Social capital has the potential to influence the success of biodiverse carbon plantings in the face of uncertainty amongst rural landholders about the need or efficacy of efforts to address climate change through tree planting. We conducted 17 face-to-face semi-structured interviews with landholders in Victoria, Australia who voluntarily participate in biodiverse carbon plantings on their land, focusing in particular on the role of social capital for understanding how ‘early adopters' can advocate for programs locally. The interviews revealed the importance of social networks and the profound impact of trusted peers on the diffusion of carbon planting schemes. These social capital dimensions are especially important for shaping ongoing participation and the ways in which participants become active agents in trusted relationships that influence the participation of others. Our results suggest that the positive impact of social networks can counteract doubts about the validity of climate adaptation responses such as carbon planting, and enable landholders to connect the program with their existing stewardship motivations. The ability for early adopters of the program to demonstrate the physical materialisation of their plantings to others was vital to this process. We propose that targeting champions and trusted peers in local communities could accelerate the proliferation of biodiverse carbon planting schemes.