aka rattlesnake hawkweed
This species of hawkweed is found in all US states east of Mississippi, as well as Quebec, and west of the Mississippi in Missouri and British Columbia. It’s endangered in Maine.
There are 37 native and 15 alien species of Heiracium in North America; at least one grows in every US state and Canadian province (except Nunavut). Some of these are naturally occurring hybrids. And several others are considered noxious weeds. And yet a few more are threatened or endangered.
This one grows up to 2 1/2 feet tall in dry, open woods and clearings, usually on rocky soils (or right on rocks).
It’s all over the Carderock area as well as the large bluff in the middle of the Billy Goat B trail. It’s easy to distinguish from other hawkweeds: on the young plant, the basal leaves have a characteristic red venation, though that color fades as the plant ages.
Several sources state that the moniker “rattlesnake” comes from the fact that it shares habitat with actual rattlesnakes.