It's a little obvious to say that "Binsey Poplars" is filled with natural imagery. After all, it is about trees—duh. And those trees are, again, pretty obvious symbols of Nature's unspoiled beauty. This kind of stuff hardly needs to be said (even though…we just said it).
What's more subtle, and therefore more interesting, is how Hopkins gets his readers to care about Nature and her trees. To do that, he's got a particular game of wordplay up his sleeve: personification. By investing the poplars with human qualities, Hopkins makes their loss seem all the more real, and all the more tragic.
Lines 1-2: The trees are described as having "airy cages," which have the power to contain and calm ("quenched") "the leaping sun." As such, they're shown to have some calming influence in the face of the hectic sun. They seem like a pretty chill (and influential) bunch.
Line 4: To say that the trees were in a "folded rank" suggests that they were lined up like military soldiers, close together. Their deaths are linked, this way, to the deaths of soldiers killed in war.
Lines 6-8: So far, personification has been used to give us a sense of these trees as powerful, calming, and noble in death. But did you know that they also liked to party? These lines personify the trees as picnickers on a riverbank, dipping their toes in the water.
Lines 12-13: The entire countryside is personified as a "slender" and "tender" woman here (recalling the figure of "Mother" Nature). The idea is that Nature is a fragile thing, in need of protection. Sure, it's chauvinistic personification, but it's for the good cause of protecting the natural environment.
Line 17: Once again, Nature is personified as a "her." Even trying to fix "her" is too much interference in the speaker's view.