Vertical Filters?

I walked through the grounds of one of the many campuses of Bristol University this morning I saw this wall of vegetation stretching several storeys into the sky. Couldn't these be used to filter CO2 from our atmosphere? They are space efficient, We have plenty of vertical surfaces in our cities, They provide habitat for birds and insects, they are nice to look at , they would only make a contribution and .....

There must be a reason why not??