Management Implications: Perennial pepperweed is one of the most important invasive plants of wetland areas in the western United States. In several undisturbed plots at two locations, we showed that its spread was almost exclusively through vegetative expansion along the perimeter of the infestation at an average of 0.85 m yr⁻¹. By comparison, the spread rate was three times greater in a site that was disked, which can explain why disking is not recommended for perennial pepperweed control. Stem densities were nearly 2.5 times higher in the interior of the infestations compared with the leading edge. Because of the reduced stem density, the perimeter of the patch is considered to be easier to control compared with the denser patch centers. Small infestations expanded at a faster rate compared with larger patches, but larger patches added more newly infested areas. We determined that the expansion of perennial pepperweed was more closely correlated with increased water availability but was negatively correlated with resident plant cover. Thus, we conclude that the most appropriate strategy for managing perennial pepperweed is to minimize disturbance of plant cover and control both the rapidly expanding satellite infestations and the perimeter of larger infestations. Concentrating management efforts on the densely stemmed, more difficult to control, and highly thatched regions at the center of the infestation might not be cost effective. These areas also have less resident vegetation. Furthermore, the ability of desirable resident vegetation to re-establish from the adjacent seedbank at the perimeter of the infestation could also prevent the need for expensive active restoration programs when focusing on the center of large infestations.

Keywords: Wetlands, riparian, spread dynamics, invasion, spatial spread, population growth

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