Knotweed identification and control

Marianna SzucsAssistant Professor

Michigan State Universityszucsmar@msu.edu

Phone: 517-353-7063

14 Nov 2019

Worldwide distribution – listed in the 100 worst invasive species

• Introduced as ornamental and for erosion control in late 1800s to NA

The knotweed species complex

Japanese BohemianGiant

Photos: F. Grevstad, Oregon State U.

Japanese knotweed (Fallopia japonica/Polygonum cuspidatum)Giant knotweed (Fallopia sachalinensis)Bohemian knotweed (Fallopia x bohemica)

https://www.eddmaps.org/distribution/usstate.cfm?sub=19655

Japanese knotweed

https://www.eddmaps.org/distribution/uscounty.cfm?sub=19655

Giant knotweed

Bohemian knotweed

https://www.eddmaps.org/distribution/uscounty.cfm?sub=59459

Distribution in Michigan

Japanese Bohemian Giant

Knotweed over the seasons

https://japaneseknotweedspecialists.com/what-is-japanese-knotweed/

Knotweed over the seasons

https://japaneseknotweedspecialists.com/what-is-japanese-knotweed/

Characteristics of knotweeds

• herbaceous perennials, can live for decades• grow 3.3 – 10 feet tall (can grow 2-4’’/day)• leaves alternate, leathery, oval• flower in Aug/Sept• reproduction by seeds but more often vegetatively by

cuttings and rhizomes• underground stems extending up

to 65 feet laterally

Photo: R. Shaw, CABI

Characteristics of knotweeds

• a) rhizomes producing new shoots• b) hollow, bamboo-like stem• c) flowers with petal-like sepals• d) fruit

Photos: Ohio State Weed Lab OSU, L. Mehrhoff, A. Rockstein, K. ChamberlainGrevstad et al. 2018

Japanese vs. Giant knotweed

Japanese Japanese

Japanese – no hairsGIANT GIANT GIANT – with hairs

Grevstad et al. 2018

Japanese – squared off base, pointy tip Giant – heart-shaped base, tapering tip

Japanese vs. Giant knotweed

Japanese – reddish stem Giant – green stem Giant Bohemian Japanese

Grevstad et al. 2018

Grevstad et al. 2018

Grevstad et al. 2018

Japanese Bohemian Giant

Leaf shape squared-off base, pointed tip

more heart-shaped lower on stems and more spear shaped at branch ends

heart-shaped at base, tapering tip, wavy margins

Leaf size 3-7'' long, 2.5‘’ wide intermediate 6-12''+ long,

4-10'' wide

Leaf underside hairless

small bumps or small, triangular hairs

long, fine, wavy hairs

Plant height up to 10' up to 13' up to 13'

Stems reddish when young, green later reddish brown always green

Flower clusters

longer than subtending leaf

same length as subtending leaf

shorter then subtending leaf

Habitat – Where to find, how it spreads

• Riparian areas – high rates of spread along waterways

• Disturbed areas - spoil heaps, alongside roads/railways

• Urban areas – gardens

• Spreads along transport corridors – mowing, snow ploughing can help spread it

• Prefers moist, open habitats

• Additional spread – contaminated soil to development sites, compost to gardens

Habitat - Urban

http://www.odonovanagri.com/Contract/index.php/contract-services-division/japanese-knotweed-treatment-eradication

http://www.japaneseknotweedireland.ie/

Habitat - Urban

Habitat – landscaping in garden

Habitat – abandoned lot

Habitat - riparian• Can impede water flow of high waters contributing to floods• Dead stems can cause blockage as swept downstream

https://www.thejapaneseknotweedcompany.com/japanese-knotweed-solutions/

https://bcinvasives.ca/invasive-species/identify/invasive-plants/knotweed

Grevstad et al. 2018

Habitat – roadsides

Randy Westbrooks Invasive Plant Control Inc. Bugwood.org

ControlVery difficult to control b/c of the large and persistent underground rhizome systemApproach/success depends on the size of the infestation

• Cultural control• goat/cattle grazing can reduce biomass – will not kill plants

• Physical/mechanical control• small infestations/small plants may be pulled/dug up• burning not recommended – underground rhizomes survive• mowing/cutting can spread plants – fragments need to be

bagged, do not compost (labor intensive)• needs to be repeated twice a month during growing season – can

reduce rhizome reserves but rarely kill plants• small infestations may be covered with light-blocking sheet

Biological control• In the native range knotweeds are attacked by at

least 186 arthropod and 40 species of fungus• Are knotweeds invasive b/c of lack of natural

enemies in the introduced range?

Biological control

• Restoring some of the balance between invasive species and their enemies

• Knotweed psyllid (Aphalara itadori, Hemiptera)• psyllids = jumping plant lice

Photo: F. Grevstad

• Tiny: size of sesame seed• Feed on plant sap – cause

twisting, curling of leaves, weakens plants

• 1 female can lay 700 eggs• Development from egg to adult

in 5-6 weeks – 2 generations/year

Biological control

Biological control

Photos by F. Grevstad

Biological control

Status: • Released in UK 2010-2013 did not establish• 2016 release used new stocks from Japan – established• Canada started field releases in 2014 in Alberta and BC

• overwintering success, no large populations present yet• US: permit for field release pending – in last stage of

process• Aug 26, 2019 public commenting period ended• Decision from USDA APHIS is expected this year• Michigan also has to approve on the state level