When gardeners talk about the zone, they are usually not referring to a new diet, defense tactics, or the mental state they reach while working in the garden. Instead, gardeners most often speak about zones in regard to a map produced by the United States Department of Agriculture. The map is commonly known as the USDA Hardiness Zone Map and classifies regions of the U.S. based on average annual minimum winter temperatures.
Thanks to a recent update by the USDA, Douglas County and much of northeast Kansas has been reclassified from Zone 5 to Zones 6a and 6b.
Experienced gardeners will tell you to take that with caution, but the change means that magnolias and dogwoods that used to be happier a little farther south are more likely to survive here.
Zones classifications range from 1 to 13, with Zone 1 experiencing the lowest average annual minimum winter temperatures and Zone 13, along the coast of Hawaii, experiencing the highest average minimum temperatures.
Gardeners should remember that changes in hardiness zones do not happen overnight, so overwintering palm trees is still unlikely here. You can, however, feel a little more comfortable about planting Zone 6 magnolias and dogwoods, and you might not worry so much about digging up the canna bulbs every year.
Updates to the map are based on weather data from 1976 to 2005. The previous version, published in 1990, used weather data from 1974 to 1986.
Gardeners (especially new gardeners) should also remember that extremely low temperatures can still affect success with plants that otherwise survive most winters. Late frosts, early freezes, humidity, insect and disease pressure, moisture availability and soil type also affect plant health and survival.
The plant hardiness zone also ignores microclimates. Microclimates are like little heat islands — maybe your front or backyard or the side of a building. These little pockets are typically created when large trees create a shelter and/or concrete, asphalt, or south-facing walls provide reflective heat. Microclimates often allow gardeners to grow plants that are otherwise not hardy in a particular region.
Most of Lawrence is now considered Zone 6a, while the rest of Douglas County is classified as 6b. This zone extends into northern Oklahoma, although there are pockets of Zone 7a along the southern border of Kansas.
Updated maps can be found on the USDA website at http://planthardiness.ars.usda.gov/PHZMWeb/,or copies are available at the K-State Research and Extension – Douglas County office, 2110 Harper St. Interactive street level maps are also available, but only through the USDA website.
The USDA attributes the changes to a longer time period being used for calculations, more sophisticated mapping systems, and a greater number of station observations. Although much of the map of the U.S. is now considered about a half zone warmer than previously listed, a few areas were reclassified as cooler rather than warmer. The USDA does not consider changes to the map to be reliable evidence of global warming.
The U.S. National Arboretum also has a cold hardiness zone map with ratings for selected woody plants. Although used in a similar manner, it should not take the place of the USDA map when making plant selection decisions.