First of all I suspect (subject to correction from even more arcane sources) that the refrain was originally "Juniper, gentian, rosemary." (Though I also remember from somewhere a plant-related name "geniver" that I can't trace.)
The meanings are pretty pungent. In at least one version of the Victorian language of flowers:
But the Herb Magickal connotations are even more fascinating:
juniper = protection, anti-theft, love, exorcism. gentian = love, power. (Four drops of a liquid infusion of gentian helps to banish doubt.) rosemary = protection, love, lust, mental powers, exorcism, purification, healing, sleep, youth.
I'd say the refrain may originally have been used for a song to make a love spell, then later was picked up for the "Jennifer Gentle" / "Wife Wrapt in Sheepskin" song.
In the spell-casting connotation the refrain would mean something like: love, protection of love, banishment of all spirits counter to love, power of love, love without any doubt, sexual love, pure love, love's healing, and possibly youth everlasting.
---i,e, a pretty good formula for casting a spell over your inamorato(-a).
Similar flower refrains in other songs might bear looking at along these lines. I also think, though, that flower names trip flowingly off the tongue and make great sounding nonsense refrains too.