of the ‘new normal.'” The National Oceanographic and Atmospheric Administration (NOAA) seems to agree, posting signs in gardens affiliated with the American Public Gardens Association saying that a shifting climate has led to changes in the areas where many plants can survive.
USDA explains that plant hardiness zone designations indicate the “average annual extreme minimum temperatures at a given location during a particular time period.” As an example, if a plant is “hardy to zone 10,” it can sustain a “minimum temperature of -1°C,” writes Wikipedia. “A more resilient plant that is ‘hardy to zone 9′ can tolerate a minimum temperature of -7°C.” Just to note, though: USDA’s map doesn’t reflect the coldest it will ever be at a location, but simply the average wintertime temperature, given “low temperature during the winter is a crucial factor in the survival of plants.”
The updated map now offers 13 zones, with the addition of two new higher temperature zones: zone 12 (50-60 degrees Fahrenheit) and 13 (60-70 degrees Fahrenheit). USDA writes that each zone has a 10-degree Fahrenheit “band,” which is further separated into 5-degree Fahrenheit zones “A” and “B.”
There have also been shifts in zone boundaries, which indicate a half-degree upward turn in all zones across the U.S. “The new map is generally one 5-degree Fahrenheit half-zone warmer than the previous map throughout much of the United States.” However, in other instances, new temperature data, more sophisticated measurement technologies, and improved accuracy have resulted in changes that make some zones cooler as well.
While not well-known among the general public, a big chunk of the U.S. economy relies on the map. Plant hardiness zones underpin crop insurance standards so farmers rely on the data. Landscape architects, horticulturalists, ecologists, and other scientists who create or restore landscapes need the data to ensure their constructed landscapes survive. There are also some 80 million home gardeners who regularly use the map.
For Catherine Woteki, USDA Under Secretary for Research, Education and Economics, the new map, which offers a GIS-based interactive format designed to be more Web friendly, “is the most sophisticated Plant Hardiness Zone Map yet.” (The map was jointly developed by USDA’s Agricultural Research Service (ARS) and Oregon State University’s (OSU) PRISM Climate Group, with the input of teams of climate and horticultural experts from around the country). Indeed, for the first time, the map enables users to search by zip code.
Still, while the new system has lots of benefits, there are also some remaining limitations. Wikipedia argues that the zones only help so much because they don’t indicate how resilient plants need to be to heat. “The zones do not incorporate summer heat levels into the zone determination. Thus sites which may have the same mean winter minimal temperatures, but markedly different summer temperatures, will be accorded the same hardiness zone.” So, those using hardiness zones will also need to check out local heat zones, along with other factors like humidity, day length, and soil moisture, to ensure a plant can survive.
Alternatives to hardiness zones include using “indicator zones” or Sunset Books’ climate zones, which identify 45 distinct areas in the U.S.
Explore the updated map.
Image credit: USDA