Despite a recent focus on engaging students in epistemic practices, there is relatively little research on how learning environments can support the simultaneous, coordinated development of both practice and the knowledge that emerges from and supports scientific activity. This study reports on the co-construction of modeling practice and ecological knowledge by following the development of one seminal disciplinary concept, plant reproduction, through third graders' yearlong investigation of a wild backyard area. Representational activity facilitated the visibility and utility of meanings for reproduction; these meanings, in turn, shaped students' subsequent modeling practice. Over time, shifts were evident in both the community's meanings for reproduction and their framing of meanings in relation to modeling activity. The analysis affirms the utility of attending to student knowledge as it is used in practice to navigate complexity and uncertainty, rather than as assimilation of declarative statements. It also provides images of productive relations between modeling and knowledge, with implications for how the two might bootstrap each other in extended classroom investigation. (Contains 5 figures, 2 tables and 1 footnote.)