Two years ago, Peter Rothbart was riding through Seattle on his bike. He came to a traffic circle. In the center was a 15-by-20-foot patch of soil where the city allows residents to garden. A man was standing there, looking down at a sorry-looking bunch of plants that had been run over and obliterated by a late-night driver. Later that evening, Rothbart went to a barbecue and overheard a woman talking about how she had an expansive lawn that she didn’t have time to take care of. “What if that guy could garden her land?” he said. “It just seemed like a good idea.”