Invaders In Your Neck Of The Woods

Three Japanese Invaders

By Lawanda Jungwirth

Our country’s earliest invasive plants originated mostly in Europe as our ancestors brought them from their homelands to grow in the new world.  Today, invaders come from the world over as they are imported intentionally as nursery stock to decorate our landscapes or are inadvertently in put soil along with those landscape plants.

Three of today’s worst invaders originated in Japan.  One of the three is already pervasive in Wisconsin, the second is big trouble in just over half of our counties, and the third is knocking at the door of our southern border.

Japanese Knotweed is a monster and it’s all over Wisconsin in riparian and wetland areas in sun or shade.  It’s a very large plant; growing up to ten feet tall with roots up to six feet deep.  Horizontally growing underground stems called rhizomes spread up to 65 feet in each direction from the parent plant sprouting along their length.  Remember  from Geometry class?  [Area of a circle equals pi (3.14159265359) times radius squared].  Anyway, using that formula, one plant can cover 13,267 square feet.  That’s almost a third of an acre!

The erect, arching stems resemble bamboo.  Stems are round, smooth and hollow with reddish-brown blotches.  You’re most likely to notice knotweed in August and September when huge upright plumes of creamy white or light green flowers bloom from the ends of the stems.

Japanese knotweed prevents streamside tree regeneration, increases soil erosion and interrupts nutrient cycling (look it up if you aren’t sure what that means).  It spreads easily.  Small pieces of root washed downstream can re-sprout into whole new plants.  It also spreads by seed.  The shiny black seeds have wings that help them float downstream.

Besides its intimidating size that physically crowds out native plants by stealing light, nutrients, water and ground space, knotweed’s roots emit chemicals that are toxic to surrounding vegetation.  It’s a real bully.

What can be done?  Small infestations can be pulled by hand when soil is moist and loose, making sure to get every piece of root.  Persistent cutting (for several years) can also control small patches.  With either method, make sure every pulled or cut piece is collected, bagged and landfilled.

Herbicide treatment works best if plants are cut and treated when they are 4-5 feet tall and treated again when the regrowth is 3 feet tall.  For detailed information on using herbicides on knotweed, get information from the DNR at

Japanese knotweed is Restricted by Wisconsin’s DNR.


Japanese Hedgeparsley is a problem in an area that covers a wide swath from extreme northeast Wisconsin down the east side of the state and across the southern counties.  It looks like a lot of other white-flowering plants, being similar to Queen Anne’s Lace, wild chervil, caraway, poison hemlock, and other hedgeparsleys.  The others are all invasive, or likely will become so in the future.  So, mistakenly eradicating any of them will be a positive.

Japanese hedgeparsley is a biennial plant.  In its first year, it develops finely divided fern-like leaves that hug the ground in a rosette.  The leaves remain green under the snow.  The second year, tiny white flowers in loose flat-topped clusters bloom in mid to late summer.  Each flower produces a pair of bristle-covered fruits that attach to fur or clothing to travel to new locations where they plant themselves.

Hedgeparsley invades forest edges, fields, fencerows, roadsides and disturbed areas.  It doesn’t much care whether its seeds land in sun or shade as they easily germinate anywhere.

Non-chemical control isn’t too difficult.  If the soil is loose, young plants can be pulled.  Older plants can be cut or mowed while flowering.  If any brown seeds are present, the cut material should be bagged and landfilled to avoid spreading seeds.

For information on chemical herbicide treatment of Japanese hedgeparsley, see:

The DNR lists hedgeparsley as Restricted in Adams, Brown, Calumet, Columbia, Crawford, Dane, Dodge, Door, Fond du Lac, Grant, Green, Green Lake, Iowa, Jefferson, Juneau, Kenosha, Kewaunee, La Crosse, Lafayette, Langlade, Manitowoc, Marathon, Marinette, Marquette, Menominee, Milwaukee, Monroe, Oconto, Outagamie, Ozaukee, Portage, Racine, Richland, Rock, Sauk, Shawano, Sheboygan, Vernon, Walworth, Washington, Waukesha, Waupaca, Waushara, and Winnebago counties.  It is Prohibited in all other counties.

As far as we know, Japanese Stilt Grass hasn’t entered Wisconsin, but it will soon follow in the footsteps of many other invaders that have crept northward from our neighbors to the south.  It first came to the United States in packing material for porcelain that arrived in Tennessee in 1919.  It didn’t begin its northward progression for many years, but now it’s racing toward us like wildfire.

Stilt grass looks like a tiny bamboo plant, growing only 12-24 inches tall.  It has smooth 2-3” pale green, lance-shaped leaves with a silvery mid-vein reminiscent of willow leaves.  Leaves turn purple in fall.

Stilt grass is taking over wetlands, streambanks, ditches, forests and floodplains.  It out-competes native plants wherever it grows, whether in sun or shade. The plants are prolific seed producers – 1,000 per plant – and they remain viable for 3-5 years.  Seeds are transported by water, in soil and gravel, and on the feet of animals and people.

Shallow fibrous roots make stilt grass relatively easy to pull.  It can also be cut at peak bloom in mid-September, before seeds form.  Chemical herbicides for grasses will control stilt grass.  Follow label instructions exactly.

Japanese stilt grass is Prohibited by Wisconsin’s DNR.

If you see Japanese stilt grass in Wisconsin, the DNR wants to know about it.  Find information about how to report it at or call (608) 267-5066.



Restricted:  It is illegal to transport, transfer, or introduce the plant without a permit.  You are strongly encouraged to eradicate restricted plants from your property.

Prohibited:  It is illegal to possess, transport, transfer, or introduce the plant without a permit.  You are required to eradicate prohibited plants from your property.


Leave a Reply

Your email address will not be published. Required fields are marked *