Guerrilla Gardening started in New York in the 1970's at a garden between Bowery and Second Ave on Houston now known as the Liz Christy Community Garden. The idea behind guerrilla gardening is to reclaim neglected land by rejuventating it through gardening.
One of my favorite ideas in guerrilla gardening are seed bombs. These allow a quick sneak attack to occur. Sometimes I imagine a similar result when I throw and apple core into neglected landscapes. Apparently these seed bombs often contain seeds as well as fertilizer which I guess I eat away with my apple cores.
Sometimes even gardeners get in trouble...
Gina Badger has some really interesting studies and installations of nature/landscape interventions and exhibitions. (I found this while searching for seed bomb photos)