Midday in Chelsea, the sun clears the rooflines for about two hours and lands square in the courtyard behind the building — and for those two hours that patch of grass is the whole outdoors. Greener Gardens Of New York Inc looks after enclosed greens like it: regular mowing, careful trimming in corners no machine can turn, edging along brick and bluestone, cleanups in spring and fall. Hidden gardens are scattered all over this island, and every one of them runs on somebody showing up. The reward is a tenant opening a door onto something green in the middle of a block of stone. Manhattan and the adjoining neighborhoods.