The objective of this article is to identify growth patterns of Japanese knotweed propagules distributed by high-water events. Along four river systems, we collected and measured Japanese knotweed propagules that had been distributed by flooding approximately 1 yr earlier. Results indicate that the size of the emergent shoot may be determined by the extent of underground growth late in the growing season, although initially it is linked to the size of the propagule. Our results show that 70% of new plants originated from rhizome fragments, and 30% from stems. This proportion is similar to regeneration rates shown in laboratory studies. We suggest that the best way to prevent the spread of Japanese knotweed along rivers is to focus control efforts on those stands most susceptible to erosion and propagule dispersal. We also suggest that an early detection and rapid response management approach can be effectively utilized to eradicate these propagules, and effectively suppress the spread of Japanese knotweed. Our data-collection method also provides evidence that control of newly distributed propagules can be effectively accomplished without the use of herbicides or heavy mechanical tools.

The objective of this article is to extend the reported period in which flood-distributed knotweed propagules may be successfully managed using only manual labor. During a second round of early detection and rapid response (EDRR) management along the Green River in Guilford, VT, we collected and measured all Japanese knotweed propagules that had been distributed by flooding approximately 21 mo earlier, in August 2011, at a single site. Our data suggest that knotweed s.l. prioritizes the growth of new stems over new rhizomes at the start of a growing season. Because the limiting factor for successful removal of new knotweed s.l. plants by hand is the size of the rhizome system, our findings support extending the time frame for EDRR management of flood-distributed knotweed s.l. into the second spring after its initial dispersal. Additionally, in November 2013, surveys of our work sites found no new knotweed s.l. plants in locations accessible to work crews. In addition to validating our EDRR management techniques, this implies that knotweed s.l. fragment viability does not extend past the second spring following its dispersal.

Japanese knotweed (Reynoutria japonica) and the other invasive knotweeds, collectively known as knotweed s.l., are significant invasives worldwide, especially of riparian areas. While R. japonica and other knotweed s.l. can reproduce sexually, their dispersal to and spread within new regions is often accomplished through vegetative reproduction from rhizome and stem fragments. Once established, knotweed s.l. can displace riparian plants, meaning that soil stability once provided by displaced roots is lost, carrying significant knock‐on implications for watershed management. We propose that knotweed s.l. rhizomes both displace roots and the structure they provide to soil, and also amplify bank‐erosion forces, especially during floods. Further, erosive forces create propagules, with larger flow events creating larger numbers of propagules and providing the vector for short‐ and long‐distance downstream spread within the watershed. Induced erosion is therefore the main driver of knotweed s.l. invasions along waterways. As some hydrological regimes shift towards more frequent and severe storm events in response to climate change, positive feedback loops may develop in these regions between existing knotweed s.l. populations, sudden riverbank failure, and increased flood‐related damage, with presumably significant impacts on riparian infrastructure. While the continued spread of this invasive could have significant riparian flood resiliency consequences if left unchecked, mindful action to control these plants is likely to be beneficial financially, socially, and ecologically within any invaded watershed.

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