Oh God, no, not another "What does lay the bent to the bonny broom" thread. We've had a heap of them, Kat, you ought to know that. I saw the question and hoped it might be overlooked until the search engine was working again and the bulk of the "repeat" requests from people who've tried looking for something and got no results had dried up...

Not your fault, MacGillickutty. Since we're here again, here are two links to earlier discussions.

LYR clarify -- Bent to the Bonnie Broom?

RE: 'twasisters' lay the bent to bonny broom

Read the discussion in the first link, in which romantic ideas about "magical herb lore" are gone over, then check the second, which contains a different (and more likely) explanation; together with a very long list of links to many other discussions and material here and elsewhere. They mostly relate to versions of The Cruel Sister, of course, which this refrain was never sung with in tradition; it was bolted on in the late 1960s, with the tune, from a 17th century English version of Riddles Wisely Expounded. That arrangement, by the band Pentangle, became very popular, and a lot of people sing it now believing it to be an authentic traditional form of the song.

One thing you need to bear in mind is that that version of Riddles was taken from the English musician and raconteur Thomas D'Urfey's book Pills to Purge Melancholy, which contained a whole raft of songs and verses traditional and modern (modern in the early 18th century, that is), a high proportion of which were comic or just plain dirty. Romantic folklorists of the 19th and early 20th centuries to the contrary, my money's on the more recent opinion.