Clay soil is much maligned---it is dense, difficult to work with, and retains so much moisture and so little air that it can literally drown many species of plants. Fortunately, there are a number of plants that have evolved to grow quite well in clay soil. Clay's high moisture content and rich supply of nutrients allow these flowers to thrive.
Helenium, or "Helen's flowers," are a group of hardy, sun-loving perennials that are at home in clay soil. They have bright red or orange flowers shaped like daisies, with vibrant yellow highlights. Helen's flowers grow in dense clumps and are generally propogated by dividing them in the spring. They can also be started from stem cuttings. They prefer damp or wet soil and will not tolerate dry soil.
Asters are a genus of fairly hardy perrenials with purple, blue, white or red flowers that bloom in late summer and fall. There are more than 250 asters and they range from a few inches to 6 feet in height. They are adapted to a range of temperature and moisture conditions, and many species do well in clay soil. They will also tolerate full sun to partial shade.
Birds and Blooms describes black-eyed Susan, or Rudbeckia hirta, as "hardy and fuss-free." The tall, tough annuals have distinctive bright yellow blooms and dark centers and grow densely in sunny locations.Black-eyed Susan reseeds well, making them extremely easy to maintain from year to year.