From Tolkien Gateway
This article describes a concept which is mentioned in J.R.R. Tolkien's works, but was never given a definite name.

The gallows-weed was a tree-hanging weed in a poem written by an unknown Hobbit-poet.

It was only ever known by this name, and its physical description is never talked about nor described. Few people who ever entered these marshes, and thus even saw these gallows-weeds, ever returned, thus there was no description of them.[1]