The C&O Canal is one of the most biologically diverse parks, especially for plant species, in the National Park System. The park has recorded over 1,500 species of vascular plants, including over 260 non-native plant species, over 100 rare, threatened or endangered species of plants in Maryland and The District of Columbia, and 1 federally endangered plant species. The number of rare plants represents one of the highest concentrations of state-listed rare plants in the eastern US. Several species are globally rare, and some occur here because they are dependent upon special habitats and ecological conditions present along the Potomac River.
Numerous ecological factors along the Potomac River create a mosaic of different natural habitats. As the park winds westward from just below the Fall Line to western Maryland a variety of geologic formations are exposed, supporting diverse native plant communities,. Areas along the Potomac River, the Potomac Gorge area for example, are subject to frequent floods, causing canopy gaps, scouring and deposition that creates a diversity of habitats and organisms. Distributions of many northern and southern plant species overlap the Potomac River. Isolated populations of western species survive where rare prairie habitat persists along the river. As a result, rare species occur here that are known in only a few other places in the Mid-Atlantic region, adding to the great diversity of this area.