Non-native knotweeds: Japanese, Bohemian, and Giant knotweed (Polygonum cuspidatum, P. x bohemicum, and P. sachalinese)

Cream flower spikes of Bohemian knotweed.
 
 

Knotweeds were introduced as ornamental plants and for erosion control. They thrive in sunny areas, especially along streambanks and roadways. There are three species of non-native knotweed that are similar in appearance. All can be difficult to control.

Description

Appearance

There are three species of non-native knotweed that are similar in appearance. They are all herbaceous shrubs that die back to the ground each winter, leaving bamboo-like stalks. Bohemian knotweed is a hybrid between giant and Japanese knotweed

Leaves and stem

Leaves are green and are attached to the stem in an alternating pattern. They are broadly oval and pointed at the tip. Leaf edges are smooth. Leaf variations between the three species are listed below:

  • Giant knotweed (P. sachalinese) has leaves that can be up to 16 inches long and have a distinctly heart-shaped base. Plants can grow up to 15 feet tall.
  • Japanese knotweed (Polygonum cuspidatum) has leaves that are about 6 inches long and 3-4 inches wide. The base of the leaf tends to be straight across and it tapers sharply at the tip. Plants can grow up to 9 feet tall.
  • Bohemian knotweed (P. x bohemicum) has leaves that are intermediate in size and shape between giant and Japanese knotweed leaves. Plants are generally taller than Japanese knotweed and shorter than giant knotweed.

Stems are reddish-brown, smooth, stout, hollow and swollen at the join where the leaf meets the stem. They look similar to bamboo. Frost-killed stems turn bronze-colored and often remain standing through the winter. In the spring, new stems re-sprout from existing roots.

Flowers

Knotweed has many tiny creamy white flowers arranged in spikes or showy drooping clusters. Flowers grow toward the ends of the stems from where leaves meet the stem and bloom in late summer. There is a small horticultural variety of Japanese knotweed called 'compacta' that can have pink flowers; this variety can grow up to 3 feet tall.

In the United States, all Japanese knotweed plants are thought to only have female flowers and so flowers do not produce pollen. Giant and Bohemian knotweed flowers can produce pollen.

Seeds and reproductive structures

Knotweeds can spread by plant fragments being moved by water or humans. Japanese knotweed plants in the United States are generally thought to all be female and so do not produce pollen. Without a pollen source, the Japanese knotweed plants do not produce seed. Giant knotweed and Bohemian knotweed both produce pollen that can fertilize Japanese knotweed plants causing the Japanese knotweed plant to produce seeds. Fruits are small (about 1/3 inch) with three wings. Seeds are triangular, dark and shiny. Seeds are typically spread by wind and water.

Roots and roots

The grass reproduces vegetatively through horizontal stems growing below the soil surface, called rhizomes, forming roots and producing new plants, eventually forming a dense mat.

Biology

Knotweed is a perennial plant that spreads primarily vegetatively to form dense thickets that suppress native vegetation. It tolerates some shade, high temperatures, high salinity, and drought, although it grows best in full sunlight where the soil has been disturbed. It can often be found along streambanks, roadways and in wetlands.

Origin and spread

Knotweeds were introduced to the U.S. in the late 1800s for ornamental purposes and erosion control. They currently grow from Maine to Minnesota and south to Louisiana, along with scattered populations in some western states. Knotweed often spreads to new sites in floodwaters – even a stem fragment only ½ inch long can become a new plant. Once established, populations are persistent and difficult to control. In Minnesota many plants reported as Japanese knotweed have been found to be Bohemian knotweed.

Refer to EDDMapS distribution maps for current distribution of giant, Japanese, and Bohemian knotweed.

Don't be fooled by these look-alikes

  • Pennsylvania smartweed, Persicaria pennsylvanica (native) – There are a number of native plants in the same plant family as the non-native knotweed. They can have pink flowers and swollen nodes on their stems. Smartweeds are much smaller than the non-native knotweeds and only grow a few feet tall. They have more flimsy stems and do not grow to be large shrubs.
  • Red berried elder, Sambucus racemosa (native) – A shrub with solid, woody stems, not hollow like knotweeds.
 
Regulatory classification

Japanese knotweed, giant knotweed and Bohemian knotweed are Minnesota Department of Agriculture Prohibited Noxious Weeds on the Control List meaning that efforts must be made to prevent the spread of seeds or other propagating parts. Additionally no transportation, propagation, or sale is allowed.

Threat to Minnesota
  • Forms dense thickets that suppress native vegetation.
  • Knotweed can pose a significant threat to riparian areas, such as disturbed streamsides, lakeshores and other low-lying areas. It can survive severe floods and rapidly colonize scoured shores. It creates bare ground under itself, which can lead to faster erosion and make stream banks more vulnerable to flood damage.
  • Knotweed can grow through building foundations and sidewalks, leading to structural damage.
What you should do

One way that invasive plant seeds and fragments can spread is in soil.  Sometimes plants are planted purposefully.  You can prevent the spread of invasive plants.

PlayCleanGo: Stop Invasive Species in Your Tracks

  • REMOVE plants, animals and mud from boots, gear, pets and vehicle.
  • CLEAN your gear before entering and leaving the recreation site.
  • STAY on designated roads and trails.
  • PLANT non-invasive species.
Native substitutes
 
Control methods

Mechanical control can be done by digging or pulling the plant by hand or with equipment such as shovels. This can be effective for small infestations in sensitive areas. Plants can resprout from stem fragments as small as ½ inch long. Mowing may spread plants to new areas and is not recommended. Do not allow plant parts to enter waterways during control efforts. Have a plan for what you will do with the pieces of plants you remove so that you do not spread knotweeds to new locations. Contact your local county agricultural inspector for disposal options. Plants can be kept on site for burning or piled and covered with a tarp for decay. Be sure to monitor the site and remove any plants that sprout from the burn or decay site. If plants must be moved off site, contact your local yard waste or compost site to see if they are equipped to compost at high enough temperatures to accept noxious weeds at their site. Transportation is only allowed to a disposal site and the MDA requires the load is protected in a manner that prevents the spread of noxious weed propagating parts during transport. It is illegal in Minnesota to dispose of plants in a landfill. See the Minnesota Department of Agriculture noxious weed disposal website for additional information.

Herbicide control is most effective when a solution of imazapyr is sprayed as a foliar spray on the leaves of the plant. Glyphosate or triclopyr are other herbicides that can be used. These are systemic herbicides that are taken up by plants and move within the plant, which can kill leaves, stems, and roots. See the Minnesota Department of Agriculture knotweed brochure for more detail. Herbicide treatments should be done in late summer early fall and can coincide with flowering. Follow-up treatments will also be necessary over the next couple of years. Do not cut down treated plants for at least one full growing season. Knotweed often grows in riparian and wetland habitats. If treating plants near water with herbicide, use only products labeled for aquatic use.

Reporting

Report new occurrences by submitting a report through EDDMapS, emailing Report a Pest, calling Report a Pest (1-888-545-6684), or contacting your local county agricultural inspector.

Resources

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