Plant Hardiness Zone Map warms up
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You didn't move an inch, but your property just got a little bit warmer.
That's what the folks the U.S. Department of Agriculture tell us, and to prove the point, they unveiled this week a new Plant Hardiness Zone Map. Developed by the USDA's Agricultural Research Service, Oregon State University and PRISM Climate Group, the maps are based on minimum winter temperatures and have long given gardeners a guideline when they purchase plants. The zones are almost universally listed in horticulture publications and on plant lists and tags.
The new map is specifically designed to be Internet-friendly and incorporates a "find your zone by ZIP code" function, leaving nothing to interpretation. Try it at http://planthardiness.ars.usda.gov/PHZMWeb/ .
The latest version includes 13 zones, with the addition of zones 12 (50-60 degrees) and 13 (60-70 degrees). The higher the zone, the higher the temperatures. Each zone is a 10-degree Fahrenheit band, further divided into Fahrenheit zones "A" and "B." The temperatures reflect the average lowest winter temperature, not extremes.
The last version of the map, published in 1990, had Pittsburgh as a Zone 5 (minus-20 to minus-10), with regions slightly to the south falling into Zone 6 (minus-10 to 0 degrees). In the new map, Pittsburgh and close-in suburbs are now a 6b (minus-5 to 0 degrees). Generally, most areas have warmed up by about 5 degrees, the USDA says.
This news is not shocking to avid gardeners, who have been pushing the envelope for years by growing plants that were supposedly not hardy here.
"One thing I have learned from growing plants in many locations in the U.S. is that plants can't read," said William Miller, Cornell University professor of horticulture.
In addition to global warming and better data collection, there are microclimates in every region, he said. Gardeners are more apt to experiment with relatively inexpensive herbaceous perennials rather than with trees or shrubs.
"One should experiment and try to push the hardiness rating. You never know what might survive," Mr. Miller said.
What this news means to area gardeners is simple: There will be more plants to choose from that will reliably thrive in our climate. What it means to gardeners who push the limits is that there are even more plants on the cusp to experiment with.
There is a downside to all of this, however, said David W. Wolfe, professor of plant and soil ecology at Cornell: "Some local favorite garden species may suffer while invasive weedy plants like kudzu are likely to expand their range northward."
First Published January 28, 2012 12:00 am
