North Fork Watershed Association
Friday, September 24, 2021

Invasive Species



Japanese Knotweed

Japanese Knotweed, otherwise known as Polygonum Cuspidatum, is a non-native plant species, and is considered invasive, and has become a nuisance to the regeneration efforts of the Dr. Walter Dick Memorial Park in Brookville.  Knotweed has not only invaded the Dr. Walter Dick Park, but has become of great concern to watersheds throughout Western Pennsylvania.  Invasive, non-native plant species, such as Japanese knotweed, are capable of displacing native plant species and disrupting the ecological balance of particular habitats.  Like many non-native species, Japanese knotweed has a great advantage due to the lack of competition, particularly in environments severely disturbed by natural, or unnatural means.  Native species that would otherwise repopulate a disturbed area are displaced by the non-native rapidly, as there has been little time for the native fauna and flora to adapt to the presence of the newcomer.  Over a period of many years adaptation could occur to some degree by native species to control the population of the newcomer, however, this would most likely occur only after the permanent displacement of some native species and an ecological upset with possibly devastating affects.

Japanese knotweed is a dioecious species, meaning it bears male and female flowers on separate plants and both are needed to create viable seed.  Fortunately, male plants are very rare in the United States so the primary mode of reproduction is vegetative through the spread and growth of rhizomes.  The rhizomes of the Japanese knotweed are capable of generating new shoots from a depth of approximately 3 feet below the soil surface and can regenerate an entirely new plant from a section of severed rhizome less than an inch long.  In addition, the rhizomes of a healthy plant are capable of breaking through several inches of pavement, which has occurred at the Walter Dick Park.  Japanese knotweed is a frost sensitive perennial and will die back in winter.  This species prefers a low soil pH and full sun, appearing to do poorly in heavily shaded areas.  The preferred habitat for the knotweed is generally wetland areas, however, this species is capable of invading croplands as well.

For identification purposes, the stalks are bamboo-like and have knobby nodes along the stems and stalks, which is a common characteristic of the Polygonaceae family, also known as the Buckwheat or Knotweed family.  Stalks of this species are hollow and can reach up to 9 or 10 feet tall.  Young shoots and leaves appear a reddish green, turning fully green as they mature.  Mature leaves will reach approximately 6 inches in length with acuminate tips (pointed) and smooth edges (entire).  It is important for residents of this and other watersheds to familiarize themselves with this plant species.  In doing so, eradication efforts can be much more effective. 

The North Fork Watershed Association is now working to control this invasive species within the watershed area.  Since the Walter Dick Memorial Parks natural areas were so severely disturbed during the flood of 1996, this species has taken a particular hold on the banks of the North Fork at the park.  As a result, the association's eradication efforts have begun on these banks with the chosen method of control being the repeated cutting of the stalks at soil level, or below, in hopes of weakening its massive rhizome.  Although herbicides can be used to control this species, the association has chosen not to utilize this method due to the possible adverse consequences associated with exposing the water to undesirable, and possibly toxic, elements.

Other's concerned with eradicating this species attempted to use a herbicide in one of the more concentrated areas, however, damage to the plant only appears to be temporary.  The first cutting effort took place on the July 11, 2002, on the beach area.  Due to the extensiveness of the knotweed, and the limited manpower, only one isolated area was cut at this time.  This same area was cut once again on August 6, 2002 with more positive results, as the total volume of the second cutting equaled approximately one quarter of the original cutting, indicating the possibility that the regenerating capabilities of the rhizome had indeed been compromised.  Between the last hard frost, which caused all new growth to die back in late May, and the first cutting was a time span of approximately 6 weeks. The time span between the first and second cutting was just under 4 weeks.  Presently, there are no formal regulations for the disposal of cuttings from Japanese knotweed.