Most annuals need well draining soil but if you add grit or cactus potting soil to regular potting soil in a mix of half and half and plant lobelia then I think you will have a good few months of color. There are also Siberian iris that will grow in water logged soil but these iris are bulb like perennial plants with a short bloom time and then long, grass- life foliage. Is there any way you could drill more drainage holes into the container? Another option might be to set shallow flowerpots into the larger container, fill the flowerpots with potting soil then surround it all with gravel. Keep growing, Marianne Binetti