Knotweeds

Identification and Management Prioritization

Minnesota Department of Agriculture

November 23, 2020

The Problem With Knotweeds

Knotweeds spread rapidly, taking over habitats and out-competing native plants, and reduce ecosystem functions. They block streams, access to areas, and can grow through cracks in pavement and building foundations necessitating costly removal and repairs. Knotweeds spread by expanding root systems, seeds and through vegetative means. Pieces of the plant can form new infestations when they’re broken, cut, or dug up and moved. Preventing the spread of knotweed by not moving plants is key to control.

Knotweeds were introduced to North American in the late 1800's as ornamental garden plants and began to be recognized as problematic in the early 1900's. Japanese knotweed is native to China, Japan, Korea, and Taiwan, and Giant knotweed is native to Japan. Bohemian knotweed is a hybrid of giant knotweed and Japanese knotweed. It has been documented in Japan and is common in North America. In their native range, knotweeds are early colonizers after volcanoes with shoots pushing through volcanic rock.

In Minnesota, all three species are listed as Prohibited Control species, meaning that efforts must be made to prevent the spread, maturation and dispersal of any propagating parts. Additionally, propagation, sale, or transportation of these species is prohibited.

Knotweed PSA (Duluth CISMA)

Identification

Knotweeds are shrub-like, fast-growing herbaceous perennial plants that can grow 10 feet tall. Plants form large, dense stands. Shoots die back to the ground in the fall after a hard frost and new shoots emerge in the spring.

Much of the knotweed present in Minnesota is Bohemian knotweed, although it's not imperative to recognize the differences between species - all of them have the same basic characteristics.

Comparing Knotweeds StoryMap

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Japanese Knotweed

Polygonum cuspidatum

Bohemian Knotweed

Polygonum x bohemicum

Giant Knotweed

Polygonum sachalinese

Compact Japanese Knotweed

Polygonum cuspidatum var. compacta

Graphic illustration of flowering knotweed plants.

Stem

Shoots are hollow, segmented, and look similar to bamboo.

Bohemian knotweed stalks — Cluster of knotweed stems.

Stems turn reddish-brown after frost and often remain standing in winter.

Red knotweed stems in winter. — Knotweed stems in winter.

Graphic illustration of flowering knotweed plants with stem circled.

Leaves

Leaves are arranged alternately and are heart-shaped to oval with a pointed tip.

Compact Japanese knotweed leaf next to ruler — Compact Japanese knotweed leaf

Japanese knotweed leaf next to a ruler — Japanese knotweed leaf

Bohemian knotweed leaf next to a ruler — Bohemian knotweed leaf

Giant knotweed leaf next to a ruler — Giant knotweed leaf

Graphic illustration of flowering knotweed plants with leaf circled.

Rhizome

Rhizomes (underground stems) can spread outwards to 65 feet. New shoots can grow from nodes along the rhizomes.

Knotweed root crown that has been dug out of the ground. — Knotweed root crown with rhizomes and buds.

Knotweed sprouting from rhizome fragment.

Knotweed sprouting from the ground — Knotweed shoot.

Flower

Knotweeds produce lacy clusters of white flowers in late summer.

flowering Bohemian knotweed plants. — Bohemian knotweed plants with many flowers in bloom.

Distribution

Current Distribution Model

Point data collected from EDDMapS; not all locations in the state have been recorded, but points on distribution map have been reported and verified.
Distribution model takes location data and environmental factors into account to create a prediction of current distribution.

Map legend showing points as locations where species has been reported, green shading as a high likelihood of finding the species, and white shading as a low likelihood of finding the species.

Management Prioritization

Influenced by:

distribution models
actual reports
economic benefits
land cover
conservation features
trails

High priority areas surround areas of high density to prevent further large infestations and loss of biodiversity.

Map legend showing yellow as a high priority, green as a medium priority, blue as a low priority, and purple as the lowest priority for management.

Infestation Examples

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General Infested Area

Knotweed dominates the ecosystem and other biodiversity is difficult to find. Management here is involved and costly.

Leading Edge

Knotweed moving from an area of high density to low density. Management hear prevents further spread to un-invaded areas.

Management is involved but can prevent further spread or large establishment of stands.

Isolated Population

Location far from other known populations.

Management here is efficient and cost effective.

Lifecycle: Treatment Timing

Download Knotweed Lifecycle PDF

View Additional Species StoryMaps

Logos for ENRRF, State of Minnesota, University of Minnesota - Extension, MNLA, and MACAI

Funding for this project was provided by the Minnesota Environment and Natural Resources Trust Fund as recommended by the Legislative-Citizen Commission on Minnesota Resources (LCCMR). The Trust Fund is a permanent fund constitutionally established by the citizens of Minnesota to assist in the protection, conservation, preservation, and enhancement of the state’s air, water, land, fish, wildlife, and other natural resources. Currently 40% of net Minnesota State Lottery proceeds are dedicated to growing the Trust Fund and ensuring future benefits for Minnesota’s environment and natural resources.