Lowcountry gets broader growing range in map

New government guidelines broaden the spectrum of what's possible to grow in Lowcountry gardens.

The Department of Agriculture last week unveiled a new plant hardiness zone map for the entire country, the first revision since 1990.

The new map reveals, as many green thumbs already knew by experience, generally warmer low temperatures for winter than the old zones showed.

No, orange groves aren't likely in the Lowcountry's future. But more people will find themselves in areas where citrus fruits and tropical plants have a chance of thriving.

"This is kind of a big deal because it's a widely used tool by gardeners," said Tony Keinath, a Clemson plant pathologist in Charleston.

Viewing the map online at www.planthardiness.ars.usda.gov, Keinath saw a few significant changes in the area's three counties.

For example, all of Charleston County had been classified as "8b," with average extreme low temperatures between 15 and 20 degrees.

Now, pockets of the county are zoned 9a, with minimum temperatures in the 20- to 25-degree range. They include downtown Charleston, James Island down to Folly Beach, a small area around Remley's Point in Mount Pleasant and a tiny tip of Cainhoy south of Interstate 526.

Almost all of Berkeley County, except a sliver along the Santee River, and Dorchester County entirely are now in the 8b zone, Keinath said. Previously, the upper portions of those counties were in 8a, or 10-15 degrees.

Keinath said what's unusual in Charleston County is that the 9a zones are not physically connected to the rest of the 9a farther south. The bulk of that zone starts below Savannah and continues down the Georgia coast.

"There may be more of them now, the so-called micro-climates," he said.

Keinath, who has lived in Charleston 20 years, said it's rarely gotten below 19 degrees at his house in Shadowmoss Plantation off S.C. Highway 61.

Nevertheless, "I'm still cautious about growing tropical things and expecting them to overwinter," he said.

Gardeners in rezoned areas may find it's easier to grow tender perennials such as salvia, plumbago and ginger, he said, along with citrus such as Meyer lemons.

The way a deep cold snap occurs affects how much plants are damaged, he said.

"If you have a gradual cooling down over two to three nights, that is less harmful than a sudden drop in temperature. The other thing is whether we have an actual frost on the plants," he said. "If the air is dry and frost doesn't develop, that's less damaging."

The nation's 80 million gardeners rely on the hardiness zones to pick trees, shrubs, grass and perennials that will survive where they live.

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 Nevada 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. Many major cities are feeling the glow: Washington, 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 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 last week at the National Arboretum in Washington.

Savvy gardeners have always considered the zone map as 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. "If you can't plant things you're not supposed to grow, you're not having fun as a gardener."

Adrian Higgins of The Washington Post contributed to this story.