@Jill Lewallen, The first place I'd begin is with the tree. Does it need care or shaping? Can it be pruned up? If it's in good shape and can't be pruned up, then look at where the shade falls at different times of day and different seasons, and enlarge the bed to take advantage of the best sun. You may be able to plant some shade-loving flowering plants under the tree as well, depending on your eco-region and what will grown in your climate. Have fun!