Giant Knotweed
Polygonum sachalinense
Washington State Classification
Class B — Control Required (Designated)
It creates dense colonies that exclude native vegetation and greatly alter natural ecosystems.
Required control in Region 1 (all western Washington counties)
Quick Reference
Identification
Growth Habit
Giant knotweed is an herbaceous perennial, strongly rhizomatous, growing over 12 feet tall. It has huge elephant-ear shaped leaves. It is similar to Japanese knotweed but its features are larger in size.
Leaves
Leaves are alternate, very large and heart shaped. They are often more than a foot long and 2/3 as wide.
Flowers
Flowers are white with a greenish tint and are in small clusters (panicles) in the leaf axils mainly on the upper portions of the stems.
Fruit & Seeds
The seed structure is a 3-sided achene that is shiny and brown-black.
Impact
It creates dense colonies that exclude native vegetation and greatly alter natural ecosystems. Established populations are extremely persistent and difficult to eradicate, even very small fragments can form new plants.
Ecology & Spread
Habitat
Giant knotweed is found along stream banks, in moist waste places, neglected gardens, roadsides and railroad right-of-ways. Please click here to see a county level distribution map of giant knotweed in Washington.
Spread Mechanisms
Reproduction
Giant Knotweed reproduces mainly by rhizomes.
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Control Methods
Mechanical
- Grubbing out small clumps when discovered can prevent new colonies from establishing but is time-consuming. Rhizomes and fragments left in the ground, or nearby, can regenerate and spread infestations. The entire root system must be removed since re-sprouting can occur from rhizomes. Frequent, repeated cutting of plants over many years can eventually kill the plant. Covering plants with a thick landscape fabric for up to 5 years may provide control. Fabric should be installed after cutting plants back to the ground, provide some give in the fabric so new stem growth won't puncture through, monitor fabric and flatten knotweed growth under it, keep fabric cleared of debris, and repair any holes.
Biological Control
- There are currently no registered biological control agents for use on any of of invasive knotweed species. Grazing may provide some reduction in plants but additional control methods are typically needed. Any grazing strategy should be carefully controlled to prevent damage in riparian areas.