I have a couple of flower beds along my drive way that I am trying to decide on a ground cover for. The beds currently have iris of many varieties, various lilies, some natives, and a miniature rose bush on one side. Despite the sound of things, there are some empty spots between the plantings that I would like to put a ground cover in. The problem is that I have heavy clay soil, I do amend when I plant, but that typically doesn't last very long before the clay is back to it's old self again. I also live in Kansas

, and as a result the ground can be water logged from spring until mid summer if not longer if there is enough rain, or if there isn't a lot of rain the ground dries out and cracks. Needless to say, it's been a struggle finding plants that can tolerate the conditions.
Any input on a plant that is readily available that can tolerate the conditions and preferably stays below 6" tall would be greatly appreciated. Thanks for any help.