I have heavy clay soil that becomes very dry in summer but geranium(I particularly like Mayflower which always flowers for a second time if I chop it back after the first flowers fade),francoa which flowers for a long time also verbena hastata ,these are all perennials but for annuals the poppies papaver somniferum come in so many shapes(doubles, frillies) and colours and the seed heads are also attractive after the flowers fade.There are of course lots of others to choose from .

A great start, though. So many geraniums are good performers, from May to October at least. With some shade from hottest sun, and a good start, heucheras. Campanulas, day lilies, grasses . . . . Worthy

Echinops ritro, with blue globes and long flowering interest, also Fuchsia magellanica and Euphorbias do well on my heavy clay.