The rooftop gardens sound lovely, but you just can’t throw a few tons of dirt on a rooftop without permits and an engineering assessment. Where technology would benefit this project would be automated watering based on temperature and rainfall. A couple of folks might like to volunteer work in gardens like this, but ultimately some professionals would have to be hired to take care of them, possibly many rooftops at a time. You can’t put this kind of money into something and then rely on volunteers’ work.