Japanese Knotweed Management in the Riparian Zone of the Bronx River

Invasive species management is key to the preservation of natural riparian habitats, including along the Bronx River in New York City. Invasive plant management techniques include mechanical removal and herbicide application; these vary in their costs, non-target effects, and effectiveness (Weston et al. 2005, Hagen and Dunwiddie 2008, Delbart et al. 2012). Controlling invasive plants in an urban environment is challenging as urban natural resource managers have limited resources and often work with volunteers to address the constant issue of managing invasive plants. One such invasive plant is Reynoutria japonica (Japanese knotweed) and a hybrid, Reynoutria × bohemica (= Reynoutria japonica × Reynoutria sachalinensis).

Reynoutria japonica was introduced in the U.S. in the 1870s from eastern Asia and is now widespread in riparian and disturbed zones (Pyšek et al. 2003, Weston et al. 2005, Barney 2006, Gammon et al. 2010). It has large stems that can reach heights of over 5 meters and it primarily expands through rhizomes, which can grow to over 10 meters in length and extend up to 1 meter below the surface (Weston et al. 2005, Engler et al. 2011, Groeneveld et al 2014). Reynoutria japonica hybridized with R. sachalinensis to form R. × bohemica, which shares many of the same physical and growth traits as the other species (Pyšek et al. 2003, Gammon et al. 2010, Rouifed et al. 2011). Hereafter, “knotweed” will refer to R. japonica, R. sachalinensis, and R. × bohemica, which are all present on the Bronx River in New York City (Frankel et al. 1999, Yau et al. 2012, D. Atha and J. A. Schuler, New York Botanical Garden, New York, NY, pers. comm.).

The New York City Department of Parks & Recreation (NYC Parks), together with the Bronx River Alliance and the New York Botanical Garden (NYBG), produced the Bronx River Riparian Invasive Plant Management Plan (Yau et al. 2012), and initiated a study to try to improve knotweed control. The purpose of our study was to test the effectiveness of two common mechanical management techniques and to examine their effects on planted native Quercus palustris (pin oak) saplings and on the herbaceous layer in a knotweed dominated area in the Bronx River bank. We examined Q. palustris because it is a fast-growing species adapted to riparian soils and is a desired component of the canopy in these riparian forests (Sweeney and Czapka 2004).

We conducted a field experiment to answer the following questions:

• • Are mechanical cutting and root and rhizome removal methods effective ways of managing Japanese knotweed populations?

• • Does the reduction of Japanese knotweed affect the growth of Q. palustris saplings?

• • How do the different Japanese knotweed management methods affect other plant species in the riparian herbaceous layer?

In April of 2010, we established 60 2 m × 2 m plots in the Bronx River Forest (BRF). An additional 60 plots were also established in the NYBG for a total of 120 plots across the two sites. We positioned plots approximately 1m upland from the edge of the riverbank of the Bronx River (Figure 1) with a half meter buffer between each plot. We planted one Q. palustris sapling in the center of each plot in mid-April 2010; all of the planted Q. palustris saplings were approximately the same size. We planted the Q. palustris saplings before treatment occurred to capture initial conditions in the plots. We randomly assigned plots to one of three treatments with an equal number of plots per treatment type. The knotweed removal treatments were: A) cutting three times during the growing season (May, July, and September), hereafter referred to as the “cutting” treatment; or B) cutting once (May) and then root and rhizome removal twice (July and September), hereafter referred to as the “rhizome removal” treatment. Cutting is defined as cutting all non-woody vegetation within the plot with hand pruners, hedge trimmers or loppers as close to the ground as possible. Rhizome removal involved digging out all roots and rhizomes to...