How to Identify

Photo credit: Invasive.org

Japanese Knotweed is a shrub-like herbaceous perennial. It forms dense stands that spread vegetatively from long, stout rhizomes and produces winged fruits that carry seeds to new areas. Though fairly tolerant of most soil and light conditions, it is often found in wet and sunny locations such as wetlands, roadsides and streambanks.

What to look for:

Hollow, bamboo-like stems that grow up to 10 feet tall
Alternate, large, oval leaves with square bases and pointed tips
Small green-white flower clusters that appear in late summer
Plants that turn brown and die back with the onset of frost
Plants that reemerge in early spring and grow rapidly

Japanese knotweed is one of the most damaging invasive species in North America. Trouble is, a lot of misinformation on knotweed management appears on many websites; inexperienced landscapers can make a bad infestation even worse.

The best available science shows that mowing, cutting, digging and/or tarping is not effective in controlling knotweed and can even stimulate root growth. The consensus among experts who work with knotweed is that the only effective control method is application of an herbicide weedkiller at a very specific time – late summer, after flowering, but before a killing frost.

We cannot underestimate Japanese knotweed. Japanese knotweed is considered one of the most resilient organisms on Earth, even surviving hot lava flows in its country of origin. The roots or rhizomes can grow more than 10 feet deep and travel laterally 35 feet beyond the woody stems. And, knotweed can remain dormant for more than 20 years, just biding its time for an opportunity to resprout.

What do you do if you have Japanese knotweed?
The first thing to do is identify the plant as knotweed. Look at the photos on this site and go here for additional assistance identifying your plant.

Do not cut, mow, or dig it. Physical methods such as mowing or cutting stems, digging plants out and/or tarping just don’t work with knotweed. Disturbance often promotes extremely aggressive lateral rhizome (root) growth.

Do not spread it. When knotweed is cut, mowed or dug, tiny pieces of root or stem can be tracked to new locations that create new stands of knotweed.
There are several ways that knotweed can spread:
1. Taking dug or cut stems and roots to a town landfill or dumping in a natural area.
2. Mowing and/or brush cutting that drags or blows knotweed beyond the existing area of infestation.
3. Transporting by machines when pieces are stuck or lodged in mower decks and wheels.
4. Moving soil that has not been sifted and contains bits of knotweed. Remember, even small roots or stems can regenerate in a new area.
5. Sharing or selling plants from your yard when pieces of knotweed are in the soil.

Do use an herbicide treatment. It should be noted that glyphosate controls knotweed but does not eradicate it. Management of Japanese knotweed is a multi-year process. And, as mentioned above, it must be done at the right time of year. It should also be noted that spraying herbicide comes with a risk to watercourses. Wetland restrictions may apply. Therefore, it is the recommendation of the Newtown Knotweed Initiative that a licensed pesticide applicator, educated in this plant’s phenology and with experience in management of Japanese knotweed, be consulted.

How environmentally damaging is Japanese knotweed?
Japanese knotweed is an invader of natural areas, displacing our native flora. When native plants are eliminated, populations of beneficial insects, birds, and other wildlife will follow.

According to reports in the United Kingdom, knotweed is causing property values to decline. It is also making it harder for first-time buyers to get mortgages on properties invaded with knotweed. UK and US courts have even started to award damages for properties encroached upon by knotweed.

To protect our properties and wild places, we need to take Japanese knotweed seriously and do everything we can to prevent further spread. It is important to refrain from mowing, cutting, digging and especially transporting to new areas.

To learn more…
Join the Worldwide Japanese Knotweed Support Group Facebook group.
Read this 2018 scientific journal article, which documented that only glyphosate effectively controls Japanese knotweed
Read a detailed Guardian article on Japanese knotweed
Read this article on the most sustainable method of dealing with Japanese knotweed