The original German of this hymn, written by an unknown Catholic author, offers dual reverence: to Mary (the rose) and to Jesus (the floweret brought forth by the rose plant). Here is a literal translation of the first two stanzas:

A rose has sprung up,
from a tender root.
As the old ones sang to us,
Its lineage was from Jesse.
And it has brought forth a floweret
In the middle of the cold winter
Well at half the night.

The rosebud that I mean,
Of which Isaiah told
Is Mary, the pure,
Who brought us the floweret.
At God’s immortal word,
She has borne a child
Remaining a pure maid.

    We can see that the first stanza differentiates the rose and the floweret; the second stanza makes explicit that the rose is Mary, who brought forth a new baby flower, while remaining ever virgin.  While this is a standard concept in Catholicism, it sounds odd to most Protestants. 

    Theodore Baker (1851-1934) was the original editor of Baker’s Biographical Dictionary of Musicians, a standard reference for more than a century, and now in its 9th edition. He translated the text into the Protestant version commonly sung today, with its focus on Jesus.  Mary has borne the Savior, but beyond that she is just like the rest of us:  together we behold the flower springing from Jesse’s lineage, Jesus, the light that will dispel the darkness. 

    Many presentations of the carol, both in print and in performance, omit the third stanza. This seems to me to be a real loss. That stanza is the turn from description to devotion, as the carol becomes a prayer. It includes the central understanding of the incarnation “truly human, truly divine,” and it recognizes that Jesus both saves us from sin and death at the end of our lives, and sustains us during our day to day struggles.

1. Lo, how a rose e’er blooming,
From tender stem hath sprung.
Of Jesse’s lineage coming,
As men of old have sung;
It came, a flow’ret bright,
Amid the cold of winter,
When half spent was the night.

2. Isaiah ’twas foretold it,
The Rose I have in mind,
With Mary we behold it,
The virgin mother kind;
To show God’s love aright,
She bore to men a Savior,
When half spent was the night.

3. O Flower, whose fragrance tender
With sweetness fills the air,
Dispel with glorious splendour
The darkness everywhere;
True man, yet very God,
From Sin and death now save us,
And share our every load.

Hiroki Royale includes the third stanza,
alas with some of the theology edited out.