Snuffy, I think you're right about the "doo," good catch. Malcolm, thanks for a much needed corrective view.
Yes, it bears repeating: we know virtually nothing for sure about magical / ritual / spell / charm survivals in song from the pagan times in which plants were so important for these purposes.
The Christianizers pretty much did a scorched-earth number on true pagan customs. It's all had to be made up fresh in the past 70 years. So if you try to google any of this, the whole mountain of recently-composed New Age stuff descends on you…fine in its way, but no good in a discussion of authentic tradition from older times.
Though I urge you, Leeneia, to scour traditional song for more of these herb and flower incantation choruses, I doubt you will find much. We're lucky to have the few we have. Most of the other such references are just riffing on flowers: "Lay the bent to the bonny broom" (a reference to withing the broom to the staff, I think, and thus sexy, yes) from "Riddles Wisely Expounded" (Child #1) and a few other ballads … "Green bush, holly and ivy" as well as "parsley, sage, rosemary and thyme" from variations of "The Elfin Knight" (Child #2) … the primrose, red rose and white lily of "The Cruel Brother" (Child #11) … "Hey the rose and the linsey/lintseed O" from "Cruel Mother" (Child #20)…Etcetera.
Evelyn K. Wells in her Ballad Tree cites a medieval precursor to "The Holly and the Ivy," without Christian references, in which the holly and the ivy battle for supremacy; the holly wins on various grounds, including having better birds to roost in its boughs...the ivy can only claim the owl. Such plant wars were big at one time IF you believe Robert Graves' thesis in his White Goddess. Many don't.
Interestingly, the cherry tree bowing to pregnant Mary in "Cherry Tree Carol" does sound like a little magic-in-song as PR for the virgin birth: the unofficial folk symbol of virginity bows to the mother-to-be.
Some versions of "Jennifer gentle" (found as a chorus in several ballads, notably versions of "Riddles Wisely Expounded," "Elfin Knight" and "Wife Wrapt in Wether's Skin" vary to "Gillyflower, gentle fair Rosemary," but are probably too recent to have any magical connotation.
My wife found the following in an article on Celtic folk customs:
"Herbs gathered on May Eve have a mystical and strong virtue for curing disease; and powerful potions are made then by the skilful herb women and fairy doctors, which no sickness can resist, chiefly of the yarrow, called in Irish "the herb of seven needs" or cures, from its many and great virtues. Divination is also practised to a great extent by means of the yarrow. The girls dance round it singing-- "Yarrow, yarrow, yarrow, I bid thee good morrow, And tell me before to-morrow Who my true love shall be."
That's a divination charm---not unlike the maslenitsa (straw figure) the Russian girls long ago used to throw into the water to welcome spring and bring new love. Not unlike picking the petals and saying "He loves me, he loves me not."
Slim pickin's, though; the above is all I've come up with, and it isn't much. It's very late for survivals of this kind. Makes you wish you could get back to the English countryside a few hundred years ago and hear what they were singing then, particularly witches and herb curers.