Inspired by the Guerrilla Gardening movement, it is a way to improve a neighbourhood by entering public and private gardens and performing any kind of maintenance that is required, such as weeding, pruning, and thinning. If one already has the tools, it is a zero-cost activity that can be done at any time.
Granddaughter: You were a long time on your walk, Grandma.
Grandmother: Yes, dear, that's because I was Guerrilla Weeding the neighbour's garden. They really need to look after their gypsophila better.