Plant Hardiness Zone Maps

by Christine Jarzab, reprinted from the November 2009 issue of 10-10-10

With autumn passing through our area, signifying winter’s approach, I paused to think about the plant hardiness zone map. In particular, I was remembering how those “hardy in zone 5” crepe myrtles my mother planted last year were not hardy. Wondering where the hardiness zones originated, I did some research, only to find that hardiness zones are not an exact science, are often misunderstood, and have caused caustic debate among gardeners.

Hardiness Zone Maps. There are hardiness zone maps for nearly every area in the world. In the United States, the first hardiness map was developed in the 1960’s by the USDA Agricultural Research Services. This map, most recently revised in 1990, identifies 11 zones, with zones being 10°F warmer/colder than their neighboring zone. Most zones are further divided into “a” and “b” zones that are separated by 5°F. The 1990 USDA map is the most widely used map, however, other organizations have also published maps. A 2006 map published by the National Arbor Day Foundation (NADF) received a chilly reception. This map relied upon temperatures recorded between 1990 and 2004. Critics denounced this map as based upon data gathered during an unusually warm period. Some predicted massive plant demise due to erroneous plant selection by big box shoppers. I suspect much of the criticism arose from the NADF’s use of the map to make a dramatic, political statement about global warming.

A less contentious and highly popular map has been published by Sunset. Although this map was historically reserved for the 13 Western states, Sunset now published hardiness zone recommendations for all of the States. The Sunset zones are considered by many to be more precise because Sunset accounts for a multitude of factors, such as latitude, elevation, and continental air influence. Sunset boasts that whereas the USDA map will only tell you whether your plant is expected to survive the winter, Sunset’s map will tell you whether your plant will thrive in your climate. Warren County’s zone depends upon the map one reads. The USDA map generally places us in zone 5a (-15°F to -20°F), whereas the NADF map assigns us to zone 6 (-10°F to -20°F). Sunset assigns Warren County to Zone 40, with low temperatures ranging from -10°F to -20°F and rainy, warm, variably humid weather that is affected by Lake Erie.

What is a gardener to do? Plant performance depends upon many factors, including temperature, day length, light, precipitation, humidity, and soil conditions. In selecting plants, gardeners need to be aware of the plants’ optimal environment and select plants that prefer the conditions present in the intended garden location. Zones are an important factor. Remember that zones identify the “average annual minimum temperatures” in each zone. These temperatures are not the lowest temperature that will ever be reached in your zone, but rather are the lowest annual temperatures recorded during a period of years. Thus, a plant may be labeled for your zone, but an unusually cold winter may kill the plant. Lastly, sometimes it is okay to ignore hardiness zones. Hardiness zones really matter only if you desire the plant to survive more than one season. Most gardeners select plants that are not hardy for their area, knowing that they will not survive the winter (i.e. the so-called “annuals”).

Resources: USDA Hardiness map: http://www.usna.usda.gov/Hardzone/ushzmap.html; National Arbor Day Foundation map:http://www.arborday.org/media/zones.cfm; Sunset Hardiness map: http://www.sunset.com

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