For cold water plants there are lots of options. In my newt tanks I use java fern, Anubias, crypts, vals, wisteria, and java moss (also duckweed, but that's more of a pest). These have all gotten down into the 50s without any issues.
Just one more tip: when you attach the background to your tank make sure each and every possible crack is filled. Newts aren't like fish and they will find any tiny space to squeeze themselves into.