Plant-microbe associations are ubiquitous, but parsing contributions of dispersal, host filtering, competition and temperature on microbial community composition is challenging. Floral nectar-inhabiting microbes, which can influence flowering plant health and pollination, offer a tractable system to disentangle community assembly processes. We inoculated a synthetic community of yeasts and bacteria into nectars of 31 plant species while excluding pollinators. We monitored weather and, after 24 h, collected and cultured communities. We found a strong signature of plant species on resulting microbial abundance and community composition, in part explained by plant phylogeny and nectar peroxide content, but not floral morphology. Increasing temperature reduced microbial diversity, while higher minimum temperatures increased growth, suggesting complex ecological effects of temperature. Consistent nectar microbial communities within plant species could enable plant or pollinator adaptation. Our work supports the roles of host identity, traits and temperature in microbial community assembly, and indicates diversity-productivity relationships within host-associated microbiomes.