"Have you ever wondered why so much public space seems to go to waste? Even apparently ‘natural’ green areas are carefully cultivated, requiring time, energy and water. However, these only produce a return if people are sitting, interacting, or playing on them. A great deal of urban green space is never used for these purposes. However, such spaces can be (and many are) subverted through guerilla gardening: the act of seeding useful plants in public spaces."
Complete Article: The Art of Guerilla Gardening