Red Lake Nation News - Babaamaajimowinan (Telling of news in different places)

By Mari Hardel
Minnesota Department of Agriculture 

September Weed of the Month: Knotweeds


September 3, 2018

The stem of the knotweed is hollow inside. Pictured is a Japanese knotweed plant.

Native to Asia, knotweeds were introduced to North America as an ornamental plant. They can spread into natural areas growing quickly and forming dense monocultures, reducing habitat diversity, and overtaking riverbanks and lakeshores. Additionally, knotweeds have the ability to grow through cracks in pavement or building foundations potentially leading to costly removal of the knotweed and repairs to the structure.

In Minnesota two species of knotweed have been identified: Japanese knotweed (Polygonum cuspidatum) and Bohemian knotweed (Polygonum x bohemicum). Giant knotweed (Polygonum sachalinense) has been documented nearby in Wisconsin and Michigan. All three species have very similar characteristics, can be spread by fragments of the root and stem, and are equally challenging to get rid of.

Knotweed's hollow, segmented stems look similar to bamboo and are green from spring to fall and red-brown in the winter. Leaves grow in an alternate pattern on stems and are round to oval with pointed tips and square or heart shaped leaf bases. Plants can grow over 10 feet tall and produce whitish flowers in the late summer on 2-6 inch long clusters. Some plants produce viable seed from these flowers.

Before beginning efforts to manage knotweed it is important to understand the structure of the plant and the means by which it spreads.

Knotweed could be described as an 'iceberg' as the majority of the plant mass is below ground. There is a vertical taproot and network of rhizomes (underground steams) that can spread horizontally up to 60 feet and send up new shoots.

Knotweed plants can reproduce from fragments to form new infestations. Cutting, mowing, chopping, or any method that has the potential to break and disperse pieces of the plant can lead to new infestations. If knotweed is reducing visibility or access, simply bend the stems out of the way. In addition, digging up established knotweeds is not recommended due to the extensive root system and potential for soil contaminated with small pieces of roots to be spread. The deep, extensive root system and the ease by which knotweed can spread make it notoriously difficult to get rid of.

Managing knotweed with a foliar herbicide spray in the late summer or early fall has been shown to be most effective. As with any herbicide treatment, using the proper chemical at the right concentration and right time of year is key to effectivity. Injecting the stem with herbicide has also been effective but is very time consuming and recommended only for small stands of knotweed. Doing some research to learn which herbicides have been most effective for knotweed management could save you time and money in the long run. Below are links to some useful online tools:

• Minnesota Department of Agriculture knotweed brochure

• University of Wisconsin fact sheet

A large Bohemian knotweed plant dwarfs a garage.

• Wisconsin First Detector Network management video

Keep in mind that eradication of knotweed is difficult and preventing spread is essential. It will likely take multiple years of treatment to eliminate a knotweed population. In addition, knotweed can seem to disappear for a few years only to send up new shoots years after management has ended. Finally, with the far spreading underground stem system, a plant may respond to treatment in one location by sending up shoots further away. It is wise to monitor the area for many years to ensure proper management of knotweeds.

The Minnesota Department of Agriculture is collecting information on public knowledge of knotweeds. Please take a moment to complete this survey (


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