‘Nettles’ pertain to the parent/child relationship. The literal meaning of the poem is, about a parent, presumably Vernon himself, and his child. The child falls into a bed of nettles behind a shed; the kid wounds himself. The poem then continues on with the parents of the child soothing him. Afterwards the parent light up the nettles, till nothing is left. The poem ends with a fact; that his child will fall into these nettles again, because
“in two weeks the busy sun and rain
Had called up tall recruits behind the shed”
I believe that between the lines, Vernon, in reality, is trying to express the topic of war through this poem. Using the nettles metaphorically.