Is this Senecio vulgaris or Erechtites hieracifolia or??? Hollow stem suggests Senecio, but can't find info about milky sap. Hairless. Leaf margins are double serrate with tiny spines(?). Leaves are not fleshy--resemble Arugula-- incl leafminer damage!
Montgomery County Maryland
This looks like a type of sowthistle, likely annual sowthistle (Sonchus oleraceus).
One of it's common names is 'milk thistle'.
Common groundsel doesn't grow nearly as tall.