I have asked this question in Jan/Feb this year - and the response was that the system that enabled this function was disabled because it produced too much lag on the server (it has to be redone). For now I recommend you to find a place with grass that *you* do not care about and harvest the grass there (this is what I did).
Eg I used a spot near a road between rocks and so on - it does not really bother me that the grass is gone there
Good luck with the grass picking and have fun!