What vegetation is the characteristic of the South?

Since the climate of the South (assuming "the South" means the southern United States) is predominantly hot and humid with warm, temperate winters that see very little to no snow, the plants that exist are primarily that adapted to such hot, subtropical climates. Many C4 ("warm-season") plants are found in the South compared with the north, and propagate themselves during the hottest part of the year, which is from May to August. Cool-season (C3) plants grow during the "winter" months of the South.

Basically, any plant that is sensitive to frost and considered an "annual" in the North (upwards into Canada, except around Vancouver which rarely sees long periods of snow cover like much of Montana and Wisconsin does), is grown down south. Crops include cotton, sugarcane, oranges, and okra. Tomatoes are considered a perennial down South because of the consistent frost-free periods throughout the year. Other plants found down South that cannot be grown further north (native or invasive) include elephant grass, bamboo, Bermuda grass, bahai grass, big bluestem, little bluestem, buffalo-grass, sudan grass, lespedeza, blue grama, cheat grass, cypress trees, sawgrass, West Indian mahogany, wild orchids, and many, many others.

To give you a further idea, you won't find spruce trees, lodgepole pine, Douglas fir, saskatoon (Ameliancher alnifolia) prickly/common wild rose, wintegreen, dogwood, snowberry, buffaloberry, high-bush cranberry, western porcupine grass, plains and foothills rough fescue, chokecherry and many other plants down in the South.