It came up in a pot where I'd sowed Gaylussacia dumosa...
Doellingeria? Leaves are all alternate? Check the FNA key and see if it fits.
Aaron, glad you could put a name to this DWC (damn white composite, borrowing from the famous DYC phrase). Going through FNA Asteraceae is daunting, even if just at the family level. With only 3 species of Doellingeria (infirma, sericocarpoides, umbellata), it shouldn't be too hard to arrive at an ID. Off hand, it looks like D. infirma to me. All of these lived previously in the genus Aster.
Thanks all! It's now covered in snow, so I'll probably have to wait for next year to do a proper ID!