Ask Mr. Smarty Plants

An oak tree that old is a valuable landscape asset, so your first question should probably be: "What will not harm the tree?" Oak trees do not like competition. Their heavy shade in summer, and the year-round presence of the root system are disadvantages for plants beneath the oak. Furthermore, the mature oak is known to have the quality of allelopathy, which means they emit substances that will inhibit the growth of plants beneath them; these substances can come from roots, twigs, or leaves. You say you plan to plant 10 to 12 feet from the oak, we assume that is from the trunk. A tree that old likely has a pretty impressive canopy, and the roots can extend out up to three times the circumference of that canopy.

Next, your idea of making a raised bed would possibly permit you to grow some plants, but it would not be good for the oak. These far-reaching roots we described reside mostly in the upper 12 inches of the soil. That characteristic is to ensure the roots have easy access to moisture, nutrients and oxygen. Piling soil up on those roots might not damage the tree right away, but in the long run it could begin a die-back of the most affected areas, eventually becoming an unattractive liability. If you dig holes for shrubs in the ground level, without raising the bed, you will both be destroying roots of the tree and putting roots of the shrubs in competition with the roots of the tree, whch is advantageous to neither.

We don't like to sound so negative, but we also would hate for you to waste time and resources putting in plants that will not thrive and possibly damaging your tree, as well, in the process. One possibility might be to put down a good quality shredded bark mulch beneath the tree, and perhaps ring that mulch with shrub beds having the same mulch in them. They would blend together nicely and look natural. With that in mind, we are going to recommend some shrubs native to South Carolina that will tolerate sun or part shade. We tried to choose some that are considered "understory" plants; i.e., they should do pretty well at coping with interference from trees. Follow each link to the page on that plant to learn other characteristics and growing conditions. 

Shade or Part Sun Shrubs for Cover SC:

Callicarpa americana (American beautyberry) - 3 to 5 ft. tall, deciduous, blooms white, pink May to July, part shade

Comptonia peregrina (sweet fern) - 2 to 4 ft., blooms white, green May to August, part shade

Hydrangea arborescens (wild hydrangea) - 3 to 6 ft., deciduous, blooms white, green June to August, part shade

Hypericum prolificum (shrubby St. Johnswort) - to 3 ft., deciduous, blooms yellow June to August, part shade or shade

Symphoricarpos orbiculatus (coralberry) - 4 ft., deciduous, blooms white, green April to July, part shade or shade

From our Native Plant Image Gallery:

Callicarpa americana

Comptonia peregrina

Hydrangea arborescens

Hypericum prolificum

Symphoricarpos orbiculatus