Nearly all plants have developed symbiotic associations with microbes above- and belowground. These symbionts often alter the ecology of their hosts by enhancing nutrient uptake, increasing stress tolerance, or providing protection from host enemies. Understanding the dynamics of symbiosis requires testing how ecological factors alter not only the fitness consequences of the symbiosis, but also the rate of symbiont transmission. Here we asked how changes in the biotic and abiotic context alter both the costs and benefits of interactions between grass hosts and symbiotic fungal endophytes and rates of symbiont transmission. First, we assessed how shade and the presence of endophyte symbiosis affected host plant growth across six grass species. Our results demonstrate a novel benefit of endophyte symbiosis via the amelioration of shade stress. Second, we examine how interactions between a fungal endophyte and its grass host change along a gradient of water availability and in the presence versus absence of soil microbes. We show that benefits of the symbiosis were strongest when water was limiting. Together, our results highlight the context dependent nature of grass endophyte symbioses.