All that plants need to survive is light (not even sunlight), CO2, water and some nutrients. They don't even necessarily need soil. Plants can even grow in space. With that in mind it is actually surprising that we have not started vertical farming on the big scale yet. To me it is clear that in future our building fronts and roofs should either generate electricity / energy or have or be made of plants.

Facade greenery are so far mainly seen as a genius way to improve or preserve biodiversity. Giving bees and other insects some living space back. Bringing some nature into the city and to its inhabitants. And since climate change is making summer hotter especially in cities, it is also a great way to cool the buildings (and the city) down in a natural and ecological way.

But in addition of course plants in cities can also provide food. The only reason why we haven't used that potential yet is probably because so far we didn't need it . Our modern society in the western world acts as if we had way too much food already (we are wasting tons of food as you might already know). While others in other parts of the world have been struggling.

But because people in rich countries are doing fine we are not using the full potential of our possibilities. Even if we could afford it financially and technologically. When we start to reduce food waste and use the potential of vertical and urban farming there will be more food left for the poor  and world hunger will not be 'a thing' anymore. Even with twice as many people.