The stanza in question is comparing the rose to a rock, suggesting that the flower has solidified in this garden as if frozen in a momentary state of being. 

The idea of the crystallized moment is expressed throughout the poem. Not only is the rose frozen in a sort of perfect state of stillness (and, implicitly, a fullness of being that is static and perhaps sterile), but the speaker of the poem is also frozen in this moment.

If I could stir

I could break a tree—

I could break you.

H.D. often used references to myth and to religion and so we may see this poem as being connected to the tale of the Garden of Eden. In some versions of the Garden of Eden story, God is depicted as coming at the breezy time of day to visit with Adam and Eve.

Read in this way, we might see the wind that is called to break the heat as a metaphor for God.

Cut the heat—

plough through it,

turning it on either side   

of your path.

If we see the entire poem as playing on an allusion to the Garden of Eden, we can see the treatment of the rose as a statement on the state before the Fall. In that state, as in Plato’s vision of the plane of the forms, things exist as inert but perfect versions of themselves.

In H.D.’s “Garden,” the state of inert perfection is not preferable to the dynamic and fruitful time after the Fall. The poem asks the wind to come so that fruit can grow. 

Thus the solidified rose is a symbol of a static or crystallized state of being.