In April the small back yards behind Union Square wake up all at once — forsythia flaring against chain-link, crocus in a strip of dirt two feet wide, everything crowding toward the light. Gardening with Jenny works at exactly that scale, where a lawn might be the size of a rug and still deserves a straight edge and a clean line along the walk. Mowing, weeding the beds, trimming what has sprawled over the path, hauling out whatever winter left in the corners: on a block of triple-deckers it adds up quickly. Somerville hands you close quarters and generous sky, and a yard here can carry more green per square foot than places with acres to spare. By the time the market tents go up on a Saturday, the little places along the side streets look tended.