According to an article on the Scientific American website, the U.S. Department of Agriculture will soon release a new Plant Hardiness Zone Map. This guide was last updated in 1990, so there’s lots of talk about how the new map will reflect changes in our climate.
No word yet on a release date, but it will be interesting to see the new map. It may, in fact, be alarming — depending on how the zones shift. Regardless of the changes, I think it’s important for gardeners to use the map as a guide. It does not by any means determine what you can and cannot grow. Most experienced gardeners take pride in the fact that they are growing a plant that’s not technically hardy in the zone. It’s called “zone denial”. Years ago, I was determined to grow Nikko Blue hydrangeas in my Vermont backyard (zone 4). I never succeeded, and then the Endless Summer cultivar came along and I lost interest in the challenge. I still have my Nikko Blue, but it grows in a pot and gets dragged to the basement to spend the winter.
The USDA map is the one most gardeners in the eastern United States rely on, and the one that most national garden magazines, catalogs, books and garden centers currently use. This map divides North America into 11 separate zones. Each zone is 10 degrees F warmer (or colder) in an average winter than the adjacent zone. In some versions of the map, each zone is subdivided into “a” and “b” regions. It works pretty well to define the garden climates of the eastern half of North America. The region is comparatively flat, so mapping is mostly a matter of drawing lines approximately parallel to the Gulf Coast every 120 miles or so. The western half of the continent is another story.
Indeed, the map has its shortcomings. In the East, the USDA map doesn’t account for the beneficial effect of snow cover, the regularity or absence of freeze-thaw cycles, or soil drainage during cold periods. And in the rest of the country (west of the 100th meridian, which runs roughly through the middle of North and South Dakota and down through Texas west of Laredo), the USDA map fails.
In the western U.S., many factors, such as elevation and precipitation, determine growing climates. Weather comes in from the Pacific Ocean and gradually becomes less humid as it moves over and around mountain range after mountain range. While cities in similar zones in the East can have similar climates and grow similar plants, in the West it varies greatly. For example, the weather and plants in low-elevation, coastal Seattle, WA, are much different than in high-elevation, inland Tucson, AZ, even though they’re both in zone 8.
The challenge to the gardener is to determine what’s hardy in his or her backyard. Use the zones as a starting point, but then start asking fellow gardeners in the neigborhood. They’ll tell you what goes and what doesn’t. Another good resource: your state cooperative extension system. After some experimentation, you might find a microclimate in your backyard that allows you to push the zone envelope, too. Come visit me in my backyard and I’ll brag to you about my crape myrtle. Not dead yet.