An appropriate flower for August after an inspection of the rock garden is Silene suksdorfii. Silene are members of the Caryophyllaceae family of which there are approximately 500 species of varying habits with characteristic five petalled flowers and the greatly inflated calyces.
Silene suksdorfii is a beautiful clear white with slightly bluish green leaves, which form a low cushion from which the flower stems extend to about 15cm, it grows in full sun in a well drained medium on the open rock garden.
In the wild it is found in North America, in Oregon and Washington State in the alpine zones of the Cascade Mountains, but in British gardens it provides a late flowering contrast to the Campanulas. The Silene in my garden came from AGS seed sown in 2002.