Green roofs, an important nature-based solutions approach, enhance urban infrastructure by providing habitat for diverse plant species, which are known to improve climate resilience through temperature regulation and carbon assimilation. However, the mechanisms by which plant interactions affect ecosystem service delivery of green roof environments have not been widely explored. In this study, we evaluated the performance of six native plant species grown in monoculture, all combined in mixture, and combined in mixture with the addition of a seventh species, Sedum spurium, a commonly used succulent on extensive green roofs (EGRs). EGRs are characterised by shallow, inorganic substrates and are more widely installed due to their lightweight design and lower cost. We quantify plant height, leaf area (LA), specific leaf area (SLA), and leaf dry matter content (LDMC), along with plant cover, and evaluate how these traits affect EGR temperature and carbon assimilation. Our findings indicate trait responses to mixture conditions through phenotypic plasticity in resource acquisition and allocation, with SLA and plant height increasing and LDMC decreasing in mixtures compared to monocultures. Mixed-species communities outperformed single-species ones in terms of plant cover and temperature reduction. Increased plant cover was driven largely by a dominant species, Achillea millefolium, consistent with a selection effect. In contrast, lower substrate temperatures, likely arising from trait differences among species, point to a complementarity effect. Mixtures with Sedum resulted in lower plant cover and higher substrate temperatures compared to mixtures without it, likely driven by Sedum's conservative functional strategy, which limited canopy development and evapotranspiration under irrigated establishment conditions. Practical implication. This study contributes to the growing body of evidence suggesting that mixed-species assemblages can outperform monocultures in EGRs, with implications for functionally diverse green roof design.