War on Weeds—weeds are everybody’s problem

 Giant knotweed (polygonum sachalinense) is a semi-woody shrub-like perennial that is native to Asia. It was originally introduced into the U.S. as an ornamental, but it has proven to be an extremely aggressive weed. It invades moist soils in disturbed wetland and riparian areas. 

Giant knotweed can grow up to 15 ft. tall, allowing it to shade out beneficial native species. Leaves are broad, ovate, and heart-shaped, growing up to 12 inches long. Stems are hollow and woody. Stems are typically red early in the growing season and turn brown in the fall. This plant produces long rhizomes (up to 18 ft. long), new shoots can emerge at nodes on rhizomes, allowing this plant to spread quickly. It produces small, compact, greenish-white flowers that form in clusters. Giant knotweed is capable of reproducing from seed and rhizomes, making it very prolific. 

Integrated Pest Management (IPM) Options: 

• Prevention— Learn to identify and do not transport seed or unknown plant material. 

• Cultural— Establish a stand of desirable plants. Bare ground is much more susceptible to weed infestations than ground that is covered with healthy plants. 

• Mechanical— Tillage, digging, mowing, or cutting are not effective control options for giant knotweed; these activities can increase the stand density. Each rhizome that is cut has the potential to become a new plant! 

• Chemical— Glyphosate is the best control option available for this weed. Apply to foliage in the fall (August- first killing frost). Applying herbicides to perennials in the fall, pulls the herbicide down into the root system, killing the whole plant, not just the foliage. Always read and follow herbicide label directions.

Justin Hatch, University of Idaho Extension Agriculture Educator in Caribou and Bear Lake Counties. 208-547-3205 [email protected]