In the version that I first heard (and the version I sing), the rose grew out of William's grave because his was the love that was true. The briar grew out of Barbara's grave because her stiff-necked pride had blighted the course of true love.

Although a rose is actually a kind of briar because, botanically speaking, a briar is characterized by having a thorny stem.

Also, far be it from me to quibble with Robert Graves, but in almost all versions I've heard, the witch idea just doesn't cut it. She and William are buried beside each other "in the old church yard," and as I understand it, you can't bury a witch in consecrated ground.

Don Firth