TreeSong is that a city-planted tree? If so, be advised that they can come at any time to dig up the tree or remove your plantings. But throwing some marigolds or petunias in there to add some color would be nice.
If it's your tree I'd be sure to check that raising them soil won't kill the tree. I'd try planting drought tolerant annuals there first since they won't develop deep root systems. You can also plant spring bulbs but I'd stick to low growing ones like crocus so people aren't tempted to pick the flowers.
Once the tree is well-established you could try planting perennials like daylilies that will provide color but aren't very deep rooted. But if it's a city tree try not to plant anything you really care about there. The city can & will destroy your plantings (and even the tree) if they need/want to.
