Last Wednesday the USDA released its newly redrawn Plant Hardiness Zone Map. The map is based on the average lowest winter temperature in a given area—not the lowest temperature ever—and is calculated from data collected over the last 30 years. The new map was compiled at Oregon State University using GIS software.
Plant Hardiness Zones are used by gardeners, nurseries, farmers, or just about anyone who wants to know if a particular plant will thrive in a particular area. They are also used by scientists studying changes in animal distribution or the spread of invasive species, insects, and plant pathogens. A variety of federal and state agencies also use the information for projects and predictions.
The USDA is careful to say that the new map is not useful for studying climate change, and it points out the many of the changes in the map are due to better technology and better data manipulation. However, it is interesting to note that many areas in the United States were bumped into a warmer zone, and two new zones were added at the high end of the scale. The new zone 12 has average low temperatures of 50-60°F, and the new zone 13 has average low temperatures of 60-70°F.
It seems to me that if the changes were due solely to better data collection, the changes would occur in both directions—some areas would drop into a colder zone, some would rise into a warmer zone, and some would remain the same. But according to the sources I’ve read, the changes were nearly all in the warmer direction. And if two warmer zones had to be added—not just one—it means that some areas have average low winter temperatures that are more than 10 degrees warmer than previously calculated. That’s hard to explain by bad data.
The warmer map has several implications for beekeepers. For one thing, flowers may bloom earlier than they used to, especially in northern areas, and the growing season may be slightly longer. The mix of available forage may change as plants, previously confined to the south, slowly expand their distribution. The bad news is that warmer winter temperatures allow the spread of insects such as small hive beetles and wax moths that are routinely killed by freezing temperatures. Even the northern spread of Africanized honey bees can be accelerated by slightly warmer winter temperatures. All very interesting . . . and all very messy.
Follow this link to the interactive map where you can enter your zip code to find the details of your hardiness zone.