You don't hear a mower often in Manhattan, so when you do — a July afternoon, a lawn behind a building down near the harbor — people slow down for it. Mowtown LLC makes that sound for a living: mowing, trimming, edging, and the cleanups that come around with spring and fall. City turf takes a beating from feet and shade and dry wind off the water, so the job is as much upkeep as it is cutting. A flat, even green in the middle of all that concrete is worth every bit of the trouble. Manhattan and the surrounding boroughs.