New Plant Hardiness Zone Map Released
Posted: February 9, 2012
Plant hardiness zone maps divide the country into zones based on average annual minimum winter temperatures, using data collected over a period of years from weather stations throughout the country. The purpose of the map is to provide the gardener and grower with a guide for determining if a plant is likely to survive the winter in her garden or field.
Each numbered zone represents a 10-degree Fahrenheit range, and the lower the number, the colder the zone. Zone 1, with average annual minimum temperatures below -50°F, is interior Alaska and Canada; Zone 10, with average annual minimum temperatures ranging from 30 to 40°F, is southern Florida and coastal California. Each zone is further divided into 5-degree Fahrenheit “a” and “b” zones, with the “a” being the colder portion of that number zone.
The 1990 map had ten hardiness zones, from 1 to 10, plus Zone 11 for essentially frost free areas with an average annual low temperature above 40°F. These zone designations were based on weather data recorded between 1974 and 1986, about a 13-year period.
The new 2012 map adds Zones 12 and 13, to include Puerto Rico, and is based on data gathered over a 30-year period, from 1976 to 2005, at 8000 weather stations. You can find this new map at the website http://planthardiness.ars.usda.gov. For the first time, the map is available in an interactive format, so that you can enter your ZIP code to find your plant hardiness zone, zoom in to click on a particular spot to find hardiness zone, latitude and longitude, or view satellite images and road maps. Static images are also available for printing, although I found the files to be very large and time-consuming to download.
The big news for Cumberland County, which may come as no surprise to residents experiencing a milder than normal winter, is that the county is warmer than it used to be – or at least, part of it is. The 1990 hardiness zone map placed all of Cumberland County in Zone 6b, which means the average annual winter low temperature ranged from -5 to 0° F.
On the new 2012 map, Cumberland County is now in two zones: still Zone 6b for most of the county, including Boiling Springs, Carlisle, Enola, Newburg, Newville, and Shippensburg, but now Zone 7a, meaning an average annual winter low temperature of 0 to 5°F, for Camp Hill, Lemoyne, Mechanicsburg, and New Cumberland – basically, the West Shore area, except along the base of North Mountain.
Does this represent concrete evidence of global warming? Not really; climate is not the same as weather, and climate change can’t be determined on 30 years of weather data. Upon release of the new map, the USDA emphasized that many of the zone shifts are the result of more accurate and comprehensive data and more sophisticated computer programs to process the data. And although many areas of the country show northward “zone creep,” some parts of the country had colder winters.
The first hardiness zone maps were created by the Arnold Arboretum, in the 1920s and 1930s. The USDA got into the business in 1960 with its first map, revised in 1965, and based on average minimum winter temperatures for the years 1899 through 1938, with some adjustments for temperatures through 1952.
Interestingly enough, I found a copy of this 1965 map in my files, so I was able to compare the three maps – 1965, 1990, and 2012. The 1965 map shows Cumberland County split diagonally from southwest to northeast into Zones 6a (-10 to -5° F) and 6b (-5 to 0° F), so subsequent maps reflect milder average winter low temperatures and northwest “creep” of half a zone.
On the other hand, much of York County on the 1965 map was placed in Zone 7a (0 to 5° F); on the 1990 map, York County was in Zones 6a and 6b (-10 to 0° F), which reflects colder average annual winter lows; and now, on the 2012 map, York County is back in Zones 6b and 7a (-5 to 5° F), a warming trend.
What does all this mean for the gardener and his plants? For plants, minimum temperatures are important, but there are many other factors influencing winter survival not taken into account by the hardiness zone map. For the gardener, the new hardiness zone map is a great starting point, but not the final word. Gardeners have always been pushing the envelope of plant hardiness, trying to succeed at growing warmer climate plants in colder climates.
Tony Avent, of Plant Delights Nursery in North Carolina, writes about testing those hardiness limits: “Don’t be frustrated when a new plant dies, and certainly don’t give up trying to grow that particular plant. After you kill it three times, use the compost you’ve created to help grow another plant.”
Annette MaCoy
Extension Educator, Consumer Horticulture
Master Gardener Coordinator
Penn State Cooperative Extension, Cumberland County
310 Allen Road, Suite 601
Carlisle, PA 17013
e-mail ahm11@psu.edu
phone 717-240-6500
fax 717-240-6548
