Knotweed out of control: What’s a homeowner to do?

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.”

The literature shows that repeated cutting once a week at a minimum will control the plant. Like any treatment, it depends on how much you have. If you have a brand new infestation that’s just getting established, then you can go and cut it and it won’t require that much effort. If you’re looking at acres and acres, that’s not really feasible.

“You can’t just run over it with a lawn mower. You want to use a weed whacker with a metal blade or a machete. Keep in mind, you want to make just one pass through each stalk. If you mince up the plant, that will just help it spread. Another method that has some rate of success [without using an herbicide] is to do a cutting and cover with plastic.” Clear may work better than black plastic.

Here are two information sources prepared by Penn State that do rely on herbicides for eradicating knotweed. This choice is offered very reluctantly by The River Reporter, because unfortunately the herbicide of choice, recommended as most effective is glyphosate, e.g. Monsanto’s Roundup™ (or Rodeo™, which is the version of Roundup™ recommended for application near aquatic habitats) (http://tinyurl.com/mrmjm5l and http://tinyurl.com/l4tnnkz).

These two information sheets recommend using as many control methods as possible and establishing a planned program of control, maintenance and establishing new vegetation.

Control phase: cutting and treating

• Late (single) cut & treat with herbicide:

June 1, cut away foliage; re-growth will come back

only two to four feet tall

Between August 1 and the killing frost, treat with

glyphosate application

(Note: If you do not cut the knotweed by June 1, treat

with glyphosate between July 1 and first frost)

• Multiple cut & treat with herbicide:

Cut twice in the spring

Follow with a glyphosate application between August

1 and a killing frost.

Maintenance phase:

additional treating

Apply after August 1; if treatments are successful, then apply maintenance program on a two-year cycle.

A conversation with Jamie Knecht, watershed specialist at the Wayne County Conservation District, led to another method of using glyphosate. “We use the product called Rodeo™, which is the aquatic version of Roundup…. Our preferred method of using it is through injection. This way the product is injected directly into the plant stalk itself, and when the plant translocates the sugars that it produces down into its rhizomes, it also moves the Rodeo™ and is killed that way. This injection method is low-impact and very effective. The National Park Service uses this methodology on their sites as well. We completely understand that people have concerns about using Rodeo™; some of the landowners we talk with do as well [because] most of these sites are so close to water resources.”

Re-vegetative phase

Roadside sites where knotweed has been killed will revert to crown vetch after a few seasons. Trying to reestablish grasses does not always work well (works better after giant knotweed than after Japanese knotweed). But establishing new vegetation is a vital step in gaining control over knotweed. Suggestions for what to plant include: Fast-growing trees (dogwoods, willows, birches, alders, poplars including tulip poplar, sycamore, sourgum or black gum or sweet gum, aspen, black locust, blackcherry, pin oak, white pine or silver maple), shrubs (viburnums, shadbush, bayberry, chokeberry, box elder, sweet pepperbush), herbaceous vegetation (black-eyed susan, black cohosh, wild ryes such as Virginia wild rye or riverbank wild rye, blue stems, swithcgrass, Indian grass, Joe-Pye weed, New York ironweed, or fowl blue grass). This information was developed for a workshop and was prepared in 2003 by DRIPP (the Delaware River Invasive Plant Partnership).

Disposal of plants

Once knotweed has been cut, it is essential to dispose of it properly to prevent new infestations. Knotweed can be burned or buried, but it cannot be composted while “green.” If you pile it up, make sure it dries out thoroughly, but you must monitor the pile to make sure it doesn’t re-sprout or get blown into new areas or washed into a stream or pond. Placing plant remains on plastic or some other impermeable surface is recommended. If the plants are buried, some sources recommend to bury them at least 10 feet deep.

Bio-control

Finally, biologists have been testing under carefully controlled conditions whether biocontrol of knotweed is possible. They are looking at a bug called a psyllid, a tiny sap-sucking insect related to aphids (Aphalara itadori). This bug from Asia loves knotweed, but the issue is whether it also will eat our native plants and produce unintended consequences if it is released widely. The history of biological control tells us of many past mistakes where the “control” caused more trouble than the original target.

[Editor’s note: Once you have weighed all appropriate weed management options for the situation on hand, and a decision has been made to use herbicide, there are still several important factors to consider involved in using herbicides safely and effectively. This includes reading the label of the herbicide you are using completely, following the label for dosage, intended species and wearing the required personal protective equipment (PPE).

Also be sure to check your state’s registered use pesticide list.

Find the New York State list at http://tinyurl.com/3ah66k.

Pennsylvania uses the National Pesticide Retrieval Information System at http://state.ceris.purdue.edu/ (click on the map of Pennsylvania and follow the instructions). Note: Pennsylvania also requires permits from its Department of Environmental Protection AND its Fish and Boat Commission (FBC) for stream bank application of pesticides; you can find the permit on the FBC website at http://tinyurl.com/n7pdzz2).]