“Lo, How a Rose” is one of my favorite songs in this liturgical period and I just ran across what I believe explains the theology behind the images of the hymn. It’s from Jerome’s Commentary on Isaiah, taken from the 2nd Nocturn appointed for Advent 2.

“AND there shall come forth a rod out of the stem of Jesse”; From the beginning of the Book of this Prophet till the xiiith chapter, where commenceth the vision, or burden of Babylon, the whole of the vision of Isaiah, the son of Amoz, is one continual prophecy of Christ. We must explain it part by part, for if we were to take it all at once, the memory of the reader would be confused. According to the Jewish commentators, the rod and the flower would both relate to the Lord Himself. They take the rod to mean the sceptre of His Royal dominion, and the flower the loveliness of His beauty.

We, however, understand that the rod out of the root of Jesse signifieth the holy Virgin Mary. She was a clean stem that had as yet put forth no shoot ; as we have read above : “Behold, the Virgin shall conceive and bear a son.” (Isa. vii. 14.) And the flower we believe to mean the Lord our Redeemer, Who hath elsewhere compared Himself to a flower ; “I am a flower of the plain, and a lily of the valleys.” (Cant. ii. i.)

THE Spirit of the Lord then shall rest upon this flower ; this flower which shall come forth from the stem and roots of Jesse by means of the Virgin Mary. And truly the Spirit of the Lord did rest upon our Redeemer. It is written that ” In Him dwelleth all the fulness of the Godhead bodily.” (Col. ii. 9.) The Spirit was not shed on Him by measure, as it is upon the Saints. To Him we may apply the words of the Hebrew Gospel used by the Nazarenes ; “The whole fountain of the Holy Ghost shall be poured forth upon Him : ” “The Lord is a spirit, and where the Spirit of the Lord is, there is liberty.” (2 Cor. iii. 17.)