My error- The words are a translation of the related carol, Es ist ein Reis entsprungen, 15th c. carol, usually sung to Es ist ein Ros, but sung to the music composed and arranged by Herbert Howells on the recording I mention above.

At "Hymns and Carols of Christmas," A spotless Rose is stated to have the words of Es ist ein Ros, but for the same lyrics under the title version 5 A spotless Rose is blowing, the words are said to be from Es ist ein Reis ..., and the carol is listed under Low, how a Rose E'er blooming (or alternate title The Rose of Sharon).