I’ve emailed the RBG for more information (such as the name of the artist; the significance of the plant materials), and will update this post when I hear back.
Thought this might be a good idea to file away for future projects. Imagine this mimicked in concrete and mixed with commercial pavers in the central section of a patio, or as an accent in a garden wall. [I’ll add my best guess about how the details might have been picked out: possibly by dipping the materials in a dye before making the impression?]
Remind me, however, to one day confess my concrete paver story. It involved a grade school full of kids, a cement truck, and a Mount Everest of fast-drying concrete.