Californian vernal pools are unique habitats that are home to many rare and/or endemic plant species. The non-native forb Cotula coronopifolia has invaded numerous vernal pool complexes in California. Manipulative and observational studies were conducted in order to determine whether Cotula coronopifolia detrimentally affects native vernal pool plant species and to ascertain what factors might influence the abundance of Cotula coronopifolia in vernal pools. The impact of Cotula coronopifolia on vernal pool plant species was assessed by (1) analyzing spatial associations between Cotula coronopifolia and other plant species, (2) removing Cotula coronopifolia seedlings from experimental plots, and (3) planting in pots different combinations and densities of Cotula coronopifolia and two native vernal pool plant species. The spatial association analyses and removal experiment provided little indication that Cotula coronopifolia strongly competes with vernal pool plant species. However, Cotula coronopifolia did have substantial competitive effects when grown at high densities in the controlled competition experiment. Vernal pool plant surveys revealed that Cotula coronopifolia is most abundant in years with above-average precipitation and relatively few frosts. While Cotula coronopifolia was dominant in some surveyed saline vernal pools, in a lath house growth experiment Cotula coronopifolia biomass and flower production were significantly greater at low to intermediate salinities.