Silene latifolia or Melandrium dioicum
(White Campion)

Family: Caryophyllaceae (Pink)
    
Melandrium dioicum
is dioecious (male and female flowers are on separate plants). A male plant is shown in the first photo, and a female plant is pictured in the second photo. The calyx below the flower is inflated and pubescent. White Campion is a non-native plant, growing to 3-feet tall on disturbed meadows or roadsides.

Compare to Silene antirrhina, which has smaller flowers and a non-inflated, glabrous calyx.

Vegetation zone:  Plains, Foothills, Montane (4900 - 10,500 ft.)
Time of bloom:  June - September
Origin:  Introduced
Eastern Colorado Wildflowers