As head of Green Guerillas, a community-gardening group, Steve Frillmann brings together neighbors who may not necessarily be environmentalists or garden advocates. No matter the color of their thumbs, they all work together to clean up urban areas, making them a little safer, a bit more humane. In answering reader questions, Frillmann chats about digging into the community-gardening scene, growing food on urban rooftops, and dealing with hose-related squabbles.