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May 02, 2016
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Knotweed out of control: What’s a homeowner to do?

A big decision in fighting knotweed is whether to use an herbicide. If using, a special kind of aquatic herbicide must be used near any body of water and some states require permitting in advance.

By J.C.B. Huggard

Knotweed is more than a nuisance. It’s an epidemic in these parts. Dense stands of this noxious, invasive species crowd along roadsides and waterways, affecting ecosystems by pushing out native plants and limiting plant and animal species diversity. Along streams and rivers, it overwhelms native plants that help stabilize riverbanks, increasing the risk of erosion and flooding.

Every day on my way to work, I drive past a well-established and extensive stand of knotweed on Route 652 east of Beach Lake, PA. There, this summer, I have followed the progress of one very determined man who is battling knotweed on the shore of a lovely large pond. It’s an uphill fight, but one patch of knotweed seems to be diminishing.

Japanese knotweed (Polygonum cuspidatum, also called Fallopia japonica) and giant knotweed (Polygonum sachalinense) have their origins in Japan and East Asia and were introduced in the U.S. as ornamentals in the late 1800s.

Knotweed is a tall-growing (six to 10 feet), hollow-stemmed, non-woody (herbaceous), perennial plant that seems to be spreading nearly everywhere in our region. Chances are, if you’re a landowner, you may have a stand of knotweed on your own country property and you’re wondering how to get rid of it.

Getting rid of knotweed is not for the faint of heart; it requires persistence, persistence, persistence.

Following a burst of growth in early spring (March or April), knotweed’s flowers emerge in July and its seeds mature in August and September, though seeds are not the primary way knotweed spreads. More insidious are its rhizomes (underground stems), which spread easily in soil, and are so determined that they can push through asphalt, concrete retaining walls and building foundations. Even a very small piece of rhizome that is moved to another site will produce a new plant. Soil in which knotweed has grown should be treated as contaminated. To control knotweed, you must control the plant’s rhizome system. Any control efforts need to target what’s going on underground with the rhizomes—you need to target what you cannot see.

So, here’s the ecology-minded homeowner’s dilemma: whether to use an herbicide or not

“And that’s the thing,” according to Jamie Myers of the National Park Service, a biologist for the Upper Delaware Scenic and Recreational River and a licensed herbicide applicator. “You can’t kill knotweed completely without using herbicides, but you can do a pretty good job of controlling it.”