On Wednesday, the Department of Agriculture unveiled what most gardeners have known for years: a new plant hardiness zone map that shows generally warmer low temperatures for winter than the department’s previous map from 1990.
In the Washington region, the District and other communities bordering the Potomac River or the Chesapeake Bay are now in the warmer half of Zone 7, which formerly reached only as far north as Tidewater Virginia. Most of Virginia and Maryland are in the cooler side of Zone 7, with a low temperature range of zero to 5 degrees Fahrenheit. Before, the areas found themselves in the colder Zone 6, along with much of the Appalachian piedmont.
The nation’s 80 million gardeners rely on the map to pick trees, shrubs, grass and perennials that will survive where they live.
Nursery plants are sold with a zone range. The magnolia variety Pink Charm, for example, is listed for zones 5 through 8, meaning it should survive winters in gardens from South Carolina to northern Pennsylvania. Florida would be too hot for it, Maine too cold. But officials say gardeners shouldn’t start ripping out plants that don’t align with the new map.
Online only and interactive, the new map reveals wholesale shifts in zone boundaries since the last one was compiled as a wall map 22 years ago. Some areas turned out to have colder winters — the western slopes of the Sierra Nevadas and Pierre, S.D., for example.
The zones cover all 50 states and Puerto Rico and were drawn from the average winter low temperatures between 1976 and 2005 at 8,000 weather stations. In addition to the District, other major cities are feeling the glow: Chicago, Tampa, New York and Philadelphia are among those that are listed in a warmer zone.
Agriculture officials stressed that the new map is not a tool to measure climate change and that many of the boundary shifts are the product of better and more complete data and sophisticated computer algorithms.
“As sophisticated as this is, it is only a guide,” said Catherine Woteki, the undersecretary for Research, Education and Economics at the Department of Agriculture. “Nothing is better than the gardeners’ knowledge,” she said, while speaking to reporters Wednesday at the National Arboretum.
Many local gardeners have come around to the idea that the super frigid winters of the past, including ones in the 1970s that wiped out camellias at the arboretum and Southern magnolias in Georgetown, are history.
Michael McConkey, who sells fruit trees from his nursery, Edible Landscaping, in Nelson County, Va., has grown increasingly tender plants since he started there in 1987. “It’s great for the gardener, especially the edible plant gardener, because there are lots of things that seem to be working now,” he said. At his nursery 500 feet above sea level, fig tree varieties once considered marginal or temporary are now mature, large plants, as are Asian persimmons. “I’ve even got fruit off my Pakistan mulberry, and I didn’t think they should be growing around here. Everybody is picking up a zone.”
He has 10 varieties of pomegranates, a subtropical tree, that not only flower but give fruit.
He said he is keen to replace the old zone map in his catalogue with the new one. He said he knows a gardener in Raleigh, N.C., growing tangerines outdoors. “He hasn’t had winter kill in 15 years — it’s amazing.”
McConkey’s trophy plant for winter survival is the pineapple guava, “a Zone 8 plant.”
Savvy gardeners have always considered the zone map a basic guide, and as Agricultural Research Service spokeswoman Kim Kaplan acknowledged, it does not address the other fundamental constraint on plant choices: summer heat and humidity.
Other factors affect the winter hardiness of a plant, and gardeners can stretch cold tolerance through site selection and soil drainage.
“The sophisticated gardener can’t just rely on the zone map, and what’s the fun of that?” said Todd Forrest of the New York Botanical Garden (now in the same zone as Washington). “If you can’t plant things you’re not supposed to grow, you’re not having fun as a gardener.”
He noted, too, that attractive conifers that excel in the Pacific Northwest suffer on the East Coast “not because it’s too cold, but too hot, humid.”
Kaplan said the new map cost almost $500,000 and took more than five years to develop. Two earlier, unofficial versions were developed privately in the past decade: one by the Arbor Day Foundation and the other by the American Horticultural Society. They, too, showed zones creeping northward to reflect milder winters.
The new official map, which was developed with climate experts from Oregon State University, will be used by researchers trying to assess the spread of invasive plants and insects. And it is detailed enough, said Peter Bretting, one of its Agricultural Research Service architects, to allow grape growers to select an optimum location for a new vineyard.