Guerilla gardening in its original form is pretty basic - you see a place that you do not own that could use some plant life and you introduce said plant life to it without the proper permits. This is mostly harmless, even though the owners will probably want to get you in trouble for it - but if you do it to government entities, do you think they will track down people planting flowers around town? I don't.
Now, ADVANCED guerilla gardening, henceforth called SIEGE PLANTING, involves planting harmful and/or invasive plants and fungi in strategic locations.
I heard about a guy who planted death cap mushrooms throughout "inner city government housing projects", resulting in severe organ damage and death for nogs dumb enough to eat them. I heard about another guy who spread kudzu in "the bad parts of town", covering entire houses in invasive vines. I heard that neither the owners nor the local government were able to neutralise these threats in time, which caused a drain on their resources in the long run. I heard the guerilla gardeners never got caught.