Moss boosted Earth's oxygen 400 million years ago: study

Why not add moss plants to the rooftops of the big cities? Seems like a simple and easy thing to do. I mean why not what do we have to lose?

http://www.msn.com/en-us/news/techandscience/moss-boosted-earths-oxygen-400-million-years-ago-study/ar-BBvFegc?ocid=ansmsnnews11