Neighboring plants, particularly exotic plant species, can affect arbuscular mycorrhizal fungi (AMF) communities inhabiting individual plant roots. However, neighbor identity in mixed plant communities is often confounded with different plant growth forms or phenology. We hypothesized that the timing of plant establishment, the identity of the plant neighbor, and the host plant interactions can all drive patterns in AMF abundance and community composition. To test this, we planted common California grasses three weeks apart to mimic early and late germination. Specifically, we planted a native perennial grass in combination with three annual species (two exotics and one native) so that the native perennial was fully crossed with each neighbor and each timing treatment. We found that in native perennial roots, the percent of root length colonized (RLC) by arbuscules (A) and hyphae (H) depended on the interaction of neighbor identity and the timing of neighbor establishment. For example, when compared to same-species treatments, one exotic annual reduced the RLC_H in the native perennial roots when the native perennial was planted first while the other exotic annual species increased the RLC_H in the native perennial roots when native perennial was planted second. In contrast, when the annuals were planted second, RLC_A and RLC_V were higher in annual roots with a native perennial neighbor compared to a conspecific. Based on these results, we suggest that the AMF network is largely driven by the combined effect of plant community composition and plant phenology. We are currently using molecular techniques to overlay AMF species identities on these colonization results.