@kiwipaul said:
Because he is maintaining it he is saying the EC money and so you have to be careful otherwise he might stop doing it and leave it to the EC to maintain.
That would be my primary consideration. I would be tempted to simply send a letter that politely established that the area is common property, congratulated him on the work that he’s done and asked him to let the EC know if he plans any further landscaping and what form that might take.
The main reason for doing this would be to establish ownership without getting into an unnecessary fight. This owner is enhancing the look of the entire building which boosts the values of everyone’s properties. But it’s worth establishing ownership to avoid any future annexing of the common property by him or subsequent owners.