Non-native plant populations pose a significant threat to California’s native plant assemblages. Foeniculum vulgare, or fennel, is a pervasive perennial herb originating from the Mediterranean that has proven to be difficult to eradicate since its introduction to California. Some have proposed that fennel may be allelopathic, releasing biochemicals into the environment that inhibit native plants, but there are insufficient tests to support this hypothesis. In this study, we tested the hypothesis that fennel has soil legacy effects that reduce the growth of other plants after its removal. We first conditioned soil by growing fennel from seed through flowering stage. After removing the fennel and homogenizing the soil, we compared the emergence and growth of three co-occurring California native plants, Eschscholzia californica (California poppy), Achillea millefolium (common yarrow), and Stipa pulchra (purple needlegrass), across four treatments: conditioned soil, conditioned fertilized soil, control fertilized soil, and control soil.
Achillea millefolium emergence rates and growth appear to be reduced by growth in either conditioned soil treatment, but too few plants were present to allow for statistical analysis. We did not observe any impacts on E. californica or S. pulchra emergence rates in either soil treatment. Eschscholzia californica and S. pulchra dry plant biomass were significantly lower in both conditioned soil treatments, and shoot:root allocation of E. californica was significantly greater in fertilized control soil. These results offer support for the potential allelopathic effects of fennel on California native plants, and thus may have important implications for invasion management of fennel in California wildlands.