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It's startling to see a palm tree growing in downtown Seattle, just a couple hours' drive from Canada. The palm exemplifies why it's wise to look at a map of "hardiness zones" whenever you're buying and planting a tree. It turns out that this particular palm and other trees that thrive in Texas are suited to Seattle, as they share the same "plant hardiness zone."

So, what are hardiness zones? Plant hardiness zones divide the US and Canada into 11 areas based their average annual minimum temperature. The zones are divided by temperature increments; each five- to 10-degree difference in annual average minimum temperature warrants a new zone. For example, the lowest average temperature in Miami (Zone 10) is 30 to 40 degrees. In Honolulu (Zone 11), it's above 40 degrees. New York City (Zone 7) shares the same hardiness zone as parts of New Mexico and Arizona, both Sunbelt states.

Much of the nation has warmed since 1990 when the last US Department of Agriculture hardiness zone map came out, causing significant parts of many states to shift by at least one full hardiness zone, according to an updated map released in 2006 by the Arbor Day Foundation. Much of Illinois, Ohio and Indiana, for instance, now fall into a warmer Zone 6 (up from Zone 5). That puts them on par with parts of Nevada, where the lowest temperatures fall between 0 and minus-10 degrees.

Some small portions of the US actually have warmed by two full zones. To see an animation illustrating the changes, go here: www.arborday.org/media/mapchanges.cfm

Crape myrtle (Lagerstroemia fauriei), a tree associated with the South, now is considered a fine choice for parts of Pennsylvania as a result of the map update. And what about palm trees in Seattle? Actually, cold-hardy varieties (including Mexican Fan Palm) grew well even before the map update, so they're not harbingers of global warming.

For more info:

* To find your area's hardiness zone, type in your ZIP code here: www.arborday.org/treeinfo/zonelookup.cfm.

* See the US Department of Agriculture's hardiness zone map here:

* So you've decided to plant a tree? Great – one tree can remove one ton of carbon dioxide in its lifetime. Read more benefits here: www.arborday.org/trees/benefits.cfm

Wangari Maathai, "the Tree Lady of Kenya" and a Nobel Peace Prize winner, calls on every living person to plant 10 trees. That's to absorb the amount of carbon dioxide we each breathe out during a lifetime. Read more about her and her movement at Greenbeltmovement.org

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