guerˈrilla ˌgardening BrE AmE noun [uncountable]
the activity of growing plants or vegetables on any piece of land in a city that you do not own, especially land that is in bad condition because it has not been used for a long time. People do this so that cities will have more green areas and be better places to live.
—guerrilla gardener noun [countable] :
Guerrilla gardeners came late in the night and turned the area outside the building into a vegetable patch.