The fern Botrychium Lunaria, or Moonwort, was held to have a strong effect on metal. Its popular name was ‘Unshoe the horse’ as it was believed to draw the nails out of the shoes of any horse that trod on it, and a story is told by Culpeper and others to back it up: ‘I have heard commanders say, that on the White Down in Devonshire, near Tiverton, there were found thirty horse shoes, pulled off from the feet of the Earl of Essex's horses, being there drawn up in a body, many of them being but newly shod, and no reason known, which caused much admiration… ’(Culpeper's Complete Herbal and English Physician (1653), 163; 1826 edn., 98–9). Moonwort's other, more useful, attribute is that if put into a keyhole it will open the lock. Opie and Tatem given references to these beliefs back to 1591.
Opie and Tatem, 1989: 266;N&Q 11s:7 (1913), 108, 177.