The poem sets us in a garden at a madhouse, and the first surprise is that "our minds see much." The word "asylum" can mean a place of confinement, but also a place of refuge, and the poem seems to hold both meanings at once. In the garden, the speaker and those around them aren't described as broken. They're described as perceptive.
The middle lines turn toward nature. The wood, the trees beyond the garden, "sends us word," as if the natural world is a messenger reaching people who are shut away from everything else. There's a quiet sense of connection here, even comfort.
Then the poem ends on a question: "who shall take the thin / man in." The tone shifts from calm to worried. The thin man seems vulnerable, perhaps unwell or unprotected, and no one has offered him shelter. The mood is gentle but uneasy, with compassion sitting right beside doubt about whether anyone will care.