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.
I will ahve all the cracks sealed up.thanks for the reminder! They never stop exploring and thats why I have designed the intake and out flow the way I have. Glad to hear that all those palnts can acclimate well in cooler water. 50's !!!! I wasnt palnnign on keeping it that cold. Is that the temp range for breeding behavior mimicing their native lower 48 homes????