I’ve been trying to identify this wildflower, photographed on the Kleinschmidt Grade in Adams County, ID, for nearly two years.  I photographed it in June, 2011.  Anyone able to help me out, here?  Update 03/05/2013: @TheLifeBotanic (Twitter) identified it for me as a Vaccaria.

Vaccaria hispanica on the Kleinschmidt Grade in Adams County, ID – June 2011

I had thought it likely a member of Caryophyllaceae due to the unusual shape of the calyx and the 5 notched petals, attributes particularly common in Silene, but my searches had been unfruitful.  A post of this photo to twitter brought the Vaccaria suggestion from @TheLifeBotanic within a couple of hours.  Vaccaria is classified either as a genus of four species, or as the monotypic Vaccaria hispanica with four subspecies.  The little bit of research I’ve done so far implies that the monotypic classification is most broadly accepted, and all my go-to sources list it as such, so I’ll stick with it – this is Vaccaria hispanica. (Some experts have also classified it in Saponaria as recently as 1994.)  It is a Eurasian native, introduced in the United States, and historical records indicate that it has been found in all but two states (Georgia and North Carolina.)  However, it is no longer as common in the United States as it has been in the past, and is likely extirpated in some states.  It goes by several common names – Cowherb, Cowcockle, Cow Soapwort, Field Vaccaria among them.

Primary reference: Flora of North America Editorial Committee, eds. 1993+. Flora of North America North of Mexico. 16+ vols. New York and Oxford.