The poem uses a few different kinds of repetition to create rhythm and emphasis.
In the first two lines, for example, the poet uses anadiplosis:
My son aged three fell in the nettle bed.
'Bed' seemed a curious name for those green spears,
This repetition highlights a word the speaker finds rather inappropriate to the thing it's describing: there's nothing "rest[ful]" about these spiky, malicious plants.
The speaker also repeats the phrase "behind the shed" in lines 3 and 15, each time followed by a colon and an accompanying, matter-of-fact statement ("It was no place for rest," "My son would often feel sharp wounds again"). Here, the repetition suggests the significance of the nettles' location: because they grow "behind" a large object, they lie outside the speaker's view. By extension, the speaker can't protect their son from things they can't see or predict.
There's also some brief anaphora in lines 9-10:
And then I took my billhook, honed the blade
And went outside and slashed in fury with it
Anaphora creates rhythm and momentum, which heightens the emotional intensity of these lines. As the speaker takes a series of rapid actions linked by "And," the reader can feel the speaker's anger at these harmful plants.
The speaker also repeats the word "nettle" in lines 1 and 11, as well as in the title itself. The more the speaker emphasizes these nettles, the more they come to symbolize all the lurking dangers in the wider world—all the things that will cause the boy to "feel sharp wounds again."