Many gardeners seem to be born with a green thumb, but understanding plant Zones can confuse even the savviest grower. Plant Zones can be key to knowing which plants are likely to thrive in your area -- and which may disappoint. Here's a short history, as well as an explanation and details on updates to help you better understand plant Zones.
Plant enthusiasts had long observed that different plants thrived in different locations. But it wasn't until 1927 that horticulturalist Alfred Rehder related the lowest mean temperature of the coldest month of the year to plant hardiness and used 5-degree bands to roughly divide most of the country into a series of zones.
Horticulturists continued to study temperature as a way of understanding plant zones. In 1938, Donald Wayman, a horticulturalist at the Arnold Arboretum of Harvard University, used weather data from 1895 to 1935 to draw a new map based on average annual minimum temperatures. That map, known as the Arnold Arboretum hardiness map, was updated in 1951, 1967, and 1971, but it wasn't based on a uniform number of degrees in each zone. "Some of his zones had a 15-degree range, while others had a 5 or 10," says Kim Kaplan, spokesperson for the U.S. Department of Agriculture's Agricultural Research Service (ARS).
Because of the lack of uniformity in the temperature divisions on the Arnold Arboretum map, the United States National Arboretum, part of the ARS, decided in 1960 to create an official United States Department of Agriculture (USDA) Hardiness Zone Map. It was based on 10-degree temperature bands and average annual minimum temperatures, and it was designed to appeal to both gardeners and plant breeders, Kaplan says.
"For gardeners, it would tell them what they should plant in their area. For nurseries, it was a way to tell what plants would be best to sell in their area," Kaplan says. "What the USDA was trying to do was create a new standard so that everyone could communicate. If people were breeding a new variety of tomatoes or petunias, they would have a way of uniformly telling people it would or wouldn't be likely to thrive in their area."
The USDA Plant Hardiness Zone Map has been updated several times. In 1990, each 10-degree Zone was further subdivided by 5 degrees into A and B areas to better help gardeners understand plant Zones. "Horticulturalists felt that adding the refinement of A and B zones was worthwhile, especially in the areas around Zones 6 and 7," Kaplan says. "There are a lot of varieties that are on the borderline and a lot of plants that are not hardy to the next half-Zone."
The map was again updated in 2012, Kaplan says, with three big changes that affect gardeners' identification of their plant Zone. The first is a switch to a Geographic Information System (GIS)-based interactive map -- allowing a much more refined scale that will hone the borders of existing Zones and make it possible to show heat and cold islands that were never able to be shown before. "That is going to change Zones for some people, just because the map couldn't show their small area before, and with the new map, they will be able to click down to a very fine scale," Kaplan says.
Along with the interactive map will be traditional-style maps of the country, regions, and states. "But the map will be moving into the digital age for the first time," she says.
The second change is the way the Zones are defined. A mathematical algorithm was created for calculating the Zones between the areas where actual data from weather-reporting stations exists. A series of weighted factors, including changes in elevation, slope, and water proximity, was devised to create a more accurate data picture of what's influencing temperatures.
But the third set of changes is the one that's most apparent: the years of data. The 1990 map compiled 13 years of data; the newest map will have about 30 years of data and includes three new Zones -- 12, 13, and 14 -- which will be helpful in particular to tropical plant breeders. So the new map will have 14 Zones, each subdivided into A and B. "Coming up with 28 distinguishable colors may have been one of the biggest challenges," Kaplan says.
Plant hardiness Zones have always provided a recognizable standard, but they are not a guarantee that a plant will thrive or survive. "The Zones are based on average minimum winter temperatures," Kaplan says. "It's not the lowest temperature it has ever been in the past, nor the lowest it will be."
While the new map has a lot more detail -- even indicating heat islands in metropolitan areas with lots of concrete -- it won't be able to show mini microclimates within your own yard, Kaplan says. "Even though the new map will get down to a scale unbelievable compared to the 1990 map, it won't be able to indicate that dimple in your yard where frost pools first or the spot in front of a south-facing wall that's warmer than the rest of the garden -- maybe they should call those nano climates," Kaplan says.
Even if your Zone has changed, Kaplan says, that doesn't mean you should start ripping plants out of your garden. "What's thriving there now is going to continue to thrive," Kaplan says. "It could even just mean your Zone was that all the time, but it didn't show it that way on the previous map."
The USDA Plant Hardiness Zone Map and more information about understanding plant Zones can be reached through usna.usda.gov.