Schoolyards and school gardens present a rich context for students to engage with engineering and design. We describe the Designing Trellises unit, an example of a guided 3rd-5th grade engineering experience in the garden. In this unit, students work collaboratively toward a shared classroom design goal: creating 2-3 pea plant trellises for the school garden. They collaboratively design, research, prototype, build, and test their trellis designs. Students figure out disciplinary core ideas about balanced and unbalanced forces through engagement in asking questions, planning and carrying out investigations, designing solutions and other science and engineering practices. At the end of the unit, teachers and students report that students feel a strong sense of agency and pride as they ask and answer questions and create designs that are important to them and their community.