Well Russ, I must agree with Dunk.
Not only walnut but, there are alot of woods that are not good for gardens or ground cover.
This subject came up on another wood working fourm and the general thought was, "Trash it or burn it but, it wont help to retain the moisture to help the garden and it can harm alot of plants."
You are right about "Treated wood, "Don't even burn it!".
There have been cases where walnut and others have been used for bedding for horses and other animals and they have died from it.
My wife buys cider chips to put around the trees and in her flower garden and it does help to keep the weeds down but, I dont see any reason to toss it on a garden or flower plot.
Rog