April in Selden comes on slowly: a gray-green blur along the hedges, and then one week the dogwoods open and it is spring. L I Lawn Maintenance Inc starts its year right there — raking out winter, clearing beds and drives, taking the first cut of the season across the flat blocks of central Suffolk. From then on it is regular mowing, trimming along fences and foundations, edging the walkways, and holding a straight line down every yard on the street. Long Island light lies broad and low here with few hills to interrupt it, and it shows off every stripe in a fresh cut. The last task of the year is the leaves — off the grass, off the walk, gone.