My website has many posts and pages on plant topics. Here is a list of some of the pages that give basic information on native plants, especially in Philadelphia and the Mid-Atlantic region.
Natural Places to Observe Native Plants
These lists are of native plants of Philadelphia and the Delaware Valley. These plants have lived here for thousands if not millions of years. They are automatically suited to grow in this region. No worries about suitable zones, soil types or hardiness, their compatibility to the climate is built right in. Just match the moisture (moist or dry) and the light conditions (shade or sun) and you are good to go.
Basic Information: Philadelphia Nature
Common trees of PA (free, downloadable pdf)
Terrestrial and Palustrine Plant Communities of Pennsylvania (free, downloadable pdf)