This poem's sensuous imagery makes the speaker's vision feel as intense, compelling, and strange as a real dream.
Take, for instance, his vision of the ladies endlessly circling single roses on his "red-rose tree." As they whirl round those roses "like a dance of snow," they seem small and light as fairies. Just a few lines later, though, the ladies of the past have suddenly become titans, "great shapes" so big that the speaker has to beg them to "stoop" to listen to him. This shifty scale captures the matter-of-fact Alice-in-Wonderland flexibility of dreams, and symbolically gets at the speaker's frustrations, to boot: either these ladies are too small and ephemeral or too big and grand, but either way, they're out of his grasp.
There's a similar dream-logic in the moments when the speaker addresses the ladies' roses, one by one. These visions always take up a single neat tercet, and their intense imagery seems to fill up the speaker's whole field of vision for a minute: he becomes utterly absorbed in the petals hanging "loose and bleached" from the rose of the past, the "ruby-rimmed" heart of the rose of the present, and the sunrise colors of the rose of the future, like the "first streak of a new morn." These focused moments open new acts in a dream-drama, again drawing attention to the obvious symbolism of these roses' different phases of life.
The poem's most heated imagery comes in the speaker's address to the ladies of the present, whom he begs for passionate embraces in vivid physical terms. He's "burning" with desire, he tells them, and desperately needs to "plunge" into their arms—or something of theirs, anyway. His images of being held and contained—"bur[ied]" like a bee in a flower, "girdle[d]" (or encircled) as if by a belt—make this passage frankly sexual. As the ladies' changing scale suggests their inaccessibility, this passage of burning and enveloping shows just how tantalized and frustrated the speaker feels. This is a dream of desire burning all the hotter because it can't (at least at present) be fulfilled.