The speaker says the garden gives them nothing, and then adds, almost in surprise, that they are glad. It's an unexpected reaction. We usually expect a garden to offer something: beauty, comfort, a lesson, a harvest. Here its refusal to give anything feels like a relief.
The second stanza hints at why. The speaker imagines a time "after love or great disorder," when emotions have been spent and life has been messy or overwhelming. The phrase "in an unknown nest" suggests a place where no one knows them and nothing is expected, somewhere small, hidden and safe. There, they "could be quiet."
The title, "Horticultured," is a playful twist on "cultivated," and it hints that the speaker is being shaped by the garden's stillness. The mood is calm and slightly wry. The poem suggests that sometimes the best comfort is a place that makes no demands, where you can simply stop and be quiet.