Reynoutria japonica (Japanese knotweed)

Japanese knotweed leaves and flowers are identifiable features of this plant. Photo credit: Liz West (NCSU Plant Toolbox).

Japanese knotweed is prevalent all across WNC. Originally native to eastern Asia, it was brought to the U.S. primarily for its ornamental characteristics and ability to rapidly establish itself in loose soil. It is most commonly found in areas of high erosion and along waterways where segments of the plant spread via underground rhizomes. Japanese knotweed can quickly displace native plant populations, especially in riparian areas, making it a highly problematic invasive plant to our area. Due to Hurricane Helene and the incredible amounts of flooding seen across our area, Japanese knotweed is a weed of primary concern. Flooding events serve as the primary spreading method for this plant.

Identification

Japanese knotweed is a perennial weed that colonizes quickly with stands that can get 3-20’ wide and 10-13’ tall at maturity. The hollow stems have nodes that are similar to bamboo in appearance and are usually reddish-brown in new growth fading to a pinkish-green as the stems age. While bamboo typically has leaves that are long and lance-shaped, that persist year-round, Japanese knotweed leaves are alternate, heart-shaped, deciduous leaves with entire leaf margins (no serrations) and are about 3-6” long and 1-3” wide. Leaves can emerge red and then fade to green. Spike-like flower structures, creamy white in appearance (5-petals) and 3-6” in length, bloom from the leaf axils in late summer to fall. This plant is so tenacious, that underground rhizomes can damage foundations, roads, and buildings. Look for the hollow stems with the characteristic red/pink nodal bands and heart shaped leaves as a key ID identifier for this weed.

Japanese knotweed stems are typically reddish-green in color. Seen above is spring growth from underground rhizomes. Photo credit: Swale Crouch (NCSU Plant Toolbox).

Control

Japanese knotweed can be controlled, however it takes repeated efforts (usually 2 years or more), time, and patience. Typically, the best strategy is to cut stems in early to mid summer (June) and wait a period of 8 weeks before treating with a non-selective herbicide. By cutting the foliage, especially in riparian areas, one can better direct the herbicide spray and minimize off-target drift onto desirable plants in the surrounding area. It is critical to wait 8 weeks and treat the resprouted growth in the fall so that the herbicide is transported with the sugars in the plant to the rhizomes as the dormant period begins. Typically, glyphosate is the best product to use in treating Japanese knotweed. It is important to note that due to the nature of Japanese knotweed being near bodies of water, an aquatic formulation of herbicide might be needed which requires a pesticide license to apply aquatic products. Always read the label to determine if surfactants are recommended, (use aquatic safe products), and what specific types of site the product can be used on.There are no silver bullets in the control of tough invasive plants, but active and consistent management can lead to the reduction of populations over time. Japanese knotweed is no different, and it is important to start treatment before seed enters the seed bank in the fall. Be mindful of drift when using any herbicide product and always follow pesticide label directions! It is important to understand how to use a product safely, and where you can use that product. Note that in certain areas, like aquatic sites, right-of-way, or forested areas, products have to be specifically listed for use in those areas in order to utilize an herbicide. If you have questions about product recommendations, or how to manage Japanese knotweed, feel free to reach out to your local county Extension office to receive guidance. For more information about invasive weed management, visit our website and continue to learn more as invasive plant BOLO’s are published.

Resources:

NC State Extension Plant Toolbox- Japanese knotweed

Japanese knotweed factsheet- Penn State Extension

Invasive Plant Management: Forming a Gameplan After Helene- NC State Extension