What do you do to implement urban naturalization if you don't have a yard?
Plant on your roof!
San Francisco is leading in naturalization of "much lost green space" by implementing rooftop gardens and making use of tons of the architectural space that the city provides. Portland has been seen following suit on such buildings such as the Broadway dorm and the new student housing building University Pointe. Both are participating in LEED sustainability and are looking to be role models for all buildings to come in the surrounding portland area.
There are many ways that you can make your home or building sustainable in an urban setting but here is a link to a rooftop approach that can be implemented with the proper planning and construction.