Oleander and tropical palms are in, but lilacs and Norway spruces may be on their way out.
So say local garden experts who’ve noticed that the area’s warmer winters are bringing changes to the region’s landscaping.
Constance Cleveland, a plant buyer for Behnke Nurseries in Potomac, says she has “definitely noticed a warming trend” that is affecting what grows best in local gardens. Varieties of plants purchased from a grower in Richmond, Va., increasingly seem to thrive better upon delivery than the same plants purchased from a grower in much cooler Connecticut, she says.
That’s not news to Richard Olsen, a research geneticist working in Beltsville for the U.S. National Arboretum. Warmer winter temperatures mean “there are definitely things that we [gardeners] can grow now that we haven’t historically been able to,” he says.
The region’s warmer winters are reflected in the revised USDA Plant Hardiness Zone Map, which was released in January 2012 and outlines the country’s temperature zones.
USDA spokeswoman Kim Kaplan of Silver Spring says the new map, which was last updated in 1990, reflects weather conditions that were recorded with more accurate tools. To update the map, the USDA team used data from 1976 to 2005 that recorded the average lowest winter temperature for a location, and an algorithm to factor in conditions such as proximity to bodies of water, the slope of the land and prevailing winds.
On the new map, zones for average minimum winter temperatures have generally shifted five degrees higher than they were on the 1990 map, which used data from 1974 to 1986, according to the USDA.
Gardening experts caution against reading too much into the revised map, saying it reflects subtle changes that have been occurring for years. In fact, the USDA was criticized in press reports for its reluctance to ascribe the changes to global warming when the map was released last year.
It is “a weather map, not a climate map” and should not be seen as “a guide to long-term climate trends,” says Kaplan, who helped create the revision.
However, Bethesda resident Rick Piltz, founder and director of Climate Science Watch in Washington, D.C., views the map as part of a bigger picture.
“Human-caused global climate disruption is now an integral component of observed weather patterns and changes in ecosystems,” he says. “For example, there are observed trends toward shorter winters, earlier spring thaws and plant flowering, and delayed onset of fall.”
Regardless of what the changes signify, the USDA map is an important tool for nurseries in determining which plants to order, and for consumers wanting to buy plants that will thrive in their area’s designated zone.
Though it’s not likely that the local climate is warming enough to support a bougainvillea—that ubiquitous heat-loving plant found in true Southern climes—Olsen says there are hardy varieties of tender broad-leaved evergreen shrubs, such as gardenias and camellias, that no longer need to be grown in the shelter of a porch and can be planted in the open.
Other plants that may fare better in the region now include oleander, a bright, flowering plant commonly seen dotting Southern landscapes, and tropicals such as elephant ears, banana plants and palms that have been more common in warmer climates.
And, Cleveland says, some plants formerly considered annuals—black and blue salvia, variegated vinca vine, lantana, Easter lilies and loosestrife—are now likely to return each year instead of dying off.
But there’s a downside to the changes wrought by warmer winters. Olsen says some cool-weather lovers, including lilac, Norway spruce and Norway maple, may no longer thrive without a cold winter. Just as a cold snap during the spring blooming season can kill a warm-weather plant, consistently higher temperatures can distress certain plants and eventually kill them, too, Olsen says. Lilacs, for example, will become stalky and bloom less in a warmer climate.
Also, milder winters allow diseases such as powdery mildew to thrive because temperatures don’t drop low enough to kill them, Cleveland says. And a local trend toward more sudden and violent thunderstorms, versus long and steady summer rains, doesn’t help plants, she says, because the downpours run off instead of sinking into the ground to benefit the roots.
“I’m hoping that [these conditions] will be a reason for people to start thinking of gardening in different ways,” Cleveland says. She says gardeners should replace their “perfect, manicured” ornamental gardens with drifts of native plants, including black-eyed Susan, mountain laurel, native ferns and honeysuckle that require little or no maintenance to thrive.
For their part, experienced gardeners are finding they don’t need the USDA’s revised zones to guide them in deciding which plants will thrive.
“I already know my zone is generally colder than the rest of the area because I back up to Rock Creek Park,” Kaplan says. “As a result, I don’t have enough blacktop or concrete to hold the heat.”
Cleveland suggests that even less experienced gardeners should rely on what they know about their gardens and yards, rather than strictly adhering to the USDA map.
“There’s still a lot of wiggle room,” she says.
Richard Olsen, a research geneticist working for the U.S. National Arboretum in Beltsville, says new, hardier forms of broad-leaved evergreens and some tropical plants may grow here now that winters aren’t as cold. Among them:
- The beautiful camellia is an evergreen shrub that can thrive in the region’s mild winters, even though it’s a Southern plant that traditionally has grown best in warmer climes. New, cold-hardy varieties need less protection than original cultivars, Olsen says.
- The gardenia, another traditionally Southern plant, is visually stunning and smells good, too. Bright white flowers bloom against dark evergreen leaves in spring and summer. Though older varieties may struggle, Olsen says new, hardy cultivars should grow well here.
- Typically grown in warm Southern climates, Japanese aralia is often found in protected spots in the Bethesda area. But this elegant, broad-leaved shrub with its little sprays of white flowers that bloom in October may fare just as well in yards and gardens with the warmer winters.
- Indian hawthorn, a shrub that flourishes in the South, now can be grown locally. It has star-shaped pink or white flowers that precede the low-growing plant’s dark blue winter berries.
- Oleander, a traditional Southern beauty, boasts showy pink or red flower clusters at the end of leafy, erect stems. Olsen suggests a cold-hardy variety to ensure that it will survive.
Karen A. Watkins is a freelance writer living in Bethesda.