Japanese knotweed season

The long, warm days of summer are my favorite time of year. Plants seem to like it too as every sort of plants from the corn in the fields to the prairie perennials are putting on quite a bit of height this time of year.

But there is one particularly large and fast growing perennial that gets very noticeable this time of year as it reaches up to 12' in height. Unfortunately, it is also an extremely aggressive plant that can crowd out just about any perennial or shrub. This plant's roots are massive and trough, they can tear through asphalt and even building foundations. So this species is not only a threat to natural communities, it can also cause some extensive damage to infrastructure.

I'm speaking of course, of Japanese knotweed (Fallopia japoinica, a.k.a. Polygonum cuspidatum).

Athena, who stands 5'8" is dwarfed by this colony of Japanese knotweed.

This species is also extremely difficult to control. Its root system is so massive that its difficult to get enough of any herbicide chemical to translocate down to the root from the relatively small above ground foliage. I said relatively small, yes, the roots are just that big that the 10-12' stalks are only a small portion of the biomass!

Furthermore, any small fragment of root or stem, as small as a pea can take root and form a new plant. So it is often spread readily by mowers along roadsides and by flooding, erosion and ice impacts along stream and lake shorelines. And digging out every little pea sized piece of root is, of course a labor-intensive and tedious process. Its also somewhat resistant to herbicides.

Regular mowing, every two weeks can starve the roots, but this process can take years. Smothering can work as well if you mow it first, but you have to cover the entire colony so no leaves are sending energy to the root mass, including new leaves that will inevitably sprout after the colony is covered.

We like to use the cut, wait, and spray technique described in the below video, but we use Milestone herbicide instead of glyphosate (Round-Up®, etc.) as the glyphosate has proven to be only mildly effective against Japanese knotweed.

Good Oak has our own Weed Identification and Control Sheet about Japanese Knotweed that should help you, well, identify and control this species.

The Wisconsin DNR has listed Japanese knotweed as Restricted, meaning it is illegal to buy, sell, give away, or barter Japanese knotweed in Wisconsin and eradication is encouraged.

Unfortunately, some gardens still share this species, often mistakenly calling it "Mexican bamboo". Some people also try to establish this plant for edible purported medicinal purposes. Though it can help with inflammation, and possible aid in treating Lyme's Disease, most medical applications for this plant have proven to be false when studied rigorously. Really, for those die-hard herbalists, there is really no need to establish it on their own property,  wild populations in our area are frequent enough that they could supply an enormous demand.

Locally, we have a massive infestation of Japanese knotweed along the Southwest Commuter bike path in Madison, some colonies along Starkweather Creek, rural roadsides in Fitchburg, various small patches in the parks of Shorewood Hills (and getting smaller each year due to Good Oak's efforts), along the Military Ridge trail at the McKee Rd./Verona Rd. intersection, between Old Middleton Road and the railroad line immediately west of the underpass/on-ramp onto University Ave... and many, many other places. Still, several dozen colonies is better than several thousand, and aside from along the Southwest Commuter path, these patches are all less than an acre in size.

So be diligent, if you find this massive weed on your property, get it under control! If you find it on public ground contact your city/town officials to report it as many municipalities, including Madison and Shorewood Hills, are making an effort to keep knotweed in check before it can spread further and cause damage to infrastructure.