Hi, Well it really depends on what kind of trees you have. Maples, for example, have shallow root systems and aren't easy to plant under at all because most plants will die. Same goes with conifers such as pines and spruces. Don't even bother planting under them. With other species, it depends on how much shade they throw. If they are big, trees that throw a lot of shade, you'll need to focus on shade-loving perennials such as hostas and ferns. If it's a smaller, ornamental tree like a dogwood or redbud, you could grow a slightly wider selection of shade lovers such as bleeding heart, Virginia bluebells, lamium, vinca, epimedium, etc. You can check all these shade-loving perennials in our Plant Encyclopedia on this site.