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SLELO PRISM

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SLELO PRISM

ST. LAWRENCE EASTERN LAKE ONTARIO PARTNERSHIP FOR REGIONAL INVASIVE SPECIES MANAGEMENT

Invasive Knotweed

Invasive knotweed (Reynoutria japonica Houtt. var. japonica) is a perennial herb with bamboo-like stems. It typically grows in thickets 3-6 feet tall but can reach as high as 15 feet. Native to Asia, Japanese knotweed was introduced to North America in the late 19th century and now can be found in most states and Canadian provinces, including Alaska.

The plant is an aggressive riparian invader but can also thrive in wetlands, and along roadsides and other disturbed areas. Once established, the plant’s long rhizomes allow it spread rapidly and it can easily create a monoculture. Its extreme tolerance to deep shade, high temperatures, saline soils, and drought conditions aids in its adaptability.

Invasive knotweed spreads through the dispersal of rhizomes and broken plant fragments. Care should be taken when fishing or enjoying other activities along waterways with invasive knotweed present as plant stems can easily break off and land in the water and be spread to new areas. 

Impacts

Invasive knotweed has a long history of serious invasive impacts in Europe and it is now regarded as the most pernicious weed in the United Kingdom. Impacts in North America will likely mirror those of the UK.

Knotweed spreads quickly to form dense thickets that exclude native species, reducing species diversity and diminishing an area’s value to wildlife, especially phytophagous (plant-eating) insects and species that rely on them.

All of the dead stems and leaves can create a fire hazard during the dormant season. It is particularly problematic in riparian areas because it can survive floods and rapidly colonize scoured shores and islands.

Identification

Invasive knotweed is an upright, shrubby, herbaceous perennial that spreads primarily by seed, stem fragments, and by vegetative means with long, stout rhizomes. Seeds can be transported by water, wind, on people’s shoes, by animals, and as a contaminant in soil.

Height: 10 to 15 feet in height.

Stems: smooth, stout, swollen where the leaf meets the stem. Membranous sheath surrounds joints of the stem.

Leaves: normally about 6 inches long by 3 to 4 inches wide, alternating on the stem, broadly oval to somewhat triangular or heart-shaped, pointed at the tip.

Japanese knotweed, Fallopia japonica (Polygonales: Polygonaceae) - 5205095

Flowers & fruits: small, greenish-white flowers in branched sprays in summer, followed by small winged fruits.

Japanese knotweed, Fallopia japonica (Polygonales: Polygonaceae) - 5452654

Seeds: triangular, shiny, very small, about 1/10 inch long.

Control/Management

Mechanical control: Manual removal of established knotweed plants is usually ineffective due to the easily fragmented rhizomes. Methods such as cutting, mowing, and pulling are most effective for small or environmentally sensitive areas where herbicides cannot be used. 

It’s best to wait for flowering, which occurs in the summer, to cut knotweed to better deplete the stored energy in the rhizomes. Care should be taken as even a small piece of stem will regrow wherever it touches the soil, all plant material must be removed from the site and properly disposed of to prevent re-establishment or spread to other sites.

Cut stem treatment: In early fall, stems should be cut about 2 in. above the ground and followed immediately by application of glyphosate or triclopyr to the cut stem. This treatment is best when Japanese knotweed is growing mixed with or near other species.
Foliar treatment: A foliar application of glyphosate can be used to control large populations, but multiple treatments may be required. It is most effective to spray in late summer or early fall after cutting the stems in late spring or early summer. Note: Care must be taken when using chemical treatment near water courses.

Chemical Control: 

Cut stem treatment: In early fall, stems should be cut about 2 inches above the ground and followed immediately by the application of glyphosate or triclopyr to the cut stem. This treatment is best when the knotweed is growing mixed with or near other species.

Stem injection: Using an injection gun herbicide can be directly injected into the knotweed stem. This method can be more time-consuming but allows the herbicide to be specifically calibrated for the size and spread of the plant, and doesn’t require the removal of plant fragments as with the cut-stem approach.  Furthermore, given that knotweed stems are hollow, herbicide injected into the plant will reach the roots and rhizome of the plant very quickly. 

Foliar treatment: A foliar application of glyphosate can be used to control large populations, but multiple treatments may be required. It is most effective to spray in late summer or early fall after cutting the stems in late spring or early summer.

Note: Permits may be needed and care must be taken when using chemical treatment near water.

Resources:

Mistakes to Avoid While Controlling Knotweed

Homeowners Guide to Japanese Knotweed Control

Oneida County CCE Japanese Knotweed Fact Sheet

Photo Credits: Title photo: Emma Erler bugwood.org. Leaves photo: Jan Samanek, State Phytosanitary Administration, Bugwood.org. Flower photo: Leslie J. Mehrhoff, University of Connecticut, Bugwood.org. Seeds photo: USDA PLANTS Database, USDA NRCS PLANTS Database, Bugwood.org.

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PREVENTION
Prevent the introduction of invasive species into the SLELO PRISM.

EARLY DETECTION & RAPID RESPONSE
Rapidly detect new and recent invaders and eliminate all individuals within a specific area.

COOPERATION
Share resources, including funding personnel, equipment, information, and expertise.

INFORMATION MANAGEMENT
Collect, utilize, and share information regarding surveys, infestations, control methods, monitoring, and research.

CONTROL
Control invasive species infestations by using best management practices, methods and techniques to include: ERADICATION (which is to eliminate all individuals and the seed bank from an area), CONTAINMENT (which is reducing the spread of established infestations from entering an uninfested area) and SUPPRESSION which is to reduce the density but not necessarily the total infested area.

RESTORATION
Develop and implement effective restoration methods for areas that have been degraded by invasive species and where suppression or control has taken place.

EDUCATION & OUTREACH
Increase public awareness and understanding of invasive species.

INNOVATION
Develop and implement innovative technologies that help us to better understand, visualize, alleviate or manage invasive species and their impacts or that serve to strengthen ecosystem function and/or processes.

Rob Williams                              rwilliams@tnc.org                     Program Director

Megan Pistolese megan.pistolese@tnc.org
Outreach and Education

Brittney Rogers brittney.rogers@tnc.org 
Aquatic Invasive Species

Robert Smith       robert.l.smith@tnc.org 
Terrestrial Invasive Species

Zachary Simek    zachary.simek@TNC.ORG     Conservation and GIS Analyst

During this time the best way to contact our team is via email.

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