Many of the fifty-plus invasive plant species occurring in the park established themselves decades ago and others are rapidly catching up. In the case of newcomers, we have the chance to intervene—so long as we recognize the threat.

Case in point: Japanese knotweed (Reynoutria japonica, Fallopia japonica, or Polygonum cuspidatum), an invasive so destructive that Great Britain began enacting laws to control its spread forty years ago. Knotweed is widely distributed throughout the United States and is a top invasive plant enemy throughout much of its range.

Japanese knotweed

colony of Japanese knotweed on a streambank within feet of JRPS boundary

Here in Richmond, knotweed has established a sizable colony on Belle Isle, has a foothold in other parts of the park, and is massing on private property just outside the boundaries of other park units.  This is a herbaceous species that spreads so densely and can tower so high that it’s easily mistaken for a shrub hedge. Its late-summer long clusters of blooms belie the harm caused by its root system.

That underground system is made up of rhizomes, underground running stems that produce new roots and stems from nodes. Once knotweed takes off, it forms long, horizontal colonies of densely packed stems that, along with its leafy canopy, extinguish all other vegetation.

Flowering knotweed

Like most invasive species, knotweed can get by just about anywhere. But it thrives in wetter soils, and streambanks or a river’s edgeuniquely accommodate its growth and spread.  A recent study found that Japanese knotweed root systems increase erosion by displacing diverse native vegetation with its own shallow roots and rhizomes that can’t hold up to flooding events. That flooding action in turn breaks off root fragments and rhizomes, carrying and depositing these knotweed ‘propagules’ downstream where they colonize new locations.

Knotweed at the river

Japanese knotweed just upriver of the JRPS

Knotweed is an even more formidable opponent because it is so challenging to control and eliminate.  If you have knotweed on your property, please do not attempt to dig it out.  Other than in the case of small, individual plants, digging knotweed is likely to contribute to its spread. Cutting alone, even repeatedly, isn’t effective.  A carefully timed and implemented combination of control methods over the course of several years is necessary for controlling, and hopefully eliminating, large stands of knotweed.  Penn State Extension provides excellent guidance on management and treatment.

If you now recognize a knotweed problem where before you saw a leafy expanse with pretty blooms, and would like more information about how to help stop its spread, please contact Laura Greenleaf with the JRPS Invasive Plant Task Force ([email protected]).

If you believe you see knotweed in the park, please do not attempt DIY control methods. Please notify the task force or FoJRP.  This is true for all invasive plant species.  The best way to help manage invasive plants in the park system is to volunteer with the task force by visiting their calendar of events.

Examining knotweed

DCR-Natural Heritage Division biologist Kevin Heffernan, DCR intern Mason Manley, and task force member Anne Wright assess a colony of Japanese knotweed on Belle Isle.