Talking to a weed as if it were a lover sounds like a joke, and the strange thing is how quickly the poem stops sounding like one. "Goatweed goatweed how you brighten / my waste places" has the breathless fondness of a pet name, with the word repeated and no comma to slow it down. Goatweed is, as far as I know, a folk name for St John's wort, the yellow-flowered plant taken as a herbal remedy for low mood. The "yellow stars" and the complaint that it plays "badly with other medicines" point that way, though the poem never says so outright. Read like that, it is about a cure that works on the mind and charges the body for it.
The poem's central idea is that trade-off, and its refusal to settle it neatly. The speaker admits the costs: the plant makes him "dizzy and anxious," and "I can't / get it up any more." Yet the last lines claim that his "dry bones / have never felt more possessed of life." Loss and vitality arrive together, the way they do in a real love affair. "Like any lover" is the hinge of the comparison. The plant is not a tidy solution but a presence that unsettles him and also makes him bloom.
The imagery turns the body into a landscape. "Waste places" is the kind of phrase you would use for scrubby, neglected ground where weeds settle first, and the speaker applies it to himself. The flowers grow "in the deep space between / my shoulder blades," so a patch of back becomes a night sky with yellow stars in it. This is where "the sun's / too weak to rise," and the line break lets that failure echo a few lines later in "I can't / get it up." The sun that cannot rise and the body that cannot rise are the same problem, and the poem sets them in the same sentence of feeling without ever pointing out the pun.
There is no rhyme and no regular meter. The poem is built from six unrhymed couplets of similar, fairly short lines, and the sentences refuse to line up with them. Full stops fall mid-line ("too weak to rise. Like any lover," "dust and pillbugs. Call me a shaman"), and phrases spill over the breaks. The result sounds like someone thinking aloud and catching himself mid-confession. The break after "I can't" is the best example: the sentence hangs for a beat, then lands on its blunt, comic phrase.
Humor keeps the poem from getting solemn. "Play badly with other medicines, such as / dust and pillbugs" treats a pharmacist's warning about drug interactions as though it were a playground dispute, and it counts dust and pillbugs among the medicines. Then comes "Call me a shaman / fundamentalist," a phrase that yokes two things that should not fit. A shaman suggests spirits in nature, a fundamentalist suggests rigid doctrine, and together they tease the speaker for his convert's zeal about a weed. "Possessed" works the same way. It means "full of" life, but it also means taken over by a spirit, and the plant is clearly the spirit.
The title, "Herbaceous," is a botanical word for plants with soft green stems rather than woody ones, and the poem ends by making the speaker herbaceous too. His "dry bones" give way to "a green husk," an image that holds two things at once. A husk is a dead outer shell, but this one is green and still growing. The poem places this change "in the otherworld," which could mean the spirit realm, the afterlife, or simply the inside of the body, and it leaves the choice open. It ends on growth rather than on a cure.
The poem matters because it finds the sacred in an ordinary, slightly embarrassing modern situation: taking a supplement, living with the side effects, and noticing that the body is also ground where other living things take hold. It speaks frankly about impotence without euphemism or self-pity, and it makes the speaker's medicine and the garden part of one story.