USDA redraws plant hardiness map

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(Host) For more than forty years, gardeners have used the USDA Plant Hardiness Zone map as a guide to tell them which plants can grow in Vermont. Now the map is being revised and, as VPR’s Steve Zind reports, some Vermonters could end up in a different zone.

(Zind) What Vermont gardener with both feet planted firmly on a patch of Zone 4 ground hasn’t gazed longingly at a list of plants that can be grown in the warmer climes of Zone 5?

Based on the draft version of the newly updated U.S. Department of Agriculture Plant Hardiness Zone map, a few of those gardeners may find themselves in transported there. Kim Kaplan of the USDA says there are two major differences reflected in the new map.

(Kaplan) “One is it uses more recent weather data. The other difference is that this will be a completely digital map which means we’ll be able to put it up on the web in great detail.”

(Zind) Since recent years have seen generally warmer winters, the new map shows that the hardiness zones have edged northward a bit. The more temperate Zone 5, which previously just grazed Vermont’s southern edge and the Champlain Valley has crept up into central Vermont. Kaplan cautions a final version of the map has yet to be approved by the USDA, and anyway, the map is just a guide. It probably shouldn’t change what Vermonters choose to plant. Leonard Perry agrees.

(Perry) “Don’t look on it as the Gospel, that’s for sure.”

(Zind) Perry is an extension professor at the University of Vermont. He says the maps are only as accurate as the data they’re based on. That data is suspect because it’s recorded at locations several feet off the ground – not down where the plants are. Perry says a researcher once took temperature readings at ground level and compared them to weather station readings.

(Perry) “Well they actually put a stick with thermometers on it from that height down to the ground and they actually found on many days a 20-degree difference.”

(Zind) If that information seems to push Vermont even further into Zone 4, there’s still hope. Many Zone 4 or colder locations have warmer microclimates that may be as small as a foot across. Perry says finding those microclimates around your house can be the key to growing less hardy plants. He’s found several at his place in Milton.

(Perry) “On my own property, I have three zones: 3, 4 and 5.”

(Zind) Perry says because the new map is based only on recent weather information and not long term historical data, all it takes to change Vermont’s expanded Zone 5 back into Zone 4 is a couple of cold winters.

For Vermont Public Radio, I’m Steve Zind.

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