The Zone Debate

Until recently, the USDA put out a new Hardiness Zone Map every 15 years. In 2005, a new map was scheduled to be released from 16 years of climate data studied by the American Horticultural Society. The AHS draft was published in The American Gardener's May-June 2003 issue.

The USDA reviewed the map and decided that it would not work, because, among other reasons, it was not based on a long enough time period. Some worry that the USDA rejection of the new map was influenced by the current administration and the politics surrounding global warming. The USDA is working on another update with 30 years of data, but there is as of yet no schedule for its release. The USDA map from 1990 is still the current “official” map

After much demand for a more current map, the National Arbor Day Foundation released their own update, the 2006 Hardiness Zone Map. Similar to the AHS update, and from the same US National Climatic Data, it shows much of the country moving a zone warmer. Their animation of the zone changes from 1990-2006 has quite an impact.

The Good and the Bad
A change in hardiness zone may seem like a good thing, in that we can grow a wider variety of plants, but with the plants we want to grow will come the ones we don’t want. There may be increased problems with invasive pests and weeds. Also, the climate warming is not limited to warmer winters. There is an increased potential for extreme heat. See the AHS Heat Zone Map

Any hardiness zone map should be used as a starting point, with plant decisions based on local experience. Know your local microclimates. Upset customers may result from trying borderline hardy plants that may not make it through the occasional hard winter.