I live in Michigan zone 5. I have 2 Black Walnut trees in the front yard. I know that the tree's leak a poison in the ground. It's shade with 2 hours of sun. The grass is very patchy. I would like a butterfly bush, red twig dogwood plants that look great from Spring to frost. The only thing that has lived are the tulips. Should I raise the flower beds? Please a list of plant's that will thrive with them darn tree's would be great.