The following video by Mathieu Young—a photographer and GOOD contributor—offers a portrait of Scott, a guerrilla gardener in Los Angeles.
[vimeo][/vimeo]
Scott's right that a few gardens won't change the world, but a bit of greenery sure does improve the experience of living in the city. And filling blighted areas with native plant species not only beautifies those spaces, but also ensures that the plants can survive with little or no maintenance.
You can see more of Scott's work around Southern California here.