April 2016


Invasive Watch: Japanese knotweed (Fallopia Japonica or Polygonum Cuspidatum) and other invasive knotweeds


Japanese knotweed, also known as crimson beauty, Mexican bamboo or Japanese fleece flower, is a plant with stout, reddish-brown, canelike stems growing 4 to 10 feet tall. Its leaves are 4 to 6 inches long by 3 to 4 inches wide, egg-shaped and narrow to a point at the tip, and have slender stalks attaching them to the stems. Knotweed plants produce minute greenishwhite flowers during the summer, soon followed by small winged fruit. The plants die back at the end of the growing season.

Other invasive knotweeds include bohemian knotweed (Polygonum X bohemicum), Himalayan knotweed (P. polystachyum) and giant knotweed (P. sachalinense).

Japanese knotweed was introduced as an ornamental from Asia, is now currently found in 36 states and is listed as a noxious weed in at least four Western states. It spreads rapidly from rhizomes. Seeds are distributed by water, by fill-dirt and, to a lesser extent, by the wind. It grows very aggressively, forming dense thickets. Once established, it outcompetes native vegetation and alters natural ecosystems. It’s often found growing along rights-of-way, roadsides, wastelands, old home sites, waterways and low-lying areas.


Invasive knotweeds as a whole are extremely persistent once established, and complete eradication is very difficult. Annual follow-up treatments are almost always required for at least two or more years. Eradication is more difficult, and more follow-up treatments are required for older knotweed infestations.

Optimum results are obtained when applications are made to plants that are 3 to 4 feet in height and when high volumes (100 gallons per acre or higher) are used. Apply Milestone® specialty herbicide at 7 fluid ounces per acre as a broadcast application or apply as the spot treatment rate of 14 fluid ounces per acre. It’s important to thoroughly wet all of the foliage — but not to the point of runoff. Monitoring and making multiple re-treatments will be necessary to control resprouting, but should never exceed 7 fluid ounces per acre as either a broadcast or spot re-treatment.