At low tide the flats behind Tybee open up — oyster rakes bared, mud shining, the smell of salt strong enough to taste — and the island feels twice as wide as it did at noon. Low Tide Lawn Care starts its mornings around then, mowers running while the temperature is still reasonable and the streets belong to walkers and pelicans. The route covers mowing, edging and trimming, with the clippings blown off before the sun gets high. Sand and salt work against a lawn out here, so the cut is kept sensible and the borders kept sharp to make up the difference. By the time the tide turns, a half-dozen yards are done and the day still has plenty left in it.