The poem is chock-full of images of the natural world. We've got flowers, rain, references to the seasons and all that good stuff. Unlike some works that use this theme to show man duking it out with natural forces, "somewhere i have never travelled,gladly beyond" shows man and nature in a kind of harmony. The speaker actually describes himself as a flower, opening and closing with the seasons. So you could say that this isn't a poem about man and nature; it's a poem about man as nature.
Easy bloom, easy shut. The poem uses the image of a rose opening and closing with the seasons to show how the speaker is totally cool with the ups and downs that his lover makes him feel.
The poem almost implies that human beings are more powerful than nature, because it's constantly saying stuff about the lover's power existing outside of this world.