Los Angeles hides hundreds of public stairways in its hills, concrete flights that climb between bungalows in neighborhoods like Silver Lake and Echo Park, where bougainvillea spills over retaining walls and nearly every yard tilts. Garden Design & Landscape tends lawns in this city of uneven ground, where a patch of grass might sit on a terrace, a hillside shelf or a narrow strip beside the steps. Mowing and trimming on terrain like that asks for sure footing and a steady hand, and the payoff is grass that sits level and neat against the pitch of the hill. In the late afternoon, when the light goes long and gold across the Silver Lake Reservoir, a well-kept yard on a stair street is an open-handed gift to everyone climbing past.