That's not difficult.
The difficult thing is to trim the branches regularly so that it won't get too heavy and drop on passerby below. Tree doctors are also required to monitor the health of each individual tree otherwise it might get blown away or break off in the event of a strong rain/wind.
You have to consider as trees grow bigger every year it will also get heavier. Such a large amount of trees on a building will definitely place great pressure on the structural integrity of the building. Not to mention water absorbed by the trees leaking into hairline cracks and make the crack bigger.
Did I see trees planted on small balconies?
![No :no: :no:](/styles/default/xenforo/smilies/cute/no.gif)
That will just amplify the challenges I identified above and increase the cost of maintenance.
What's the purpose of the project again? Environmental concerns? With such high maintenance? Might as well allocate the maintenance costs to adopt renewable energy.