I think you need to get permission. Not everyone likes the same things, such as female gingko trees planted near the bus stop. Or the "natural" gardens some folks put in to replace their lawns. Some of us like having a fire break around our houses, and don't appreciate the rats, mice, and snakes who flock to those urban "wildlife" plantings. Perhaps spaces are no longer maintained because of traffic hazards or soil contamination. I'm sure there are guidelines in different areas for street plantings so they don't become an issue. Find out if there are gardening groups in your area who volunteer to maintain public spaces. If not, perhaps you could get one started.
