Knotweeds: The Plant, The Problem, The Solutions

The Problem

What makes Knotweeds such a problem?

Japanese, Bohemian, and Giant Knotweed easily top the list of most problematic Invasive Species in North America. Not only do they share common invasive traits as other invasive plant species (aggressive, allelopathic, dense, no natural predators), but knotweeds also have extensive root systems (10 feet deep and extending 30+ feet out), and can regenerate from very small stem and root fragments.

Knotweeds pose a serious threat to our native ecosystems by easily outcompeting native plants through aggressive growth and chemical warfare (allelopathy). In the end, the consequences are extensive:

  • Erosion along waterways
  • Dense growth impassible by various wildlife (think turtles)
  • Dense growth passable with difficulty by other wildlife
  • Limited plant diversity, leading to...
  • ...limited food supplies for native species of insects, birds, and mammals
  • ...limited habitat for native species of insects, birds, and mammals
  • Additionally: Knotweed can encroach on agricultural land, increasing costs for farmers.

Knotweed also takes advantage of weaknesses in infrastructure, where it can cause damage, such as growing through and expanding cracks in roads, drains etc.


The Plant
Identifying Japanese, Bohemian, and Giant Knotweeds

Although there are three types of knotweed, the main difference between the three is their size.

Japanese Knotweed is the smallest at a "modest" 9 feet tall. as a result, its leaves are generally smaller as well: around 6" long.

Giant Knotweed--as its name suggests--is giant. Up to 15" tall, with leaves up to 16" long.

Bohemian Knotweed is a cross of Japanese and Giant Knotweeds. As a result, its size falls somewhere in between.

Refer to the photos at the top of the page to see examples of these shared, visual characteristics to look for when identifying knotweed:

  • Spade shaped leaves that Zig Zag along the stem
  • Hollow stems with visible "nodes" where new stems sprout, making this plant often mistaken for bamboo. These nodes are also the only NOT-hollow part of the stem.
  • Very densely growing stems and thickets with little to no other herbaceous plant species around
  • (~September) flowers on little stems that grow periodically along the main stems
  • Brown stems after a frost and throughout the winter and early spring.
  • Brown winter stems that retain their seeds for a while, giving a lacy appearance.
  • (Spring) asparagus-looking sprouts with reddish stems, to begin with.

Knotweed Identification Video with Hillsborough County Forester


The Solutions
Management Techniques for Homeowners

Before getting into the details, keep in mind these general rules of thumb, unique to knotweeds:

  • Cutting and trimming only invigorate the plant. Don't cut unless you're going to apply herbicide in September and need to make sure it will be short enough to reach with the foliar spray.
  • Don't dig past the first year of infestation. The roots are too deep, making it ineffective, and likely to spread instead of contain. Digging also leaves root fragments which grow into new plants.
  • Don't mow: Mowing risks spreading fragments, which will resprout.
  • NEVER chip knotweed!
  • NEVER compost or put in town compost/brush! Always trash.
  • You may have heard of the smothering technique. Research does not show that this method is effective. It's time-consuming, expensive, and in the end the Knotweed will win.

Effective Management

...requires multiple years of monitoring and repeating treatments. If treatments are skipped for a year, any regrowth will build back up to its original extent.

We've provided the steps below for property owners to follow, in order to come up with an appropriate plan for their property. Before selecting an approach:

  1. Confirm species and map extent (include possible rhizome spread up to 30 ft). Use multiple identification methods (Google Lens, Seek ID, photo comparisons) if unsure.
  2. Stop the spread:
    1. never move soil or plant fragments off-site without approved containment. See our page on invasive plant disposal;
    2. clean tools/vehicles after working near infestations.
    3. Ensure that any landscaping companies working at your site are aware of the infestation, and follow best practices to avoid spread
  3. Select the plan below that best fits your site and objectives
  4. Develop a restoration plan, to re-plant the area with hardy natives to help prevent re-establishment of invasives in the future. Trees and tall plants that provide shade may be best in this case, as Knotweed prefers part-full sun.

Management Options

Small, Patch Excavation: Carefully excavate the entire root crown and rhizome network. If the infestation is less than a year old, this process may successfully be done by hand. If it's older, machinery may be required. Ensure all plant and root fragments are carefully bagged and disposed of in the trash, or dried on pavement and burned completely. Monitor site every year, and repeat efforts if/when growth reappears.

