A revised winter hardiness map just released by the National Arbor Day Foundation pushes much of the U.S., including parts of Oregon, into a warmer planting zone.

The foundation developed the new 10-zone map based on 15 years of data (1990-2005) from the National Oceanic and Atmospheric Administration's 5,000 national climate data cooperative stations around the country.

Granted, 10 of those years proved to be among the warmest winters on record. Still, this map is already resonating with people concerned with global warming, an issue that has also captured the attention of the conservation-minded foundation in recent years.

Home gardeners have long depended on the familiar U.S. Department of Agriculture hardiness-zone map to decide which plants are likely to survive where. But escalating climate change has sparked controversy in the once-staid world of hardiness zone maps - most recently in 2003, when the USDA sponsored a revised map by the American Horticultural Society that indicated a northward creep of hardiness zones very similar to what is shown in the new NADF map.

The USDA ultimately rejected the 2003 map, explaining in part that it was based on too few years of data. The governmental agency is in the process of creating a new version of its hardiness map using updated mapping technology and an extended set of meteorological data. Their new version will include 15 primary plant hardiness zones (the current map has 11) to reflect the growing interest in tropical and subtropical plants.

In the current USDA map, the entire western third of Oregon sits solidly in zone 8, where the lowest winter temperatures range between 10 and 20 degrees. The bulk of central Oregon straddles zones 5 (-20 to -10 degrees) and zones 6 (-10 to 0 degrees), with a few spots in the high Cascades perched in zone 4 (-30 to -20 degrees).

But in the National Arbor Day Foundation's map, released just four weeks ago, a brand new zone - zone 9 (20 to 30 degrees. for their winter low) now engulfs much of the Oregon coast. It starts just north of Newport and extends all the way down to the California border, jutting west as much as 40 miles in some spots.

Much of the Willamette Valley remains in zone 8, but the zone is now wider, extending farther east into parts of Hood River, Wasco, Jefferson, Deschutes, Jackson and even Klamath counties.

In a few higher-elevation parts of normally winter-chilly eastern Oregon, there are areas that have jumped all the way from zone 4 to zone 6 in the revised NADF map.

Across the country, significant portions of many states have shifted at least one full hardiness zone in this new map. Much of Illinois, Indiana, and Ohio, for example, have shifted from Zone 5 to a warmer Zone 6. Some areas around the country have even warmed two full zones.

It's an unsettling trend, even when you take into account the relatively short time frame covered by the data.

On the one hand, a warmer climate allows gardeners to experience more plant diversity. But at the other end of the spectrum, native species are likely to find their growing seasons shifting - and their life cycles out of sync with pollinating insects - if warming trends continue.

Tree species native to Oregon such as Douglas fir, Western red cedar and Western hemlock could suffer if warming trends continue, according to Alby Thoumsin, a certified arborist and president of the Eugene Tree Foundation.

Over the past decade, Thoumsin has seen a marked decline in overall tree health with those three species in certain parts of Western Oregon.

"We've had extended drought, flooding - all classic signs that relate back to our warmer winters," said Thoumsin. "If this keeps up, we will have some sort of species retreat related to climate, where trees will simply die out before hopefully reappearing in a more hospitable environment."

But with a consensus of climate scientists in agreement that global warming is underway, Thoumsin appreciates the effort by the NADF to encourage tree planting as a way to reverse the trend.

Interested in learning more? Go to www.arborday.org and click on the Hardiness Zone link. You can then enter your zip code to find your hardiness zone, and learn how planting a single tree can help remove more than a ton of carbon dioxide from the air over the tree's lifetime. With atmospheric CO2 a leading contributor to global warming, planting trees is a simple, cost-effective measure that anyone can easily undertake.

My favorite hardiness zone map is still the one published by the folks at the Sunset Publishing Corporation and laid out in all of its full color glory in the latest Sunset Western Garden Book, last updated in 2001. You'll also find it on the web: www.sunset.com or www.monrovia.com.

Sunset classifies 33 western zones, numbered from harshest (Zone 1) to mildest (Zone 33), and organized by region from north to south. The boundaries of each of these unique zones are a function of six geographic and climactic factors: latitude, elevation, ocean influence, continental air influence, mountains and hills, and microclimates. Taken together, these factors determine what will grow well in your garden and what won't.

This intricate, complex and highly flexible climate map is especially helpful because it doesn't just focus on cold-tolerance alone.

The new NADF map is sure to provoke spirited discussion among gardeners, arborists and environmentalists. In the meantime, remember this: at some point between October and February, a freakish cold snap is sure to appear out of nowhere just when you - and your tender plants - least expect it.

Yes, a warming trend is very likely on its way. But I'm keeping my row covers and old space blankets in a handy pile by the shed door.

Just in case.

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The National Arbor Day Foundation is a nonprofit organization of nearly one million members, with a mission to inspire people to plant, nurture, and celebrate trees.