I am looking for something that I can use that is all natural as an invisible fence instead of going to the expense of purchasing a fence. I live in an apartment but would like to block off part of the yard that my dog and his friends play in, but something that no one knows about, so there would be no cause or mess to the other tenants or building supervisor. I am basically looking for something that will keep them in a safe place to play and I don't have to keep chasing after them when they head to the road. I am sure that there has to be an herb or something that will help do this and I can just sprinkle it as to the limit of where they can go. Any help would be greatly appreciated.