Japanese Knotweed–a fearsome plant
In the world of weeds, there are few that are the equal of Japanese knotweed (Polygonum cuspidatum). By any measure: aggressiveness, economic impact, and near impossibility of control, Japanese knotweed is a fearsome invasive plant .
The excellent book “Invasive Plants of the Upper Midwest” (Czarapata, 2005 , p 73-75) describes Japanese knotweed thus:
“Japanese knotweed spreads rapidly by rhizomes forming large, dense thickets that eliminate native vegetation and wildlife habitat. The rhizomes are strong enough to penetrate pavement. Knotweed thickets are particularly problematic along waterways . . . It is also one of the most troublesome weeds along railroad rights-of-way because it becomes a fire hazard during the dormant season. Once established, Japanese knotweed stands are extremely difficult to eradicate.” “Established” means it’s been growing in one area for years and is expanding the patch size by moving into new areas.
The plant is such a concern that Science magazine published an article on the plant’s economic impact in England, and the Wisconsin Department of Natural Resources (WIDNR) has developed a control options fact sheet. The upshot is that there are few viable control methods that work at the large-scale of infestation that we see along the SW Bike Path.
Japanese knotweed likes to grow along riverbanks, pond edges, low-lying areas, woodland edges, storm water channels, and in yards. You probably would not want this plant to show up in your backyard. Yet, that is just what is happening to some residential backyards along the SW Bike Path as the weed creeps in from its extensive hillside thickets.
David Thompson’s blog “Saveourstream” describes the Japanese knotweed situation along the SW Bike Path as a “BIG” problem. There is an established infestation extending about 1.5 miles from Allen Street to Parman Terrace. If you use the bike path often you have probably noticed the extensive stands just east of Glenway along the backside of the Glenway Golf Course, the Forest Hill Cemetery and the adjacent woods. What you can’t usually see is the Japanese knotweed growing in the storm water channel below the bike path. In this stretch, garlic mustard can barely find elbow room.
Let’s evaluate this situation and one possible scenario for potential downstream impact. Japanese knotweed rhizomes and broken stem pieces are moved by water–storm water will do. Where does the SW Bike Path storm water end up? In Ho-Nee-Um Pond at Arbor Drive/Monroe Street, into the Arboretum and on to the Lake Wingra shoreline and wetlands. Japanese knotweed populations have not yet established here—that we know of—but the potential consequences for our Lake are enormous.
For established stands like the one along the SW Bike Path, there are two viable options according to Czarapata (2005, p 74) (and pulling out the roots is not one of them.) The first option is repeated cutting of at least three times per season for several growing seasons. This mechanical control technique has been reported to result in a modest reduction of stem density, depending upon the situation.
The second option is herbicide application. An herbicide may be either be applied to the leaves, cut stems, or injected into the living stem. Results vary depending upon the herbicide used, timing of application, and the nature of the infestation.
What’s Being Done Locally?
The City of Madison Engineering hired a contractor (NES Ecological Services) last August to control Japanese knotweed along the SW Bike Path. You may have noticed the clear-cutting of Japanese knotweed’s bamboo-like stems. The contractor used various means to cut all, or most, of the stems. There is very little evidence that any herbicide was applied to the foliage, cut stems, or as an injection.
The first few weeks of this growing season (shown below) are that the root crowns of most plants survived last summer’s cutting. Plus, each cut stem has produced at least two new re-sprouted stems. Right now, the situation is at least twice as bad as it was before control work was begun. We do not know if the contractor plans to return for more control efforts this spring or not. Stay tuned.
The apparent, and possibly unrealistic, goal of the City and contractor is eradication of Japanese knotweed. The apparent management strategy was a mass assault, a clear-cut to crush the plant into submission.
An alternative, achievable goal in this situation would be to contain the population within current boundaries and prevent the spread of the plant to new areas by first attacking the outlying patches of Japanese knotweed, say the ones at either end of the bike path and those that have sprung up in back yards. When the expansion is halted and the population stabilized, the managers can continue to tighten the noose. This small-scale approach could achieve the desired outcome of preventing the problem from becoming worse, but would need lots of hand work, persistence, and on site supervision. Volunteers could be recruited to tackle small patches in their section of the bike path. This volunteer efforts would help the City supplement its limited resources.
This small-scale approach would also let the City build upon a record of successful pest plant management and, would have the added benefits of education, outreach, and public engagement.