Aster

Between the internet, two large wildflower books, and two hours of my time... I still don't know what particular kind of aster this is. It could be a Frost Aster or a Southern Saltmarsh Aster, but sometimes identification can be hard when the plants are mowed short. There have been similar plants growing about 3 feet tall on the side of the road, but the lack of leaves on the short plants means I can't even be certain if they're the same kind.
This particular kind of aster has flowers about 1/2 inch wide and is compound. The ray flowers are a light lavender (though some of them are almost completely white), and the disc flowers are yellow. Two more helping things: when the ground is dry the petals curl back and make the flowers look like little buttons, and the stems are purplish.

I've seen pictures of this kind on the web but this is always noted as "unidentified aster" or "unknown aster," so maybe it's a variant.

They're very common plants, so it's good enough to be able to recognize them as asters.