Broadleaf evergreens, allowed to grow in a graceful form rather than being sheared into mushrooms, blousy hydrangea, fragrant plants, unabashed mixed colors of azaleas. Old brick and sand pathways, ferns on the substantial front porch, working shutters at the windows.

yes, to expand on what rhizo mentions about plants, every southern garden should have a gardenia or two for fragrance. those big, floppy southern indica azaleas, esp. 'george l. taber', are also omnipresent but might be out of zone for you. and an old, arborescent crape myrtle that has not been chopped back to stubs is nice to see for months of summer color.

To me, the definitive plants are azaleas, dogwoods and redbuds. Our gardens are full of "passalong" plants, like hydrangeas, daylilies, crinums, amaryllis, and such. We save seeds as if our life was dependent on it! Heirloom veggies filll our veggie gardens and we share the harvest too. I've never been shy about walking up to a southern gardener's house and asking for a cutting, seeds or start of a plant. They are largely THRILLED to share and I've made many a lovely garden friend that way. So in short, the southern garden is defined, IMHO, by sharing.

Depends on the region specifically in the South/zone but all of these are pretty Southern I think (though, not all can be grown everyone in the South). List is going to be heavily biased towards Deep South, but that's usually people's idea of a Southern Garden.

Trees:

Pecan

Dogwood

Magnolia

Crape myrtle (if it's a good garden they won't prune them to nubs! I hate that so much absolutely ruins the lovely elm-like loose canopy and interesting peeling bark)

Pines

Southern Live Oaks

Red cedar.. or whatever kind grows here

Wax myrtle (Florida specifically?)

Peaches & Plums

Shrubs/Larger Flowers:

Hydrangea (native oakleaf or chinese ones)

Azalea (native woodland/graceful ones or hot pink and white cultivated ones)

Hostas (especially large dark foliage ones with white flowers)

A lot of indiscriminate hedges and whatnot

Dahlias

Even some types of roses

Smaller flowers:

Zinnias

Lantana

Spiderflower, ornamental onion-relatives

My grandmother who is from a long line of people living in rural Georgia also is obsessed with begonias and daylilies but I'm not sure if they're "quintessentially Southern" (I know daylilies are popular where I go to school in New England)

You might see columbine and fuchsias too

Vines:

White jasmine vines

Trumpet honeysuckle (native)

Chinese honeysuckle (don't recommend growing this one, it's horribly invasive despite it's beauty)

Trumpetvine (native)

Muscadine grape (native, if a tree isn't being covered in kudzu it's being covered in this vine)

Other:

Liriope/lilyturf (dark green or variegated most common)

Vegetables/Crops/Herbs:

Okra

Pawpaw (uncommon but not unheard of in cultivation)

Cucumber

Watermelon

Rabbiteye blueberry

Anything heirloom, traditional, and locally grown fits here

Style/Look:

Overall design is a bit of a juxtaposition between the very formal and the lush, wild, and 'unruly'. This is due to the intense sunlight and humidity which enables plants to grow quicker and allows epiphytes like 'Spanish moss' to proliferate versus what I believe is an inspiration from noble/formal English gardens. I say the second because wealthier Southerners often aligned more with the English in the Revolutionary era than their northern counterparts and I think this is reflected in the gardening styles and in certain customs (most obvious is their love of tea). I see plenty of gardens that make use of both copious amounts of trees and more "wild greenery" with stately paths and geometric hedges. Yard ornaments that are more grand than rustic, like stone grey and white fountains are common.

Basically, think of a lush woodland clearing with a bunch of manicured hedges and a fountain or statue in the center. Add in some Spanish moss and any of the plants mentioned above and it is very 'Southern'.

Hope that helps and sorry for being so long-winded haha