Large patch, more than 50 feet away from waterways and wetlands, and not directly next to public right-of-ways: Planned, repeated herbicide treatments may be carried out by the property owners with 2.5% glyphosate solution with a surfactant. This should be done just after flowering in September to maximize effectiveness and avoid pollinator harm. Monitor site every year, and repeat if growth reappears. Spraying may take 2-5 consistent years of treatment for complete eradication.

Small or Large Patch Excavation, within 50 feet of waterways: Excavation, including hand-digging, is generally prohibited this close to waterways, due to laws that are in place to protect waterways from erosion. Excavation may be permissible with the right permits and a dredge and fill request with the Department of Environmental Services. Continue to monitor the site in subsequent years, and repeat treatments if growth reappears.

Small or Large Patch, repeated management with goats: Goats will absolutely chow down on knotweed. This will never eradicate the knotweed, but it will keep it low. When considering this option, look into local goat herds and the cost. It can be costly for large areas, but it is a good option to consider especially near waterways, where you want to avoid chemicals and erosion concerns from digging. Depending on the time of year, the Knotweed will likely need to be hacked down for the goats to eat. And they may need to come every few weeks to keep the knotweed in check. This option may also be combined with a chemical option, where repeated, regular goat grazing helps reduce the plant's root store during the spring and summer, and in September you can switch tactics to herbicides with potentially higher success.

Small or Large Patch Spraying, within 50 feet of waterways: NH Law requires you to hire a Certified Pesticide Applicator for spraying within 50 feet of a waterway or wetland, who will need to file additional permits, and who can use pesticides rated for wetland and waterway which are not commercially available. Continue to monitor the site in subsequent years, and repeat treatments if growth reappears. Spraying may take 2-5 consistent years of treatment for complete eradication.


New Hampshire-Specific Calendar for management

Season


Steps

Winter (Jan - Feb)

map patches, plan removal approach, contact local experts (UNH Extension, Merrimack Invasive Species Committee) for guidance.

Spring (March - April)

begin careful excavation where appropriate when soil is workable; avoid moving contaminated soil.

Summer (June)

initial cutting (if using cut/spray strategy) to promote regrowth for later translocation treatments. Avoid mowing due to fragment creation.

Fall (September, after flowering until hard frost)

Apply systemic herbicides to maximize translocation to rhizomes; avoid spraying flowering patches when pollinators are active. Repeat in subsequent years.



Q&A

Common questions and misconceptions

Don't Deer and Honeybees love knotweed? Doesn't that make it a valuable species?
Yes they do. However, both deer and honeybees have tons of other native species they can eat.

I've heard glyphosate is what we're supposed to use on Knotweed. Isn't that a dangerous chemical?

Yes and no. Glyphosate has been a topic of news lately, and not all of the claims about it are accurate. It's important to read the original research (using the CRAAP test ) to determine the value of the research you're reading) or to find the least-biased source of information that you can find on the topic.

We will say:

  • Glyphosate is highly effective against Knotweed. Subsequent years will require less and less herbicide until the patch is fully dead.
  • To stay safe, follow the label. It's also the law.
  • Know the most effective time to spray, for the greatest effect.
  • Spray on calm, windless days, when rain is not in the forecast.
  • Hire a professional for sensitive areas.
  • Weigh the pros and cons of your specific location. Are non-chemical methods an option?

Glyphosate is dangerous for wetlands! Why are we using it to manage Knotweed?

Yes, Glyphosate is dangerous for aquatic life. So is Knotweed, if we ignore the problem. Here are a few things to consider:

  • There are forms of glyphosate permitted for use near wetlands and waterways.
  • Use of any pesticide near waterways requires a special license, and often extra permits.
  • When an individual or organization (like a conservation commission) are deciding how to manage knotweed on their land, they need to weigh the pros and cons of each option. For example:
    • Where are we managing? Is it valuable habitat that we need to protect, or less valuable, such as roadside?
    • Is it a small patch of knotweed that can be regularly monitored and managed by cutting and digging? Or is it a large patch that can't be mechanically managed?
    • Is machinery an option?
    • What is the long-term effect of each option? For example, leaving the knotweed may reduce habitat and increase spread. Spraying and continued monitoring may eradicate the Knotweed over time, and the ecosystem will recover.



Local Resources to consult

  • Hillsborough County Forester, Mike Gagnon
  • US Forest Service
  • New Hampshire Department of Agriculture, Markets & Food — Japanese Knotweed Best Management Practices (BMPs) and disposal guidance.

Resources


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