I inherited a similar situation when I purchased my current home, only difference, my Sweetgum was in the front yard, and highly visible to everyone passing on the street!
At first, I tried to grow grass under it, which was a total failure. I finally concluded that a flower bed with shade tolerant plants was the only solution. So I constructed a raised bed, after collecting rocks from construction sites all around the area, to build a perimeter barrier.
I dumped tons of ground bark, compost and top soil to raise the level about 4", rented a tiller and tilled the whole bed to a depth of about 10", incorporating the native soil with the ammendments. I probably cut a few tree roots in the process, but it didn't seem to affect the tree.
In that bed were planted, evergreen azaleas, Hostas, Ferns, 2 species of Mahonia, Campanula, Trillium, Bloodroot, Selaginella, Toad lilys, Helleborus, Epimedium, Hardy Begonias, Cyclamen, Bletilla Orchids, Asarum, 4 Japanese maples, a Star Magnolia, Hydrangeas, Columbines, 3 native Azaleas and a member of the Aralia Family, Acanthopanax sieboldianus 'Variegatus'.
On the side that receives morning sun, there is a KO Rose and 2 species of Iris. I have just added 2 species of Lycoris there. It is crowded, to be sure!
I placed 200ft. of soaker hose in the bed and covered it with a layer of compost, so that water can be added when necessary.
No fertilizer is added and a fresh layer of compost and leaf mold each year, provides nutrients for the plants and the tree.
I'm sure there are other methods of growing plants under your tree and you will likely receive other recommendations.
Try all of them, to determine which is most successful for you.
Good luck!
Rb

Wow razorback33, can you post a pic?

Cultiv8er, all the plants you're trying to grow are sun loving. The plants razorback is growing like shade. That could be part of the problem.

Gardening within the root zone of a large tree is always a challenge, especially one like the Liquidambar (sweet gum), with an extensive network of surface and feeder roots. The roots outcompete smaller plants for any available soil moisture and nutrients and the tree when in leaf will often shield rainwater away from reaching the underplantings.

Plants best suited to this type of environment are those that will tolerate dry and typically unfavorable soil conditions. Whether they need to be shade tolerant or can be more sun loving will depend exactly on how much light reaches this planting area but they will all need to share the characteristic of being drought tolerant. And they will need help initially to get established, with frequent deep watering and perhaps some nutrient supplementation through fertilizer or a layer of compost.

One of the things you could try - in addition to a better suited plant selection - would be to enlarge the size of the bed. Using the sweet gum as an anchor for one end of the bean shape, draw the shape out so that the opposing end is well clear of the canopy. While the roots will still be present, the area will be more exposed to the elements, receiving more direct sun and rain. You could also raise the soil level in this area, which will give the new plantings a better start than digging directly into the rooty, nutrient-starved existing soil. You just need to be careful not cover the entire perimeter around the base of the tree with additional soil, as this can deprive the tree roots of oxygen and eventually kill it. But raising in it one particular area well away from the trunk will have no adverse effect. And a larger planting area will carry more impact than a smaller "ring" around the base of the tree.

There are lots of drought tolerant perennials (and quite a few shrubs) that will grow in this situation, especially in a zone 7 climate. Google up some lists or visit a local nursery for their recommendations. Smaller sized plants will establish easier than larger, more mature ones (and they are far easier to plant when digging through compacted, rooty soil). Most annuals tend not to be the slightest bit drought tolerant so you may not have as much success with them. And often the plants best suited for these conditions are not the showiest with respect to flowering so you may need to dial down your expectations and focus on foliage or other long term interest.

Good luck!

Never raise the soil level under a tree or shrub, especially a well established tree. As for planting underneath a Sweetgum, use native sedges and native plants that like partial shade. Use smaller plants (plugs or 1 quart pots).