Ecosystems are structured by networks of interactions among species, but this hypothesis has rarely been tested in plant communities. Indeed, the structure and functioning of plant interaction networks have remained elusive so far and the mechanisms underlying their origin and maintenance remain unknown. By developing a novel approach that integrates the ecology of plant interactions with network theory and using spatial pattern analysis, we show that plant communities are organised in spatially variable and complex networks. Specifically, we found that positive plant interactions promote the formation and cohesiveness of large networks. At small spatial scale, where positive mutual interactions prevailed, the network was characterised by a large connected component. With increasing scale, when negative interactions took over, network structure became more hierarchical with many detached components. These findings shade new light on the complex networks of interactions occurring in plant communities.