The speaker looks back on the last days of a childhood summer, when going barefoot was normal and the ground felt safe. Running across a freshly mowed field, the children discover it isn't. The stubble of cut goldenrod and the thistle barbs hurt their feet, and the mower has left behind its own small damage: nests of baby voles, "pink as erasers." The phrase "the hard way" works literally, since the field is hard on bare feet, and also as a lesson about learning through pain.
The tone is wry and observant, with a quiet sadness underneath. The pain lingers "into September," just as school begins, and the yellow buses and waxed hallways replace the open field. The similes of pencils and erasers lead the reader there too, as if school were already creeping into the landscape. The mood is nostalgic but unsentimental: freedom ends, and small injuries mark the change.