Out where Lowell runs up against the Lowell-Dracut-Tyngsborough State Forest, the yards borrow their canopy from the woods next door — big shade, thin grass, leaves by the bushel. UNITED BEST TREE SERVICES INC keeps those lawns cut, trimmed and edged on a schedule that respects how slowly turf grows under old maples. Spring means clearing a winter's worth of twigs and road sand before the first mow; autumn means going back again and again as the woods let down their load. In between there is the plain weekly pleasure of a mown lawn with the border cut clean against the driveway. Green up to the treeline, tidy up to the curb.