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Generate shadows without trees: vegetable awnings, a new way to generate shadows in the city.
(www.singulargreen.com)
I found these kind of cool as an idea, as they can be added rather easily to existing cities and provide some greenery without taking up space, while providing shade:
I also have concerns about nutrient cycling, every rain and over watering is going to wash nutrients into the street below. This is an interesting idea, but could use more refinement.