"Nettles" begins with a straightforward description of a fairly ordinary incident. The speaker recalls how their three-year-old son "fell in the nettle bed." Nettles are stinging weeds, and a "bed" of them is a patch where they happen to grow.
After this matter-of-fact statement, the speaker muses on the word "bed," which they consider "a curious name for those green spears." After all, it's not particularly "rest[ful]" for anyone who wanders into it! Comparing the nettles to "spears" makes sense since they're tall and pointy. But the speaker expands on this metaphor by calling the nettles a "regiment" (military unit) "of spite." It's as if the speaker imagines these nettles as an enemy army. The comparison makes this incident feel a lot more charged; clearly, there's more going on here than a kid getting scratched up by some plants.
The fact that this incident took place "behind the shed" hints at its broader implications. The boy got hurt while wandering out of the speaker's sight; he may be only three, but he's already old enough to land in a thorny situation the speaker couldn't have predicted. Undoubtedly, then, the speaker is thinking ahead to all the ways the world might endanger their child. The speaker describes the nettle patch as "no place for rest," but they might as well be describing the whole world!
The poem is made up of a single stanza (or four quatrains, if you divide it up based on its ABAB CDCD EFEF GHGH rhyme scheme). It's written in iambic pentameter, meaning that its lines generally contain five iambs (feet made up of an unstressed syllable followed by a stressed syllable: da-DUM). This commonplace meter gives the poem its steady, familiar rhythm. Together, the meter and rhyme scheme give the poem a form similar to that of an English sonnet—only with three extra lines. This resemblance may be intentional, as "Nettles," like traditional sonnets, deals with love and pain. (For more on the poem's form, see the Form, Meter, and Rhyme Scheme sections of this guide.)
Besides rhyme and meter, these lines use some other notable sonic devices, including repetition, alliteration, and assonance. For example, the word "bed" at the end of line 1 repeats at the start of line 2; this anadiplosis forces the reader to slow down and consider the word's implications. Heavy /eh/ assonance in line 1 ("fell in the nettle bed") draws attention to the incident that has prompted the poem. Meanwhile, the harsh /s/, /sp/, and /r/ alliteration of lines 1-4 ("son," "seemed," "spears," "spite," "regiment," "rest")) conveys the speaker's distaste for the plants that harmed their child.