We don't have county-specific lists, but you can use the resources linked below, which list many species that occur anywhere in Maryland, to narrow-down which grow naturally in the Mountain Zone of the state. They are not exhaustive lists, since they focus on "garden-worthy" species that tend to have showy aesthetic attributes and/or those which are more readily available for purchase, but they're a great starting point.

• Native Plants for Wildlife Habitat and Conservation Landscaping: Chesapeake Bay Watershed by the U.S. Fish and Wildlife Service
• Native Plants of Maryland by UMD Extension
• Landscaping with Native Plants by the Maryland Native Plant Society
• Native Plants for Northern Virginia (still applicable for much of Maryland)

Species can also be looked up on databases like Maryland Biodiversity Project, which tracks where species observations were made by county.

Miri