From BobVilla.com…
Both urban and suburban areas are filled with unused lots and neglected plots of land. Some plant lovers looking to add greenery to these areas have opted to take matters into their own hands. Guerrilla gardening is the practice of planting flowers or edible plants in neglected private or public spaces. It’s a sort of horticultural graffiti that can serve as a form of protest, though it also simply serves the purpose of cultivating life in places that have been abandoned.
Historically, guerrilla gardeners have operated under the cloak of darkness, working at night to avoid detection, though today the practice is more commonly accepted.