Expert Insight: Invasive Knotweed

March 3, 2021

Japanese Knotweed is present everywhere along the Upper Delaware River and its tributaries. Left unmanaged it continues to spread along the river and everywhere it can find suitable conditions all the while crowding out native riparian vegetation leading to habitat loss, soil erosion, and bank destabilization.

Friends of the Upper Delaware River (FUDR) is undertaking research to document the extent of knotweed colonization and its impacts on soils and aquatic habitat. This year they are also establishing 3 demonstration sites where landowners can see different options for you to manage, control, or even eradicate knotweed using a variety of low-cost techniques. 

On January 28th, in partnership with FUDR, Penn State’s Shaver’s Creek Environmental Center, and Shippensburg University’s Center for Land Use and Sustainability, we held an informational webinar on knotweed in the Upper Delaware River Watershed and the demonstration sites. You can view a recording of the webinar here.

During the forum, the audience had an opportunity to ask questions. For all the questions we couldn’t get to during the webinar, panelists Dr. Eric Burkhart of Penn State’s Shaver’s Creek Environmental Center and Steve Schwartz, the Knotweed Project Consultant, have since weighed in. See below for their expert insight!

I’m above the Pepacton’s Downsville Dam on the East Branch near Margaretville… I’d love any help I can get. Thanks for this.

Steve responds: Follow our website, and facebook page for more info on knotweed management and site demonstrations

Is digging and selling for resveratrol a possibility?

Steve responds that yes, there are herbal product companies that are purchasing knotweed rhizome.

How many years do you plan to follow the Skinners Falls site?

Steve responds: We have funding for one year. If we can raise additional funding the plan is for two or more years.

The test plots appear to be long and fairly narrow. How will you deal with drift from foliar herbicide application to adjoining test plots? Wouldn’t it better to separate the herbicide plots from the others?

Steve answers: There will be a strip between each plot and foliar is sandwiched between injection and covering.

Landscape architect and riverfront land owner here with half of the property covered with knotweed. Are there partnerships or grants available for landowners to implement these management techniques? Are the products / service providers that were referenced (Geo textile, herbicide, goat rental, herbicide applicator?)

Steve responds: What state are you in? Some federal funding through NRCS may be available, through county conservation districts. Some state level funding is available from time to time though nothing right now. We will share suppliers once we get the project up and running

What about eradication possibilities on state lands, such as state forests, that border the Delaware in several places? Is the State onboard with this problem and trying to solve it?

Steve answers: We have support from NYS DEC for the Skinners Falls project. We suggest advocacy to the state agencies to better address the problem. In NY, CRISP is the agency most involved with knotweed. In PA, there is no equivalent organization.

Our property has a lot of knotweed. We are trying to get it under control by cutting it down constantly and letting our goats and sheep eat it. Is it good to cut down along the river bank as well?

Steve said “Yes!”

How about plowing it up and picking the roots out?

Steve responds: Plowing it might only spread it as the rhizomes are very durable and any rhizome fragments might propagate and then you have the problem of where and how to dispose of all of the rhizomes and root fragments you remove.

How effective is digging it out?

Steve responds: If you have isolated plants it is very effective. In a mature stand it is impossible to get all of the rhizomes and radial roots at one time so it will require repeated management and then you have the problem of disposing the rhizomes without further spreading the plant.

When you mention herbicide. Is there a brand? Or should we keep away from herbicides? (sic)

Steve responds: This glyphosate product is aquatic-labeled and available at Home Depot. It also gives specific instructions for knotweed on pages 9 and 13 of the label. But, it is very important that anyone using herbicide can properly understand and follow all instructions on the label. I haven’t found aquatic-labeled glyphosate products that are ready-to-use (no mixing required).

While I can see how roundup injection would be more effective than foliar applications, is it practical as a landowner to use a foliar application with a handheld tank sprayer right next to the stream? Doesn’t seem environmentally sound unless one is a prescribed distance from the stream. In the alternative cutting and tarping right next to the stream seems like the most practical and safest practice (until I find a neighborly goat farmer)

Steve responds: Aquatic-labeled herbicide products can be used for foliar applications at the water’s edge. The active ingredient glyphosate is very effective on knotweed.  However, the short answer is we are not promoting using it casually, only providing this an option with precautions and warnings that it should be applied by a licensed professional.

What keeps knotweed in check in its native habitat? Are there any biological controls? And is its original habitat only Japan or other regions of temperate Asia? On the west coast they are having success with biological control. Has it been evaluated here in the east?

Steve responds to both questions: There is a psylid from Japan that has been trialed in the US, UK, and Canada with little success.

Do you restrict the goats to the knotweed area so they ONLY eat that? i.e., is knotweed something they eat only when everything else is already consumed?

Steve said: The goats will restricted to test plots which have mature stands of knotweed. Our goat farmer reports that they do eat knotweed and that they seem to like it as it is a novel food source for them. We will try to protect any trees and saplings growing in the plot so that the goats don’t eat the leaves and bark of plants we want to see compete with knotweed.

Any research on planted or natural grass as a deterrent? I’ve seen where lawns grow strong and are absent of knotweed.

Steve replied: Repeated mowing of lawns will deter knotweed. We plan to try sowing various grasses and bushes/trees such as willows as well to see how they can compete.

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