Japanese Knotweed

Fallopia japanica

Japanese knotweed along the Peepsock stream
 Image credit: Matt Kelly

Characteristics

Japanese knotweed, a terrestrial herbaceous plant that is in the Polygonaceae family, invades along rivers, steams, roads, and in disturbed areas or where dumped. It prefers full sun and rich soils but is able to grow in a wide variety of conditions.

  • typically grows to be 10 feet tall with stems that are hollow like bamboo
  • has spade-shaped leaves with pointed tips and flat bases that are smaller than giant knotweed leaves
  • produces flower stalks that contain numerous clusters of small green-white flowers
  • develops a massive rhizome system below ground that allows it to spread rapidly and be very difficult to remove
  • broken off stem and root fragments are able to re-root and create new plants

Note: Can hybridize with giant knotweed and produce a species called bohemian knotweed.

For more information visit MISIN.

 

 

 

Japanese knotweed leaf and flower
(image credit: Sigrid Resh) Japanese knotweed leaf and flower
Japanese knotweed flowering in the fall
(image credit: Sigrid Resh) Japanese knotweed flowering in the fall

KISMA Management Practices

Japanese knotweed is very difficult to remove from an area after it has been introduced because of how aggressive it is. It’s able to spread through rhizomes and stem/root fragments. Make sure to remove all plant material from soil contact to prevent re-rooting. KISMA has been managing a quarter-acre knotweed area in Kearsarge for the past four years with very positive results (see images below).

  1. cover portions of the knotweed plant with tarps or repurposed carpet pieces to reduce photosynthesis and to act as safe spots to place removed plant material to dry in sun
  2. repeatedly pull or cut aboveground knotweed plant material every 2-3 weeks throughout the growing season over multiple years
  3. remove plant material in a safe way by placing on cement or on tarp/carpet to dry in sun or bag the plant material
  4. do not allow any fragments to come in contact with the soil
  5. revisit the site every year afterwards to be certain that the knotweed was completely removed

Note: The purpose of this removal strategy is to starve the rhizome system of any carbohydrates that would be gained by photosynthesis and will require multiple years of surveillance and removal of plant material to be successful.

Japanese knotweed drying on pavement after cutting
(image credit: Sigrid Resh) Japanese knotweed drying on pavement after cutting
Japanese knotweed piled on tarp to dry away from contact with soil
(image credit: Sigrid Resh) Japanese knotweed piled on tarp to dry away from contact with soil

Examples of Areas Managed for Giant Knotweed around KISMA

Kearsarge site of Japanese knotweed infestation in 2017 when treatments started
Kearsarge infestation of Japanese knotweed in 2017 when treatments started
Kearsarge infestation of Japanese knotweed in 2020 after 4 years of treatments
Kearsarge infestation of  Japanese knotweed in 2020 after 4 years of treatments

Native Alternatives 

Some popular native alternatives to Japanese knotweed are those from the dogwood genus, such as red-osier dogwood. These are beautiful native plants that provide a habitat for native pollinators and other native wildlife. Also many other native understory species are good alternatives as they can help prevent knotweed spread by filling out areas knotweed could grow in.

Sources:

“Japanese knotweed (Fallopia japonica).” MISIN, 2020 
“Recommended Plant List.” NMISN, 2017