I don't know about the grass. I just know it did not sound like fun having to clean the fake grass and the tray under it.
Gracie went through a period when she would pee next to the pee pad after she had already used it a couple of times. I guess she doesn't like the idea of getting near the pad once she has used it twice even if they where she went is on the other end.
Anyway, I started giving her treats every time she did go on the pad and she started being much better at doing it right. I also started changing the pad more often. She may be a dog but she is sure particular. I know if your gone during the day that is not always an option. They do sell liquid that you can put on the pee pad that is supposed to have a stronger attractant in it.
I have a neighbor who uses a littler box for her little dog and it seems to work fine. I can't do it because we have cats. Hope Peppermint decides to cooperate soon.
I think they use a liquid attractant for the fake grass, too.