The Pennsylvania flora includes 3,091 species of vascular plants. Sixty-nine percent of these are plant species that evolved in eastern North America along with other indigenous plants and animals. The remaining thirty-one percent consists of plants that evolved elsewhere and have been brought to Pennsylvania through the actions of humans.
Plant introductions have been both deliberate and accidental. Many cultivated plants were brought by the earliest European settlers. Most of our agricultural crops and ornamental garden plants originated in other parts of the world; the search for useful plants is a continuing process. Accidental introductions also have a long history and continue to occur as seeds and other propagules "hitch a ride" in ballast water of ships, airplane cargo holds, or on people's shoes.
While many introduced plants have remained confined to cultivated fields and gardens, others have spread unchecked into the surrounding landscape. For instance, purple loosestrife (Lythrum salicaria), a plant from Europe, has been able to dominate wetland habitats. Sources of information on invasive plants of our region include: Invasive Plants of the Eastern U.S., Mid-Atlantic Invasive Plant Council, and the Invasive Plant Atlas of the United States.
In Pennsylvania and other mid-Atlantic states, problems caused by invasive, non-native plants have been exacerbated by deer overabundance. White-tailed deer evolved in eastern North America along with the local flora; they recognize native plants as suitable food. These large herbivores do not eat most non-native plants as long as native species are available. This results in the proliferation of non-native species in heavily browsed landscapes and has led to the complete dominance of the forest floor by Japanese stiltgrass (Microstegium vimineum) or garlic mustard (Alliaria petiolata). A variety of cascading effects on ecosystem function brought about by invasive, non-native plants have been documented.