Hybridization can play an important role in the evolution of invasiveness. Eurasian watermilfoil (Myriophyllum spicatum L.) is a widespread aquatic invasive plant species that hybridizes with native northern watermilfoil (Myriophyllum sibiricum Kom.). Previous studies have found mixed evidence for whether hybrid watermilfoil (Myriophyllum spicatum × sibiricum) and pure M. spicatum differ in vegetative growth rate and herbicide response. While several studies have emphasized variation in these traits among M. spicatum × sibiricum genotypes, variation within M. spicatum has not been considered. Therefore, it is unclear how much genetic variation influences invasive traits and management outcomes within M. spicatum versus between M. spicatum and M. spicatum × sibiricum. If M. spicatum × sibiricum genotypes are always more invasive than M. spicatum genotypes, simply distinguishing taxa may be sufficient for identifying lake management priorities; however, if significant phenotypic overlap is observed between taxa, distinguishing individual genotypes may be important for tailoring management strategies. We performed replicated trials of a vegetative growth and 2,4-D assay to measure clonal variation in growth rate and herbicide response of M. spicatum and M. spicatum × sibiricum. Our results indicate that M. spicatum × sibiricum exhibits higher average vegetative growth than M. spicatum, regardless of whether it was treated with subsurface applications of 2,4-D. We did not observe interactions between taxon and treatment or between genotype and treatment. Despite differences between M. spicatum and M. spicatum × sibiricum in average vegetative growth, there was substantial overlap between taxa. For example, we found that the fastest-growing genotype of pure M. spicatum did not differ significantly in average growth from the fastest-growing M. spicatum × sibiricum genotype. The potential for overlap between these invasive Myriophyllum taxa suggests that distinguishing and characterizing genotypes may be more informative for management than simply distinguishing between M. spicatum and M. spicatum × sibiricum.

Additional active ingredients are needed for use in aquatic systems to respond to new threats or treatment scenarios, enhance selectivity, reduce use rates, and mitigate the risk of herbicide resistance. Florpyrauxifen-benzyl is a new synthetic auxin developed for use as an aquatic herbicide. A study was conducted at North Carolina State University in which 10 µg L−1 of 25% radiolabeled florpyrauxifen-benzyl was applied to the isolated shoot tissue of 10 different aquatic plant species to elucidate absorption and translocation patterns in these species. Extremely high levels of shoot absorption were observed for all species, and uptake was rapid. Highest shoot absorptions were observed for crested floatingheart [Nymphoides cristata (Roxb.) Kuntze] (A192 = 20 µg g−1), dioecious hydrilla [Hydrilla verticillata (L. f.) Royle] (A192 = 25.3 µg g−1), variable watermilfoil (Myriophyllum heterophyllum Michx.) (A192 = 40.1 µg g−1), and Eurasian watermilfoil (Myriophyllum spicatum L.) (A192 = 25.3 µg g−1). Evidence of translocation was observed in all rooted species tested, with the greatest translocation observed in N. cristata (1.28 µg g−1 at 192 h after treatment). The results of this study add to the growing body of knowledge surrounding the behavior of this newly registered herbicide within aquatic plants.