Anything you plant under your maple will be competeing with the tree roots for moisture and nutrients. Nothing will do well there unless it is planted in a pot and the pot sunk into the ground. Then you'd have the problem of cutting tree roots to plant the pot. They do make circular pots for going around a tree if you can find them or you could do a triangle or square with window boxes. They could sit on top of the ground and be hardly noticable once the impatiens spill over the edge.