September brings the ball fields back to life around Somerville — whistles in the late afternoon, the grass taking one more growth spurt before it quits for the year. Mundy Lawn Maintenance keeps mowing straight through it, because the last cuts of a season are the ones that decide how a lawn winters. Trimming along walks and fences, leaves taken up before they mat down, beds raked out clean. Then the quiet months, and the spring cleanup that starts the whole cycle again. It is unglamorous week-in work, and the yards on the route show exactly where it has been.