After knocking this together very quickly for the weekly challenge, I’ve decided to continue with it.
I have, by and large, eliminated the problem of grass growing out of rocks (there are still a couple, but much better). To do this I did an orthographic top view render of just the ground plane and the rocks, the ground with a black diffuse and the rocks with white, took it into gimp, inverted the colours and cranked up the contrast, and used that image as a density texture for the grass.
Also done some work on the ground plane texture itself, and altered the composition slightly to follow the golden triangle.
It is a cool little picture, but I agree the rocks could use a bit of love. Maybe just another subdivision would make them more like normal river rocks?