Vervain, or “the herb of the cross.”
Hallowed be thou, vervain, as thou growest in the ground. For in the Mount of Calvary thou was found. Thou healcdst Christ our Saviour, and staunchedst His bleeding wound. In the name of Father, Son and Holy Ghost, I take thee from the ground.
Source: Dictionary of Phrase and Fable, E. Cobham Brewer, 1894