Princeton has been called the City of Elms for as long as anyone has bothered to name it, and its streets still run deep and green under arching branches all summer. Elm City Lawn LLC keeps that green in trim — mowing the parkways and back yards, edging the walks that thread the old blocks, trimming close where the shade thins the turf. Then December turns the same routes white and the work changes shape entirely: plowing drives, clearing walkways, salting the slick spots before anyone steps out. It is a satisfying kind of year, the machines swapping over and the same familiar streets asking for something new. Summer's reward is a lawn that sets off a porch. Winter's is a driveway open before the coffee finishes.