My 92 yo MIL has a lovely yard but would like more flowers. She has a sunny area at the bottom of her yard, but it is flooded in the spring and then the clay turns to concrete in the summer. Somehow Pachysandra grows, but when I have tried to plant flowers, such as black-eyed Susans, they die. Can anyone suggest how to alleviate this problem, and what flowers to plant?

I need to add that this ground needs a pickaxe to break up when dry, I am only able to dig separate holes, due to back problems.