On the North Side the maples throw enough shade to make turf work interesting. ChemTurf Lawn Service reads a lawn the way you would read weather — where it is thin, where it is hungry, where the shade wins and the grass needs help holding on. Spring green-up is the payoff: a yard that came out of a Westchester winter patchy and gray standing even and thick by May, hedge line to walk. Mowing, trimming and edging keep it in shape once the season is running, and cleanups take away the rest. Mount Vernon is a city of small lawns, which means every square foot is on display.