A Victorian nationalist wrote the lyrics. The king of British folksingers wrote the tune. The father of modern witchcraft made it part of the Book of Shadows. And across the English-speaking world, pagans sing and dance to it every Midsummer's Day.
How good is that?
Poet Rudyard Kipling (1865-1936) first published the poem A Tree Song in his childrens' novel Puck of Pook's Hill in 1906. Folk-singer Peter Bellamy (1944-1991) wrote a musical setting for the poem (you can hear it here), retitled Oak and Ash and Thorn; it was released on the album of the same name in 1970.
Meanwhile, some time in the 1950s, Gerald Gardner (1886-1964) had written the last verse of the song into the liturgy for Beltane. How did a Midsummer's song (“Sing Oak and Ash and Thorn, me love/all of a Midsummer's morn”) end up at Beltane? Well, the cross-quarters were the original sabbats of Gardner's revived “witch-cult,” as in Murray, and the quarter-days (solstices and equinoxes) didn't come in until later. That explains the truncation of the lyrics in the BoS version as well.