Print PageJapanese knotweed is an invasive perennial weed of increasing concern. An extensive rhizome structure and shoots emerging from rhizome nodes result in rapid growth and spread. Because little is known about herbicide management efficacy on rhizomes, my objective was to quantify short- and long-term management success of an herbicide treatment on shoot and rhizome production. Japanese knotweed spread is regulated at rhizome nodes that can continue growing (rhizome extension with concomitant node production), remain dormant (no new growth), or produce new rhizome branches. Two of the three experimental sites were recently colonized and had low shoot densities (< 3 shoots m-2), while a third had been invaded at least five years before with high shoot density (> 10 shoots m-2). Isolated ramets at each site were treated in fall 2009 with two herbicides: glyphosate, imazapyr, or their combination, with a fourth ramet left untreated. Both herbicides are systemic and should target new growth in the rhizomes and shoots. Ramet density and height was recorded throughout 2010. In fall 2010, I traced and identified rhizomes that connected all observed ramets.
Results/Conclusions
Untreated plants at recent infestations re-sprouted from the site of old shoots (mean=2 shoots per crown, n=5) but did not produce new shoots from existing or new rhizomes suggesting these plants may be storing resources underground to spread in subsequent years. In the established infestation site, the crown produced at least 2 new rhizomes branches. Untreated plants were 53 cm taller (P<0.02) and had significantly more biomass (P<0.001) than treated plants across all sites. Plants treated with imazapyr and imazapyr+glyphosate herbicides did not produce new shoots or new rhizomes in the year after treatment. Glyphosate herbicide treatments significantly reduced shoot height, however populations generated an average of 0.5 (0 to 1, n= 6) new rhizome branches per plant with 0 to 6 new shoots per rhizome. While glyphosate may prevent shoot regrowth from the crown, I speculate it may be activating rhizomes to spread. Short-term reduction in shoot production may mask the reduced but still viable rhizome growth. These studies are being replicated to follow rhizome production and growth over multiple years. Long-term management of Japanese knotweed and other clonal species is dependent on targeting underground resource storage and spread. This study will further parameterize genet projection models that will assist in identifying life cycle vulnerabilities in this invasive clonal species.
