I'm completely in love with this whole concept! People are out there making Green Bombs- which are tightly packed soil balls, filled with self seeding little seeds- ready to be catapulted into dissused spaces in the city. It's kind of like plant grafitti! Cool? Hell Yes!
Also- there is such a thing as Guerrilla Gardening! Bless them. Check out this lot! 'Jettisoned onto barren, abandoned, or otherwise inhospitable land, including construction sites and abandoned lots, seed bombs are a covert-and, OK, just a tad passive aggressive-way of protesting and combating urban sprawl.'
This from Wikipedia: 'The term "seed grenade" was first used by Liz Christy in 1973 when she started the "Green Guerrillas". The first seed grenades were made from condoms filled with local wildflower seeds, water and fertilizer. They were tossed over fences onto empty lots in New York City in order to make the neighborhoods look better. It was the start of the guerrilla gardening movement.'
Tree Hugger has got behind them as well...
Get seed bombing!