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Ask Mr. Smarty Plants is a free service provided by the staff and volunteers at the Lady Bird Johnson Wildflower Center.

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Saturday - August 10, 2013

From: Chippewa Falls, WI
Region: Midwest
Topic: Invasive Plants, Shade Tolerant
Title: Plants to replace Polygonum cuspidatum ( Japanese knotweed)
Answered by: Nan Hampton

QUESTION:

I live in a heavily wooed area of Chippewa Falls, WI. Our property is covered with Giant Japanese Knot Weed. We have been trying to get rid of it for years. We are finally going to try using the dreaded, Round UP, after trying many natural techniques and solutions over the years to no avail. Even covering them up with dark AND clear plastic sheets hasn't worked. We are wondering what sort of fast growing/spreading ground cover we could plant to try to crowd out what may survive of the knot weed once we use the chemicals. What might we seed the area with that would be native to our area? We have deep shaded wooded areas, with a few depressions of standing water with hard rains. Also have shady and sunny areas up next to the house where the knot weed is prolific. Thank you for any suggestions.

ANSWER:

Polygonum cuspidatum [synonym=Fallopia japonica] (Japanese knotweed) appears on the Plant Conservation Alliance's (PCA) Alien Plant Working Group's LEAST WANTED list.  Please read their "Management Options" sections for control methods.  The USDA Plants Database has a distribution map for North America plus a list of states (under "Legal Status") where it is considered prohibited, banned, invasive or noxious.  We do understand you have a serious problem here.  There is an Invasive Plants Association of Wisconsin that has a website with a great deal of useful information, including a list of Regional and County Groups that deal with invasives in Wisconsin.

Now, with that said, finding species that will "outcompete" this very tough plant (according to the PCA's LEAST WANTED web page "can tolerate a variety of adverse conditions including full shade, high temperatures, high salinity, and drought") is not going to be an easy task.   We can recommend species that will do well in the conditions you give for your site, but you are going to have to continue to be vigilant and deligent in removing/destroying the knotweed as it reappears.  Sorry to deliver that bad news—but you probably already realized that was going to be the case.

Let's start with suggestions for the deep-shaded wooded areas with moist soil:

Arctostaphylos uva-ursi (Kinnikinnick) is a trailing evergreen shrub that makes a good groundcover in shady areas.  It will also grow in sun and part shade.

Adiantum pedatum (Northern maidenhair) and Athyrium filix-femina (Common ladyfern) are deciduous ferns.

Cornus canadensis (Bunchberry dogwood) makes an excellent low groundcover in shady woods.

Dryopteris cristata (Crested woodfern) is a partially evergreen fern that grows in sun, part shade and shade.

Hydrophyllum virginianum (Eastern waterleaf) grows in part shade and shade in moist woods and is labeled as an "aggressive grower"—a good candidate for competing with the Japanese knotweed.  Here are more photos and information from Robert W. Freckman Herbarium University of Wisconsin.

Polystichum acrostichoides (Christmas fern) is an evergreen, clumping fern.

Chasmanthium latifolium (Inland sea oats) is one of the few grasses that grow well in shade.  It is said to be aggressive in spreading.  Here is more information from Missouri Botanical Garden.

Carex plantaginea (Plantainleaf sedge) is an evergreen grass-like plant.  Here is more information from Illinois Wildflowers.

Lycopodium digitatum (Fan clubmoss) is an evergreen, low growth plant with a spreading habit.  Here is more information from Illinois Wildflowers.

You can search for more possibilities by doing a COMBINATION SEARCH in our Native Plant Database.  Choose "Wisconsin" from Select State or Province, "Shade" from Light Requirement, "Moist" from Soil Moisture and both "0-1 ft" and "1-3 ft" from Size Characteristics.  You, of course, can change or add characteristics for your search.  For instance, choose "Sun" from Light Requirement for the areas near your house.

Prairie Nursery in Westfield WI has Native Ground Covers for sale as well as other native plants.  Prairie Moon Nursery in neighboring Minnesota has native seeds and seed mixes for sale (e.g., Shady Woodland Seed Mix for Wet Mesic to Dry Mesic Soils).  You can find more nurseries and seed companies that specialize in native plants in your area by search in our National Suppliers Directory.

 

From the Image Gallery


Kinnikinnick
Arctostaphylos uva-ursi

Northern maidenhair fern
Adiantum pedatum

Common lady fern
Athyrium filix-femina

Bunchberry dogwood
Cornus canadensis

Crested woodfern
Dryopteris cristata

Virginia waterleaf
Hydrophyllum virginianum

Christmas fern
Polystichum acrostichoides

Inland sea oats
Chasmanthium latifolium

Plantainleaf sedge
Carex plantaginea

Fan clubmoss
Lycopodium digitatum

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