Like a perfect rose, this poem is well-put together. It's got a very regular pattern that makes it easy to follow. For example, just consider the rhyme scheme at work here: ABAB. The end of every other line features a perfect end rhyme (in stanza 1, for example, it's "met" with "wet," "chose" with "rose"). It seems like this structure is creating a nice symmetry for us the reader, which of course we'd expect in a lovey-dovey poem about a rose. Still, all is not well in Lovesville, which is where the meter comes in to play.
"One Perfect Rose" contains three quatrains (four-line stanzas) and uses two different meters in each stanza. The first three lines of each stanza are written in a meter called iambic pentameter, while the final line of each stanza is written in iambic dimeter. No, not diameter, as in a circle. We mean dimeter, as in two iambs. Let's break this down to show you what we mean.
Up first: iambic pentameter. Basically, a line of iambic pentameter contains 5 ("pent-" is Greek for five, as in pentathlon) iambs. An iamb (pronounced like I AM) is a two-syllable pair, the first being unstressed which is then followed by a stressed syllable. If you say "allow" out loud, you'll hear an iamb: da DUM. For example, check out line 1:
A single flow'r he sent me, since we met.
Since we have five iambs here, you should hear this pattern: da DUM da DUM da DUM da DUM da DUM. There you have it dudes and dudettes, iambic pentameter.
Okay, now that that's out of the way what about that dimeter deal? Well "di-" means two, so a line of iambic dimeter contains two iambs. Can you find which lines have this meter? Here's a hint: all three in this poem are identical. 10000000 percent the same. Think refrain, or lines 4, 8, and 12:
One perfect rose.
Now, for an answer to the burning question you're asking yourself, why two meters? Well, this is a poem about dissatisfaction, irritation, and all that jazz. The first three lines of each stanza are fine and dandy but then we get a short line, which sort of upsets the pattern, right? The switch to a line of dimeter at the end of each stanza messes with the flow of the poem. Darth Vader would say it's a great disturbance in the force, but we'll say that it's an annoyance that is just like the speaker's annoyance. By using this metrical break, she makes us feel her pain!