The tall garden phlox need rich, well-watered soil and pampering, according to the experts and the books.
Yet the old farm and dooryard varieties cannot read, and they grow where they want to. I have one that came up in the middle of a quince bush. Others persisted in forgotten places against the house. One four foot tall variety ,yet to declare its color, is growing in sand in the dry garden.
And then there are these,obviously self-seeded, and sprouting out of cracks in a granite retaining wall.
The granite must protect the roots and keep them cool and moist, although one is growing in a pile of sand at the entrance to a woodchuck burrow.
Plants always astonish me.
Gardening On Granite 