Silene latifolia (formerly Melandrium album), the white campion is a dioecious flowering plant in the family Caryophyllaceae, native to most of Europe, Western Asia and Northern Africa. It is a herbaceous annual, occasionally biennial or a short-lived perennial plant, growing to between 40–80 centimetres tall. It is also known in the US as bladder campion but should not be confused with Silene vulgaris, which is more generally called bladder campion.
http://www.agric.gov.ab.ca/app107/loadPest?action=display&id=61
Hairy pouch-like calyx with dark green or reddish purple veins. Clusters of white flowers with five deeply notched petals. Plants covered in short hairs. Grow to 1m (3.3 ft).
When are they blooming in Banff National Park? End of June at the earliest. Full bloom usually between mid-July and early August. Bloom/seeding continuing on until early September.