Here's one of the interesting things about Imagist poetry: since it concentrates exclusively on the subject (or object) it's trying to discuss, chances are that it'll spend a good deal of time describing that object's appearance. In this case, the rose gets depicted in its living conditions: and believe us, they're pretty down and dirty. Don't be fooled by appearances, though: this poem is about more than just looks. As it turns out, looks can be a way to explore other aspects of life, as well.
Although the rose in this poem is tough, it's not beautiful.
The rose in this poem acquires a sort of beauty through its strength and tenacity.