September Weed of the Month: Giant Knotweed

September's Weed of the Month is giant knotweed (Polygonum sachalinense). Giant knotweed has hollow, bamboo-like stems and grows over 12 feet tall, making it the largest of the invasive knotweeds. It forms tall, dense thickets that degrade habitats, particularly along riparian areas where it facilitates erosion and flooding.

Giant knotweed is native to Asia. It was brought to North America in the 1800s and planted as an ornamental, for erosion control, and as livestock forage. Giant knotweed escaped cultivation and has become a serious invasive plant in many states.

Giant knotweed has distinctive characteristics. The tall, hollow stems have swollen reddish nodes arranged in a zigzag pattern. Leaves are heart-shaped, dark green, and 12 inches or more in length. Small white flowers in the leaf axils bloom from July to October. The root system of giant knotweed is extensive, growing deep into the soil and producing thick, dense mats of rhizomes. Giant knotweed may be confused with a closely related species, Japanese knotweed (Polygonum cuspidatum) but can be distinguished by the leaf base: giant knotweed has a heart-shaped leaf base and Japanese knotweed leaves have a flat base.

Giant knotweed is adapted to many habitats. It can be found in disturbed areas, along roadsides, gardens, stream banks, and riparian areas. The rhizomes allow giant knotweed to spread quickly and aggressively. Root fragments can be spread by equipment and water, making management a challenge.

• Do not plant giant knotweed as an ornamental and control existing plants. If you find this plant, immediately report by calling the Arrest the Pest Hotline at 888-545-6684 or arrest.the.pest@state.mn.

• Management efforts should focus on eliminating root growth. Mowing in combination with herbicide application will produce the best results.

• Always clean and inspect equipment after working in an infested areas to reduce the transport of plant fragments to new areas.

• Herbicide treatments 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.