Knew I had read an explanation for this refrain somewhere.
This is from the notes to 'Riddles Wisely Expounded' from MacColl and Seeger's 'The Long Harvest (Vol. 2)
Jim Carroll

"The refrains of the various versions in Child 1 are, almost without
exception, of mystical or magical origin, mentioning either wondrous
plants or combinations of numbers. In our version B, for instance, the
bent (or rush) and the broom form what is believed to be the survival
of an incantation against the fiend, for bent is widely used both in
England and on the Continent in charms against the evil eye; twelfth-
night broom is held to be most potent against witches and spirits-
Together they make an invincible protection. (See note to Child 2 on
REFRAINS.)"