Effective interventions are needed to bolster students' argumentation capacities, an area in which they consistently struggle. Quality Talk (QT) is an approach to small-group classroom discussion shown to support students' oral argumentation with preliminary evidence that it may also bolster students' written argumentation. Teachers often must adapt interventions to their local context, balancing needed flexibility with sufficient adherence to fidelity to reach expected efficacy. The present study was conducted over one school year with two fifth-grade teachers and their 46 students. In Phase I, two participating teachers implemented a refined version of QT, and we examined the effects on students' oral and written argumentation performance. While typical gains in students' oral argumentation performance were evidenced, students' written argumentation did not improve to the degree expected, particularly in terms of performance with unfamiliar texts. In Phase II, both teachers reincorporated a component of QT (i.e., regular post-discussion written argumentation practice) they had adapted in Phase I, and one teacher added a new written argumentation scaffold designed to further bolster students' transfer from oral to written argumentation. By the end of the study, students from both classes evidenced growth in written argumentation, but the students from the class receiving the writing scaffold outperformed comparison class students with large effects. Findings underscore the importance of including regular post-discussion written argumentation practice and illustrate the added value of a new written argumentation scaffold, while also contributing to a better understanding of how to balance flexibility and fidelity for efficacious QT implementation.