and time to toss some seeds into empty lots - instructions on the link above
(and some gorgeous pics from the site http://www.guerrillagardening.org
Here is how Wikipedia defines it:
Guerrilla gardening is gardening on another person's land without permission. It encompasses a very diverse range of people and motivations, from the enthusiastic gardener who spills over their legal boundaries to the highly political gardener who seeks to provoke change through direct action. It has implications for land rights, land reform. The land that is guerrilla gardened is usually abandoned or neglected by its legal owner and the guerrilla gardeners take it over ("squat") to grow plants. Guerrilla gardeners believe in re-considering land ownership in order to reclaim land from perceived neglect or misuse and assign a new purpose to it.
Some guerrilla gardeners carry out their actions at night, in relative secrecy, to sow and tend a new vegetable patch or flower garden. Some garden at more visible hours to be seen by their community. It has grown into a form of proactive activism or pro-activism.