I apologize for this delayed reply to your gardening question and hope it is still helpful to you.
I am not aware of any plants that will keep copperheads or other snakes out of your garden. Those snakes like the Liriope because it is a great place to hide and search for food. A good cat or rat terrier dog seems to be the best way to "manage" copperheads and other snakes!
Thanks for the question. Best wishes for a wonderful gardening season. Please stop in again soon!
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