Tangled up in knotweed: the need for adaptive management

by Carolyn Bernhardt

September 10, 2024

a man stands in front of a tall, wide overgrowth of blooming knotweed
Knotweeds are shrub-like invasive plants with semi-woody stems and can grow up to 9 feet tall. Image courtesy of Alan Smith.

 

In various pockets across the state, a sneaky invader has rooted itself into Minnesota’s landscape, breaking into buildings, and eroding shorelines. Knotweeds, including Japanese knotweed (Polygonum cuspidatum), giant knotweed (P. sachalinense), and their hybrid bohemian knotweed (Polygonum x bohemica), are hardy environmental troublemakers. 

A research project funded by the Minnesota Invasive Terrestrial Plants and Pests Center (MITPPC) unveiled new insights into the knotweeds problem in Minnesota and developed practical advice for land managers and homeowners.

Not your typical garden weed

“Knotweeds are early colonizers after volcanoes in their native range, capable of breaking up volcanic rocks so that other plants can start to move in,” says Monika Chandler, noxious weed coordinator at the Minnesota Department of Agriculture (MDA), and collaborator on the MITPPC project. “When you put them near concrete, they take advantage of weaknesses and cracks. In the invasive plant world, knotweeds are in a class of their own for how destructive they can be."

invasive knotweed grows out from behind siding on a house
Knotweed is capable of growing through structures, like the siding of a house. Credit: Minnesota Department of Agriculture
knotweed emerges from the base of a support column in a basement
Knotweed emerges from the base of a support column in a basement. Credit: Minnesota Department of Agriculture

Alan Smith, PhD, a professor in the Department of Horticultural Science at the University of Minnesota and principal investigator on the project, says knotweeds’ rhizomes (continuously growing horizontal stems underground) grow thick like tree branches that are difficult to remove and resistant to control.

Research uncovers breadth of invasion

The MITPPC research initiative, supported by the Environment and Natural Resources Trust Fund (ENRTF), came from a growing realization that Minnesota's knotweeds invasions were far more complex than experts initially understood. By 2014, the MDA learned about unexpected spread of knotweeds in Duluth, leading to concerns that the plants were reproducing through seed dispersal rather than through rhizome fragments alone. In 2016, a survey in Duluth revealed that not only were hybrids of knotweed species present, but these hybrids exhibited a mix of genetic traits and high seed production. 

In response, the MDA collaborated with Smith, as well as Lori Seele, Forest Specialist with the 1854 Treaty Authority, formerly with the Duluth Collaborative Invasive Species Management Area, and Roger Becker, Extension Weed Scientist in the Department of Agronomy and Plant Genetics, to address unanswered questions and develop science-based management strategies.

pink flowers of P. cuspidatum var. compacta
P. cuspidatum var. compacta. Credit: Alan Smith

Researchers uncovered how widespread knotweeds are in Minnesota. They identified the presence of four knotweed species, including one previously unreported type (P. cuspidatum var. compacta). This confirmed that Minnesota’s knotweed infestation is due to a mix of several species, rather than just one. "We discovered that Bohemian knotweed is a hybrid complex that has been formed through hybridizations of all three of the now distinct species,” Smith says. “Japanese, giant, and compacta knotweed all contributed to the hybrid knotweed swarm in Minnesota and North America."

knotweed seeds lined up in a row from largest to smallest with a ruler
Variation of morphology of knotweed seeds. Credit: Alan Smith

This level of genetic diversity presented the team with a few problems. First, the potential for hybridization among species creates even more resilient knotweed species. Knotweeds are dioecious, meaning they have male and female flowers on separate plants. This type of hybridization not only increases genetic diversity, but increases the possibility for development of herbicide resistance. 

someone stands next to a line of uprooted knotweed root balls
Resarchers prepared root cuttings for planting and evaluation. Credit: Roger Becker

Second, each species and hybrid of knotweed has distinct traits, such as varying growth patterns, reproductive strategies, and responses to control methods. This leads to unpredictable and varied outcomes of management tactics. Repeating a single control strategy for all knotweeds does not work in the long-run.

Seele emphasizes the broader impact of these findings in a state full of water bodies like Minnesota. "Knotweeds are notorious for damaging stream bank stability,” she says. Their root systems carve up the soil along riverbanks and shorelines. She warns that knotweeds dominating Minnesota’s shorelines can increase sedimentation and turbidity in tributaries to St. Louis River and Lake Superior. Cascade effects from these outcomes include more erosion and sedimentation, altered aquatic habitats, nutrient run-off, and reduced water quality.

The need for collaboration and adaptive management strategies

In 2020, the MDA updated regulatory measures to add the bohemian hybrid to the listing of Japanese and giant knotweeds as prohibited control species in Minnesota. This enforces strict guidelines to prevent the sale, planting, and spread of knotweeds. Regulation has been an important step forward for the state, but more action is needed. This includes educating land managers and property owners about knotweed identification and management, as well as controlling existing populations.

a person digs near a knotweed infestation with a shed and cars in the background
Knotweed infestation. Credit: Alan Smith

This expert team of collaborators concluded that land managers must develop and implement more tailored strategies that are adapted to the current local conditions. This would require a greater state-wide allocation of resources and expertise. Becker says that knowing the current genetic and geographic diversity, as well as the plant species’ various ways of reproducing, should signal some alarm bells so that “more coordinated efforts be put toward control."

Researchers recommend land managers focus on preventing seed-based dispersal, and refine herbicide applications based on local conditions. These approaches are crucial to keep pace with the evolving nature of knotweed infestations.

Continued collaboration with local and international partners are also essential to share insights and improve management practices. "The ripple effects from everyone’s talents, risk-taking, and sweat equity, have helped us leverage resources and address a landscape-scale problem," says Seele. She calls for increased community engagement with tribal nations, land stewards, and citizens. 

"While there is conceptual support for landscape-scale stewardship, resource managers face big challenges when it comes to operationalizing, funding, and sustaining collaboration at scale. Respect, reciprocity, creativity, and cultural sensitivity are needed to meet communities where they are and learn how to connect with them best. We need everyone, including decision-makers, to bring funding, talents, and other resources to prevent the spread of tenacious knotweeds."

More information 

Managing Japanese knotweed, Minnesota Department of Agriculture

Non-native knotweeds, Minnesota Department of Natural Resources

Effective Management of Knotweed in the Midwest, a webinar by Dr. Roger Becker and Dr. Mark Renz, 54 minutes

Report sightings of knotweed to EDDMapS.org or EDDMapS mobile app 

Management of invasive knotweeds, research project


Research from the Minnesota Invasive Terrestrial Plants and Pests Center is supported by the Environment and Natural Resources Trust Fund, as recommended by the Legislative-Citizen Commission on Minnesota Resources.

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