This poem is particularly appropriate to look at through the lens of Pre-Raphaelitism.  The Pre-Raphaelite artists loved depicting flowers, especially roses, the stereotypical flowers of love.  Additionally, the Rose is so clearly a type, or symbol, of something else that looking at the Pre-Raphaelite take on symbolism helps one to understand the poem more fully.

The above picture is Dante Gabriel Rossetti's Ghirlandata.  Rossetti was a founding member of the Pre-Raphaelite Brotherhood.

Notice the hyper-attention to detail in the picture, and the realistic portrayal of these details.  In the bottom right-hand corner, for instance, we can see every leaf.  We see each string on the harp. In "To the Rose upon the Rood of Time," Yeats also gives an abundance of descriptive details (though not as many as in some of his poems).  In line four, for instance, we know that the druid is "grey, wood-nurtured, quiet-eyed," because the poet wants us to build a detailed picture of this druid in our minds, not because the information is vital to the poem.  Likewise, although lines sixteen and seventeen provide important details rather than random details, "[t]he weak worm hiding down in its small cave," and "the field-mouse running by me in the grass" build two small-scale images; Yeats pays attention to the small details. 

Rossetti's Ghirlandata is extraordinarily bright and lush.  Notice the layers of color in the pink flowers, making them seem still moist.  Notice the vibrance of the woman's hair and skin. 

Yeats gives us the same bright lushness in "To the Rose upon the Rood of Time."  In line seven, the stars are "dancing silver-sandalled on the sea."  Picture this--silver stars stand out brightly against the night sky, as their reflection on ocean waves stands out brightly against the general darkness of the sea.  To take this image further, the "s" sounds in "silver-sandalled," "dancing," and "sea" give the line a pleasing, unified sound--this is an extremely pretty sounding vision.  The technique of repeating consonant sounds is called, predictably, consonance (when these sounds come only at the beginnings of words, we call it alliteration).  Consonance and assonance (the repetition of vowel sounds) provided the pre-Raphaelite poets a way to "paint" with the sounds of their words (or create music, an analogy that might work better); these poetic tricks are their equivalent of bright colors and lush portrayals.  "[P]oor foolish," in line eleven, is an example of assonance. 

Dante Gabriel Rossetti painted the above scene, Roman de la Rose, as well.  The title, along with all of the roses used as decoration in the picture, make it particularly appropriate to pair with "To the Rose upon the Rood of Time." 

Both the painting and the poem make use of eroticized medieval ideas.  With the antiquated word "rood," Yeats refers to the Middle or Dark Ages.  Judging by the clothing Rossetti's lovers wear, they also belong in the Middle Ages.  We normally associate roses with love (an idea that Yeats brings up in line ten of the poem) and beauty (see "Eternal beauty," in line twelve).   Love (or at least a romantic kiss) and beauty, Rossetti's "Roman de La Rose" also has.  The line "a little space for the rose-breath to fill!" (14), set as an exclamation, certainly sounds heady and erotic to me, as far as poetic inspiration goes. 

Poem and painting share another connection.  Pre-Raphaelites didn't simply want to portray things realistically.  In fact, they took moral exception to doctrines of materialism, which stated that only things we can see and touch matter.  In Pre-Raphaelite paintings and poetry, what you get is more than what you see, because the poem or scene always contains symbols (or "types") pointing to something greater than themselves.  Rossetti's Roman de la Rose (Romance of the Rose) actually illustrates a medieval French romance, a story in which the rose is a symbol for the human soul.  As discussed within the poem, the Rose upon the rood of time is also a symbol--possibly for the "Eternal beauty" that exists within time, but is not touched by it.  Through the use of these types, Roman de la Rose and "To the Rose upon the Rood of Time" hold artistic and moral messages beyond the literal level.