Anyone worried about the homogenization of American culture by fast food restaurants and chain stores will be soothed by a walk in a Georgia, Florida or Louisiana garden. No matter that St. Augustine, Florida, shares the same USDA hardiness zone with Las Vegas, Nevada, and Austin, Texas. A unique combination of humidity, soil conditions and a long, warm growing season produce plants that still make the Southeast unlike any other gardening region. Several plants say "Southeast" as nothing else can.
The Southeastern United States is distinguished by growing zones that range from USDA hardiness zone 7a and b (northern Alabama, Arkansas and Georgia) to zone 10b to 11 (the Florida Keys). In zone 7, winter temperatures can dip to 5 degrees F, but killing frosts are brief (frost-free days begin in early March). Zones 10 and 11 are near-tropical and effectively without frost at all.
The University of North Carolina Herbarium lists dozens of trees familiar to Northern gardeners: oaks, pines, maples, hickories--almost all the popular tree varieties except the cold-hardiest firs and balsams. The list also includes six types of magnolia. This iconic Southeastern tree has colorful regional nicknames: cucumber magnolia, umbrella tree, sweet bay. Long-lived (varieties last from 40 to more than 100 years) and large (reaching heights up to 100 feet), magnolias perfume Southeastern gardens and Northern imaginations.
Among familiar Southeastern shrubs are six varieties of native azalea. Forest dwellers, vigorous in dappled shade, azaleas grace both gardens and mountainous areas in Alabama and the Georgia Piedmont. Azaleas with deeply curved petals and long stamens are Southeastern growers only, unable to tolerate cold winters.
Profusion characterizes Southeastern perennials. Warm zones harbor eight varieties of perennial lobelia, while Northern gardeners are familiar with red (lobelia cardinalis) and blue (lobelia syphilitica) perennial varieties but most often see lobelia only as a small-leaved, blue-flowered heat-tolerant annual.
One large class of plants characteristic of the Southeast is vines. Many are native and perennial: clematis, wisteria, honeysuckle and morning glory, in every color of the rainbow. Receptive growing conditions mean that several Southeastern pests and invasives come in vine form: Japanese honeysuckle, mile-a-minute, and kudzu are among the most difficult.
A persistent problem in the Southeast is not getting plants to grow, but rather preventing invasive, out-of-control growth. The Florida State Noxious Weed List, for example, contains roughly 100 plants that respond invasively to a long, warm growing season. Plants include Asian air potato, Chinese tallow tree, wild raspberry, African couchgrass and Indian rhododendron. Some are accidental imports; others were brought into Florida as ornamentals.