For the first time since 2012, the U.S. Department of Agriculture (USDA) has revised its hardiness zone map that helps growers and gardeners determine which perennial plants are likely to thrive in their specific zone.
The latest map, which came out late last year, incorporates data from 1991 through 2020 and shows that much of the country was reclassified into a warmer zone. Divided into 10-degree zones and 5-degree half zones, ranging from zone 1a to zone 13b, the hardiness map covers the extremely cold temperatures found only in Alaska to the warmest, found in Puerto Rico.
Based on the 2023 USDA Plant Hardiness Zone Map, Strasburg and Woodstock changed from 6b to 7a, Front Royal moved from 7a to 7b and Winchester went from 6b to 7a.
Joanne Royaltey, a program associate with the Virginia Cooperative Extension Service, said what many people tend to forget is that the map is based on the average coldest winter temperature — the coldest night of the year for each region that is being studied.
“It doesn’t show you what are the hot nights and hot days,” she said. “It’s concerned with cold weather temperatures.”
The new map reflects the averages over 30 years and doesn’t predict the weather of the future, Royaltey said.
“Only the universe or a higher power can know that, right?”
Noting that the changes to the map are due, in part, to the increase in temperatures related to climate change, a representative from the USDA said that over time plant hardiness zones will gradually move northward.
“In addition, each new map is more sophisticated and incorporates more data than previous maps — these improvements are likely the cause of some zonal changes, especially in the western U.S. Therefore, we cannot say with certainty that the changes from one map to the next are due solely to climate change, but it is likely a contributing factor,” the USDA representative said in an email.
What do these changes mean for gardeners?
“It’s an opportunity to try something new,” Royaltey said. “The warming trends with the zones open up a whole new world of possibilities to try new shrubs, trees, vegetables, perennials that maybe didn’t work in the past.”
But there’s also a downside.
“Some of the plants that we currently have may not survive because of the warmer weather,” Royaltey said. “The things that typically did very well here suddenly may be struggling, like a conifer tree or shrub. They need really cold weather. If the zones suddenly start creeping up some of the conifers we have may no longer thrive.”
For Royaltey, she’s looking forward to adding more tender perennials to her garden that in past years she’s had to bring inside during cold weather.
“It’s an opportunity to think about what these new zones are going to do for us,” she said.
With zone changes, come changes in frost dates as well as planting dates.
“Those dates may be pushed further out because of the warming trend,” she said. “That’s something that we will have to just wait and see as the months go by.”
As the climate continues to change in the mountains and valleys of the region, Royaltey said other plants, shrubs and trees might be affected as the area warms.
“The Christmas tree industry,” she said. “They need cold weather. If it keeps getting warmer and warmer, what does that mean for them?”
Thinking about fruits in the region, specifically apples and peaches, Royaltey said fruit trees may see the effects of the zone changes as time passes.
“They need a certain amount of what they call chilling hours to get a good fruit crop for the next year,” she explained. “So if we suddenly keep having warmer and warmer winters that could impact their productivity.”
Royaltey wants gardeners and farmers to know that there is nothing to get hysterical about, but it is something to be mindful of.
“Things are moving in a warmer direction,” she said. “If we think about what’s happened to us in the last couple of years, we’ve been in a drought and had restrictions. Having erratic precipitation patterns where you have really wet weather for a while and then everything fires up, which is kind of what’s happening now. It impacts the health of the plant or tree. Not only on the things you grow in your garden but the agriculture we work to produce to sustain us all and the landscaping that makes our homes beautiful.”
Royaltey said she is looking forward to new planting opportunities.
“I’m celebrating that now I can safely plant the magnolia in my garden because now it seems like the temperatures are just right.”
