Common Name: Giant Knotweed
Alternate Names: Japanese bamboo, Sakhalin knotweed
Scientific Name: Polygonum sachalinense F. Schmidt ex Maxim. and hybrids, also Reynoutria sachalinensis (F. Schmidt ex Maxim.) Nakai
Related Species: Japanese knotweed (Polygonum cuspidatum Siebold & Zucc. synonyms Fallopia japonica (Houtt.) Ronse Decr.)
Legal Status: Specially Regulated
Any person, corporation, business or other retail entity distributing Japanese and/or giant knotweeds for sale within the state must have information directly affixed to the plant or container packaging that it is being sold with, indicating that it is inadvisable to plant this species within 100 feet of a water body or its designated flood plain, as defined by Minnesota Statute 103F.111, Subdivision 4. View Minnesota Noxious Weed Law for more information.
Giant knotweed is native to Asia and was imported to North America in the late 1800s as an ornamental. It was also planted for erosion control and as livestock forage. Since its introduction, giant knotweed has escaped cultivation and is classified as a serious invasive species in several states.
- Giant knotweed is a stout perennial bamboo-like shrub that grows to over 12 feet in height. It is the largest of the invasive knotweeds.
- Stems resemble bamboo, are erect, smooth, hollow and have swollen reddish nodes arranged in a zig-zag pattern.
- Leaves are simple, alternate, ovate, heart-shaped at the base, dark green, pointed and large (generally 12 inches or more in length).
- Small creamy white or greenish flowers bloom on short racemes in the leaf axils from July to October. Flowers are either male or female.
- Female flowers produce broadly winged/paper-like fruits containing a small, shiny, brown, 3-sided seed. Reproduction primarily occurs through roots.
- Reproduction primarily occurs vegetatively through rhizomatous roots. The root system extends deep into the soil and produces thick "mats" of rhizomes that help infestations to grow very dense and spread.
- Japanese knotweed looks similar, but is significantly smaller and leaves are not heart-shaped at the base.
Giant knotweed can be found in various disturbed habitats with varying sun exposure. It can be found along roadsides and other right-of-ways, meadows, gardens, stream banks, and riparian habitats.
Means of spread and distribution
Rhizomes allow giant knotweed to spread quickly and aggressively, although seed production can also contribute to spread. Giant knotweed typically infests new areas through human intervention or transport of root fragments along stream environments especially during periods of flooding. Movement of soil from infested sites is another common way for giant knotweed to infest new areas. Because giant knotweed produces both male and female flowers, it can also serve as a pollen source for the equally invasive Japanese knotweed.
Giant 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.
- Monthly cutting or weekly mowing throughout the growing season may reduce giant knotweed stands and will reduce seed production. However, because knotweeds reproduce primarily by abundant root rhizomes, mowing is not recommended as a single management option. Pre-flower mowing in conjunction with selective herbicide applications will produce better results than mowing alone. Always clean and inspect equipment after working in an infested area to reduce transport of seed and plant fragments to new areas.
- Foliar, stem injection, and cut-stem 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.