Metaphor's generative power is commonly acknowledged, but typically studied through the impact of a chosen metaphor on the designed product. In this article we take a longitudinal approach, tracing a generative metaphor through a yearlong design process. We report a case study in which the metaphor 'Plant a Seed' provides the pivotal focus in the design and implementation of an interactive media architecture installation in the botanical gardens in Aarhus, Denmark. We show how the particular context of this site, and the repertoire of the project's participants (including their competencies and interests), provide material for constructing this metaphor. We then show how, after it is established, the metaphor is maintained through subsequent design and implementation activities; including the installation's public promotion. Within this timeline, we offer a closer analysis of the metaphor's shaping and validation during a co-design workshop. Our analysis extends current understanding of how a generative metaphor is grounded, shaped and maintained during collaborative design activities; and of how this apparently lightweight construct has the strength and flexibility to become the cornerstone framing design work over an extended period.