As evergreen notes represent a thought, or concept, the author's understanding or perspective may change over time. Although they should have an aspect of atomicity, the refinement process may tease out aspects which could themselves be extracted into their own evergreen note.
The garden metaphor describes the necessity of curation: Evergreen notes are never finished, but remain timely and relevant through the act of successive refinement (a form of Deliberate practice).
Continuing with the nature theme, Evergreen notes are never finished and require curation (or gardening) to stay relevant.