Introduced plants may compete for pollination with native plants, resulting in reduced fruit or seed set. In this study, I use several techniques to assess whether the invasive plant Euphorbia esula L. (leafy spurge, Euphorbiaceae) reduces the pollination success of chasmogamous flowers of the native plant Viola pedatifida G. Don (prairie violet, Violaceae), which has chasmogamous and cleistogamous flowers. Euphorbia pollen was found on most Viola stigmas, suggesting the potential for competition. Additionally, application of Euphorbia pollen to Viola stigmas prior to conspecific pollen reduced seed set. However, natural receipt of Euphorbia and other heterospecific pollen was not associated with decreased fruit or seed set. In the field, fruit set was higher for Viola in sites with Euphorbia despite increased Euphorbia pollen receipt, and seed set did not vary with the presence of Euphorbia. However, there was no significant interaction between the presence of Euphorbia and flower type (cleistogamous or chasmogamous), suggesting that increased fruit set near Euphorbia is unrelated to pollination. This study suggests that Viola and Euphorbia interact through shared pollinators, but there is no evidence that competition for pollination with Euphorbia reduces Viola’s fecundity or results in increased reliance on cleistogamous flowers.
