The poem opens with an image of the speaker's penpal working in his garden. Even though the penpal is the one writing to the speaker about farm life, this scene is presented from the point of view of the speaker; this is how the speaker images the penpal spends his time. Right from the get go, then, the speaker's fascination with and romanticization of farm work starts to become clear.
It is still wintertime ("February"), but when looking through the speaker's eyes, the outdoor work seems peaceful rather than cold or harsh. The penpal digs in his garden, "planting potatoes," and sees a kind of bird (called a "lapwing") return, perhaps from a long migration. The alliteration and consonance of these descriptions imbue them with a sense of musicality, elevating the penpal's humble labor. Note the hard /g/ sounds of "digging," the popping /p/ and /t/ of "planting potatoes." His knuckles, despite being subject to the cold air and soil, are "singing" as they "reddened in the warmth" of the indoors.
On a literal level, this refers to the way that people's hands turn red when coming in from the cold, due to increased blood flow. "Singing" evokes the tingling warmth that accompanies this reddening. And by personifying the knuckles as "singing," the speaker also imbues this reddening with a subtle sense of beauty and joy. This seeming joy is likely due, in part, to the relief of being inside, out of the cold. The knuckles might also be "singing" in joyful anticipation to "write" about the natural world they just experienced. Either way, the speaker instills the entire scene with a sense of comforting, welcoming warmth, even though the farm work itself was cold and probably unpleasant to perform. At this early stage, the speaker seems to set up the natural world, and farm work, more specifically, as an enviable and important aspect of their penpal's life.