Off Crescent Street the yards are small and stubborn — a strip of grass, a fence line, one tree deciding everything about the light. Green Urb Gardens likes those: mowing and trimming that respects the corners, edging that gives a little lot a defined shape, beds mulched, clippings and leaves carried away. City greenery earns its keep by being tended rather than by being large. A tidy urban yard makes an entire Waltham block feel looked after. Serving Waltham, Watertown, Newton and Belmont.