Strictly speaking, the moss pink (Phlox subulatd) is about the only American rock plant I know which is commonly cultivated the world over, even in its own country, which is the supreme test. But, here again, it is folly to draw any sharp line between plants that grow wild only on rocks and those which also grow in other situations. For instance, bloodroot will grow anywhere, yet it attains its highest beauty, I think, on rocks. The English think so, too, and spend no end of money to establish it in their rock gardens; but it is a hard plant to export, and we can always surpass England on mass effects with bloodroot.
As near as I can tell the most famous rock-loving perennials that are native to America, and reasonably common even in our gardens, are the following:
Crested dwarf iris
Purple poppy mallow Callirhoe involucrata
Wild bleeding heart
Those marked * are the only ones that grow more than a foot high. I must confess that it is not a very strong list from which to pick the big American winners, for the rose, pink, and crimson kinds are not of the most popular shades, especially in the case of the two greatest geniuses in the list. Moss pink, in its wild state, has a crude and violent colour, and our rich estate owners have already made the mistake of painting whole hillsides with it, when they could just as well have used white, lavender, and other refined varieties of it. So, too, the wild bleeding heart is a never-ending marvel, because of its exquisite foliage and profuse bloom from May to September; but every time you look at its colour you sigh and turn away.
However, we need not be discouraged about the American element in rock gardening for many reasons.