Japanese knotweed invades the bike path

Madison has a BIG problem with Japanese knotweed along the Southwest Bike Path.  It's a big weed--standing up to 9 feet tall--and a big patch, extending over a mile along the bikeway.  It's an invader from eastern Asia, and very aggressive.  It forms dense stands that crowd out most other vegetation.

Left: Large knotweed plants. Center: re sprouting quickly after being mowed.

A sinister plant...

A recent article in Science describes the plant as "sinister"and "a Goliath"--strong words for the usually staid magazine.  "What makes this plant so menacing is its ability to grow through solid concrete foundations, forcing contractors to abandon infested building sites.  In England alone, about a half-million homes are uninsurable, and in the UK, damages and removal cost $288 million a year."

In Japan, the weed is seldom a problem "thanks to 168 insects and 40 fungi that keep it in check."  But in Wisconsin without any enemies, knotweed has already become established in most counties.

According to DNR, knotweed defies control: "The rhizomes (horizontal roots that send up shoots) grow out to 60 feet.  In Bayfield, rhizomes have grown under streets to infest neighboring properties.

A business in the United Kingdom spent over $600,000 to replace its parking lot after extensive damage from rhizomes, and one Welsh family even found the plant invading its living room as it pushed up from beneath the floor!"

This knotweed shoot, a few inches tall, is resprouting from a dead-looking fragment of rhizome.

Biological controls--maybe

The Brits have been experimenting with a tiny plant louse* that eats knotweed--and only knotweed.  After a successful test last year, they plan to release the louse at eight sites in the UK this year. The US may follow with tests of the louse in northeastern states in 2012.  But it will be years before we know if this biological control really works.

Spread by construction projects

In Madison, knotweed infests 1.4 miles of the SW Bike Path between Allen St and Parman Terrace.  The biggest patch is just east of Glenway St., where it extends for .4 miles along the golf course woods and cemetery.  It's mostly on public property, but is starting to invade back yards.

Knotweed is also found in the Westmorland Greenway (between Gately & Chatham), and in the Hillcrest-Upland Greenway, where it was spread by the recent sewer work.

When the bike path was constructed, knotweed already present in a few yards was spread all along the path by construction equipment.

Then, when the watercourse below the woods was riprapped, knotweed was spread further along the ravine.  Seeding of native plants done by the contractor barely took, because the seed matting kept rolling back up and was never monitored or replaced.

When the City performed maintenance (using a bobcat) on the channel perpendicular to the bike path last summer, the weed was spread again.

Now the weed grows for a substantial distance along the stormwater channel, which could become an invasion route to Lake Wingra.

Left foreground: Knotweed patch in front of stormwater inlet, leading to L. Wingra.



The Bike Path Committee

Sandy Stark, Chair of the DMNA Bike Path Committee, says they have been working on the problem: "We have one small test patch going with 2+ volunteers, on a two-year schedule of timed cutting (by hand) and herbiciding, well off the path area where dogs and folks and kids wander."

"We're experimenting on knotweed control, based on the advice of Arboretum research staff--we follow their advice on when to cut and when to apply herbicide to the cut stalks. It allows a small number of folks to do the work, including hauling, and keeps a low profile until we gather results we can then take back to the city for a larger proposal."

Sandy says the DMNA Path Committee will meet later in June and come up with some strategies for selecting projects and getting city approval for knotweed projects.

The Westmorland Greenspace Committee is aware of the problem, but too busy with other projects this summer to deal with the sinister weed.  The Friends of Lake Wingra would also like to see the weed eradicated before it spreads to the lake.

Control methods vary...

...depending on the size of the patch and what other plants are nearby.

If you have just a few plants in your yard, you can probably eradicate by frequently cutting it back and applying Roundup, or covering it with a tarp.  Mowing or trying to dig out the rhizomes is ineffective, because remaining root fragments simply resprout.

For dense stands, Mark Horn advocates cutting the stalks in early summer.  The plant regrows, but in a shorter, more compact form that's easier to spray.  Then herbicide is applied in late summer.  Finally, erosion control mats have to be applied, to protect the steep slopes of the bike path from erosion while new plants take root.

For areas where knotweed is just getting established, spraying would kill existing plants like lilacs, so another method is needed.

Larger stands along the bike path will be very difficult to eradicate.  It will take a serious, coordinated, and professional effort--lasting 3-5 years. 

The City needs to take Japanese knotweed seriously.  The bike path--along with parallel stormwater channels--is a corridor for the spread of this destructive weed.  So far, City projects and maintenance have only contributed to its spread.   It's time to come up with a professional plan. 

The role of volunteers

Because this infestation large and the weed is so aggressive, the City needs to provide the backbone for a strong attack on this Goliath weed.

Once the weed has been knocked back by professional eradication, volunteers can watch for re sprouting, and do the final mop up, which will require several years of vigilance.

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Knotweed (left) is growing all along the stormwater channel.  It was spread by construction that built the sediment filter (under the manholes).



Knotweed grows to 9 feet tall.  By mid-may, it was half grown.  Some people mistake it for bamboo, since it's hollow and has segments.

Knotweed is mostly on City land on the N side of the bikeway, but it is invading some back yards next to the bike path.

Slide show of the knotweed infestation on the SW Bike Path.
DNR pamphlet on Japanese knotweed and its control.
Good description of a variety of control techniques, including tarps, here.

*  The 2-millimeter-long plant louse is also called Aphalara itadori.