I would first look at what is causing the shade... tree's sometimes leech chemicals around their base that prevent plants to grow there. Pine trees are a great example as they leech turpentine and not very many things will grow in that environment. There are shade loving plants such as ferns, moss, and maybe even some orchids... If you plan a garden outward from the trees, in the lighter shaded areas flowers like Pansy, viola, begonia and even hydrangea may grow. Identifying the trees causing the shade would be my first step.