Ossining sits on old rock — gray outcrops shoulder up through lawns and driveways, and there are yards here shaped entirely by whatever ledge refused to move. Kai Stone Landscaping Corp works with that instead of against it, mowing and trimming close around the stone, edging the grass to follow the shape of the ground, mulching the beds that fill the gaps. May is the month it all pays off, ferns unrolling in the shade of the ledge and the turf coming in thick beside it. Grass and stone in the same frame, each making the other look better. Properties in Ossining and the towns nearby are kept that way all season.