Giant Knotweed | Keweenaw Invasive Species Management Area (KISMA)

Fallopia sachalinensis

giant knotweed leaves and stems — Image credit: Sigrid Resh

Characteristics

Giant knotweed, a terrestrial herbaceous plant that is in the Polygonaceae family, invades along waterways, roads, forest openings, and disturbed sites where dumped.

grows to be 12 feet tall with hollow stems resembling bamboo
has broad heart-shaped to spade-shaped leaves that are larger than your hand, and larger than the similar Japanese 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 Japanese knotweed and produce a species called Bohemian knotweed.

For more information visit Midwest Invasive Species Information Network (MISIN).

(image credit: Sigrid Resh) tall stem of giant knotweed

rhizome segment of giant knotweed — (image credit: Sigrid Resh) root segment of giant knotweed

KISMA Management Practices

Giant 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, as well as seed production. Make sure to remove all plant material to prevent re-rooting. KISMA has been managing a half-acre knotweed area along the Portage trail and at the Quilt House for the past four years with very positive results (see images below).

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
repeatedly pull or cut aboveground knotweed plant material every 2-3 weeks throughout the growing season over multiple years
remove plant material in a safe way by placing on cement or on tarp/carpet to dry in sun or bag the plant material
do not allow any fragments to come in contact with the soil
revisit the site every year 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.

use of carpet to cover giant knotweed with re-sprouts that need to be cut — (image credit: Sigrid Resh) use of carpet to cover knotweed with re-sprouts that need to be cut

(image credit: Sigrid Resh) giant knotweed piled on tarp

Examples of Areas Managed for Giant Knotweed around KISMA

portage trail giant knotweed before treatments started — Portage trail giant knotweed **before** treatment started

portage trail giant knotweed in 2018 after 2 seasons of treatments — Portage trail giant knotweed in 2018 **after** 2 seasons of treatments

Quilt House giant knotweed before treatment started — Quilt House giant knotweed **before** treatments started

Quilt House giant knotweed in 2020 after 4 seasons of treatments — Quilt House giant knotweed in 2020 **after** 4 seasons of treatments

Native Alternatives

Popular alternatives to giant knotweed include native dogwoods, like red-osier dogwood (Cornus sericea) and flowering dogwood (Cornus florida), since they are beautiful and provide for native pollinators and other native wildlife. Also, understory plants like native ferns, bushes, and trees are great alternatives because they fill up space knotweed could use to spread while also providing for native wildlife.

Sources

Giant knotweed (Fallopia sachalinensis) MISIN, 2020

Recommended Plant List NMISN, 2017