Chenopodium capitatum (Chenopodiaceae) - This member of the goosefoot family was introduced from Europe, and is ediible. The photograph is the plant with fruit, not in flower. The berries are reputedly nutritious but flavorless (I didn't try them, having not been able to identify it). It can be found in the more northern areas of the USA, and in Canada. This photo was taken Labor day weekend, when it had already gone on to the fruiting stage; it blooms in summer.
(Photographed in Canada on St Joseph's Island)
Thanks to Susan J. Meades for identifying it!