If you are in Georgia, you are probably in hardiness zone 7 or 8. In that mild a climate, it is safe to plant well into fall. In fact for a plant fully hardy in your zone, you could likely plant throughout winter, provided the soil is workable. We do here :-) Fall planting is actually preferred in many situations over spring planting and many local nurseries and garden centers promote this fact with fall planting sales.

As to specific plant choices, it might be helpful to know what type of tree you are attempting to underplant. Some trees develop an extremely dense system of surface feeder roots that makes planting nearly anything underneath very problematic. I'd try to avoid anything that requires frequent division (too much root disturbance for the tree) and will tolerate whatever light and moisture conditions the tree allows. Generally, recommended tree underplantings focus on plants well-suited for dry shade but there are individual exceptions based on your specific conditions.

My KO roses were planted in September 05. No problem as long as you water until established. Perennials do well from being planted in the fall in my zone. It gives the roots time to get established over the winter since energy isn't going into the blooms.

Cameron

Here is a link that might be useful: my KO roses