>What exactly is "guerilla gardening"?
It's when you garden in highly mobile, dispersed way.
For example graphing a fruit tree branch on a tree in a forest so that you have food to come back to that no one knows about.
It could also mean planting crops on land that doesn't belong to you.
The most practical suggestions I've see for guerrilla gardening are planting opium poppy seed plants using seed bombs, (wads of mud with seeds in them).
Apparently poppies are very robust and can grow in hostile environments. It'd be useful not only for quick fund raising, but if you convinced enough people to do it it in your city you could choke your local intelligence presence of drug money.