Bohemian Knotweed

Common name: Bohemian knotweed
Scientific name: Polygonum x bohemicum (J. Chrtek & Chrtková) Zika & Jacobson [also Reynoutria x bohemica Chrtek & Chrtková, also Fallopia x bohemica (Chrtek & Chrtková) J.P. Bailey]
Synonyms: Japanese bamboo, Mexican bamboo, Japanese fleece flower

Legal status

Prohibited Control

Efforts must be made to prevent seed maturation and dispersal of plants into new areas. Additionally, no transportation, propagation, or sale of these plants is allowed. Failure to comply may result in enforcement action by the county or local municipality. 

Background

Knotweeds were introduced to North American in the late 1800s as ornamental garden plants and began to be recognized as problematic in the early 1900s. Giant knotweed hybridizes with Japanese knotweed to form the Bohemian knotweed species. Much of the knotweed present in Minnesota is Bohemian knotweed. In their native range, knotweeds are early colonizers after volcanoes with shoots pushing through volcanic rock.  Similarly, shoots can grow through pavement and building foundations necessitating costly removal and repairs.

Description

• Knotweeds are shrub-like, fast-growing herbaceous perennial plants that can grow 10 feet tall.
• Shoots are hollow and segmented, leaves are arranged alternately and heart-shaped to oval with a pointed tip.
• Plants form large, dense stands. Rhizomes (underground stems) can spread outwards to 65 feet.
• New shoots can grow from nodes along the rhizomes. Shoots die back to the ground in the fall after a hard frost and new shoots emerge in the spring.
• Knotweeds produce lacy clusters of white flowers in late summer.  

Habitat

Bohemian knotweed is often found in sunny areas within intentionally planted landscapes, along roadsides, and in riparian areas such as stream banks. Knotweeds thrive in a range of soil types and light levels although will not be as robust if growing in shady conditions.

Means of spread and distribution

Bohemian knotweed can spread both vegetatively and by seed. Rhizomes allow knotweeds to spread quickly and aggressively and new colonies can form from very small stem or rhizome fragments. Plant parts can be moved by natural means, such as waterways, and often through human activities such as moving knotweed plant parts to new location or moving soil contaminated with rhizome fragments. 

Impact

Bohemian knotweed forms tall, dense thickets that shade out and displace native vegetation, degrade habitat for fish and wildlife, alter waterways, and facilitate erosion and flooding. Once established, it dominates native vegetation and is a significant problem in riparian areas where stream-side tree growth is greatly reduced. Research studies have also shown that giant knotweed produces allelopathic chemicals from its roots, which prevent other plant species from growing and competing with it.

Prevention and management

• Do not plant giant knotweed as an ornamental and eradicate existing plants on your property. A sound management plan is necessary to manage this species and will take a commitment of several years to ensure that the population has been eliminated. Because this is a perennial species with a vast underground network of spreading roots, management efforts should focus on eliminating root growth. Prevent soil movement containing knotweed rhizome fragments to uninfested areas. Management plans that include restoration of sites by fostering existing or newly planted site-specific native plant species can also sustain management well into the future and prevent new populations from developing.
• Foliar and stem injection application of herbicides that translocate their active ingredients into the root system can be very effective. Treatments will need to be repeated for several years to eradicate a population. If using herbicide treatments, check with your local University of Minnesota Extension agent, co-op, or certified landscape care expert for assistance and recommendations. There are several businesses throughout the state with certified herbicide applicators that can be hired to perform chemical applications. Giant knotweed also grows commonly in riparian and wetland habitats. If treating plants near water with herbicide, please be aware of the state pesticide laws and use only products labeled for aquatic use.
• Do not cut or mow knotweed which increases spread and can stimulate growth. Bending stems out of the way can be useful if knotweed blocks pathways.
• Always clean and inspect equipment after working in an infested areas to prevent transport of plant fragments and seed to new areas.
• Manual or mechanical means of control and NOT recommended.
• Knotweed lifecycle and treatment timing graphic