Check out a blurb on Guerrilla Gardening and a how-to video for Guerrilla Growing Paw Paws on public land by Quad Cities Edible Landscapes' Chris Rice.
Guerrilla Gardening is planting where you don't actually have permission, whether road ditches, along bike trails, abandoned lots or public parks & open spaces. Guerrilla gardeners often see these spaces as neglected and ply their trade as a way to improve the space. Sometimes vegetables and fruittrees are planted for food, while other times sunflowers and other flowers are used to beautify a space. Fruit bearing branches can be attached to ornamental pear and crabapple trees by Guerrilla Grafters. Even moss can be used as a form of "Green Graffiti." Guerrilla Gardening can be used as a political act to encourage local food production, bring attention to urban blight or promote changes in land use policies.