Snow is the one word the poem never uses, yet it is the first thing most readers see. A whole hillside goes white, and "prematurely" tells us the season has jumped ahead of itself. Then the last line overturns that picture. "Fall webworms" are caterpillars that spin silky tents over the branches of trees in late summer and autumn, so the whiteness is not frost or snow but webbing. What looked like winter arriving early is a swarm of small creatures going about their business. The central idea is how easily appearances mislead us, and how the correction can be stranger and more interesting than the mistake.
The poem is built to produce that mistake. It is three short lines, and if you count syllables you get five, five, and three. That loosely echoes the haiku pattern, but the last line is cut short. The reader expects one more beat and gets a drop instead, like a step that isn't there. Nothing rhymes, and there is almost no punctuation apart from one colon at the end of the second line. That colon does a lot of work. It works like a pointing finger, or a magician pausing before the reveal. Everything before it is impression, and everything after it is explanation.
The key words are chosen with care. "Turns" is a verb of change, and it catches the hillside in the middle of becoming something else. It also carries other meanings: seasons turn, and so does hair when it goes white. That second sense makes "prematurely" more interesting. We usually use that word for hair or for age, so the hillside takes on a faint human quality, as if it had aged overnight. This is an inference from the wording, not something the poem states, but the pairing is hard to ignore. "Prematurely" is also the longest word in the poem and fills most of line two. It is a judgment word too, and it comes from a human point of view. The webworms are not early by any calendar of their own. Only the observer thinks something is ahead of schedule.
Sound ties the lines together. The "w" of "whole," "white," and "webworms" runs through the poem, so the eye and ear are carried toward the ending before we know what it is. There is also a near-echo between "turns" in the first line and "worms" in the last. The vowel and the closing "-rns/-rms" sounds match closely enough that the first line seems to contain its own answer. The change the hillside is going through is the work of the worms.
The final line also does something clever with "fall." Read as a season, it fits the autumn mood the poem has been building. Read as part of a creature's name, it is suddenly small and specific. Then "worms" closes the poem on a humble, slightly unpleasant word, after the grand scale of "whole hillside." The poem moves from landscape to insect in two words. That shift in scale is part of its surprise. It is startling that something so small can change how a whole slope looks, simply by being numerous.
What makes the poem worth reading is how little it says. It borrows the habits of haiku in English: a sharp image, a pivot, a seasonal clue, and no commentary. It does not tell us whether the white hillside is beautiful or a blight. The tents can be ghostly and lovely from a distance, and the poem lets the word "worms" sit uneasily next to that beauty without settling it. It leaves us holding two views at once: the clean, quiet image we first imagined, and the crawling, feeding reality underneath it.
The poem also makes a quiet point about looking. Our first reading is shaped by what we expect, and here we expect snow. The last line shows us that expectation at work and then corrects it. In the space of three lines the poem makes us both mistaken and better informed, which is a good deal to ask of fourteen syllables.