I'd to think both Seeds of Love and Child #2 predate Frazer too, but whereas Shakespeare was quite deliberate in his Jacobean incantations, it's still a fancy rather than witch-lore per se. There's nothing to indicate that the makers of folk songs were being quite so self-consciously occult in the meaning of their imagery, much less that such a tradition exists as such beyond the usual medicinal associations of certain flora, a lot of which was still extant when I was kid & probably is today (dock for nettle stings, dandelions for a diurectic, comfrey for broken bones, hawthorn leaves as an appetite suppressant, etc etc). Is the riddling of Child #2 an indication of the occult I wonder? Or just two lovers being smart with each other - more Much Ado than Macbeth? As for Seeds of Love, the flora serves to euphemise a load of knob-gags.