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.
Nomenclature: Knotweed sensu lato; Japanese knotweed; Fallopia japonica (Houtt.) Ronse Decr.; Polygonum cuspidatum Siebold & Zucc.; Reynoutria japonica Houtt.
Management Implications:Once established, stands of the invasive species knotweed s.l. requires several seasons of intensive effort to eliminate, often requiring the use of hand tools, heavy machinery, or herbicides. This is due to its ability to regenerate from very small pieces of stem or rhizome. Currently, management techniques focus on eliminating established stands, rather than new plants. After the flooding caused by tropical storm Irene in Vermont in 2011, land managers anticipated that knotweed would become widely distributed across floodplains throughout the state. A coordinator was hired for the summer of 2012 to arrange the removal of as many new plants as possible, by only manual labor. Over the course of the summer, 5,000 to 6,000 plants were eliminated over approximately 30 acres (12.1 ha). It was quickly recognized during initial removal efforts that a better understanding of the relationship between the sprout above ground and lateral underground growth could be a quick and simple assessment tool that would generate more efficient and site-specific strategies for eliminating new knotweed plants. Though our study does not reveal a consistent relationship between the sprout above ground and lateral underground growth for knotweed s.l., there may be a window of time during which such a relationship does exist. We also found that stem and rhizome fragments retained their ability to regenerate and grow up to 13 mo after tropical storm Irene. We also found that 86% of new plants were growing from propagules buried under less than 4 in. (10 cm) of sediment. Given this relatively shallow burial depth, we suggest that whole plants can be easily removed by hand within a year of the dispersal event. When properly integrated into a long-term management strategy, these techniques may significantly reduce the need for more costly, and potentially environmentally disruptive, treatments in the future. We suggest that every new knotweed dispersal event is an opportunity to use early detection and rapid response (EDRR) procedures that take advantage of the window of opportunity when manual labor is an effective means of knotweed control.