The riddle ballads are, IMO, the most interesting carriers of folklore in the tradition, and among the oldest survivals though many of them have been rationalised, and 'blandified' because their original meaning has been largely lost – 'Scarborough Fair' is pretty typical of this happening, though, when well sung, it can still be a powerful song - try and find the recording of Scots Traveller, John McDonald singing 'The Elfin Knight' (forget S and G – pop-pap).
This is a summing up of the genre from collector, Lucy Broadwood, taken from Lowry C Wimberley's 'Folklore in the English and Scottish Ballads (1928) – essential reading for anybody interested in ballads.
Jim Carroll

"Says Miss Broadwood:
On studying this type of riddle-ballads one cannot fail to be struck by the extraordinary frequency with which "plant-burdens" occur in them. Both abroad and in the British Isles one meets still with so many instances of plants being used as charms against demons, that I venture to suggest that these "plant-burdens," otherwise so nonsensical, are the survival of an incantation used against the demon-suitor. That it should have disappeared from many versions of the riddle-story (when the dialogue only survives), is most natural, seeing that to mention an evil spirit's name is to summon him, in the opinion of the superstitious of all countries. Every one of the plants mentioned in the burdens above quoted is, as a matter of fact, known to folklorists and students of the mythology of plants, as "magical." That is to say, from the earliest times they have been used both as spells by magicians, and as counter-spells against the evil powers who employ them. ...
It is perhaps hardly necessary to remind our readers that, from early times, the herbs or symbols efficacious against the evil eye, and spirits are also invariably used on the graves of the dead, or during the laying of the dead to rest.
Miss Broadwood's observations on the magical properties of the plants represented in the burdens of The Elfin Knight and Riddles Wisely Expounded may be summarized as follows: parsley, used by the ancient Greeks at funerals, and on graves, and employed magically in Germany, the British Isles, and in Europe generally; sage, a magic plant in England, and proof against the evil eye in Spain, Portugal, etc.; rosemary, called "Alicrum" or "Elfin Plant" in Spain and Portugal, is worn there against the evil eye, burnt against witches in Devonshire, and everywhere associated with funerals and death; thyme, a chief ingredient in a recipe (ca. 1600) for an eye-salve for beholding without danger the most potent fairy or spirit, and associated with death and the grave in England; juniper, sacred to the Virgin in Italy and France, and especially potent against evil spirits; the gentle (thorn or bush), the name used all over Ireland for the large hawthorns which are regarded as holy and sacred to the "gentry"—"gentle people" or fairies who inhabit them; holly and ivy used magically from the earliest heathen times, holly being particularly abhorred by witches in England and other countries of Europe; broom, most potent against witches and spirits, and per contra, often used by witches in their spells; the bent or rush, protective against the evil eye, and, as Miss Broadwood points out, doubly powerful when combined with the broom, as in the refrain, "Lay the bent to the bonny broom." We may dis¬miss the subject of the incantation refrain by quoting a note from Scott, which goes no little way toward proving Miss Broadwood's point that our plant burdens are incantations directed against evil spirits;
The herb vervain, revered by the Druids, was also reckoned a powerful charm by the common people; and the author recollects a popular rhyme, supposed to be addressed to a young woman by the devil, who attempted to seduce her in the shape of a handsome young man:~

"Gin ye wish to be leman mine,
Lay off the St. John's wort and the vervine."