There are a wide variety of non-native invasive plant species that have taken hold in Madison County. These plants cause harm to the environment, as well as potential harm to human, animal, and plant health in the preexisting environment. Some of these damages can be caused by overtaking beneficial native plants in the ecosystem, act as vectors for diseases and other invasive species, such as invasive insects, and are unhealthy or potentially toxic to native animals. In this series, we will discuss identification and control of these prevalent invasive plants.

Identifying Japanese Knotweed

Japanese knotweed (scientific name Reynoutria japonica) is a herbaceous perennial weed that originated in Eastern Asia. Though it is not related to bamboo, the structure of Japanese knotweed is very similar due to the hollow, sectioned stem with swollen nodes, making it appear almost jointed. It can grow up to 10 to 13 feet tall in dense patches that choke out other more beneficial plants, such as native species. New growth of the stem is a reddish-brown color when it reappears in the spring after it dies back to the ground in the winter. As the plant continues to grow, the more mature bottom sections of the stem turn more woody. The leaves of this invasive species can range from about 3 to 7 inches long and about 2 to 4 inches wide, with a spade-like shape that draws to a point at the tip, their placement on the stem alternating. Small flowers, usually less than an inch in size, bloom in clusters during the late summer to fall, and can range in color from white to a cream/tan to a green shade.

How It Spreads

This invasive species is extremely prolific, spreading itself through strong root systems created by underground stems known as rhizomes. These rhizomal root systems can cause structural damage to the foundation of homes, walls, and roads while allowing Japanese knotweed to spread in dense patches with rapid growth. In situations where Japanese knotweed is damaged, each jointed section of the plant’s stem can sprout a new root system when in contact with the moist soil or water, making it more difficult to contain and control the spread. It thrives in areas with moist soils, and is often spotted along riverbanks and creek beds throughout the county, especially in areas that receive six or more hours of sunlight throughout the day. Where Japanese knotweed takes hold, it chokes out other plant life and creates a monoculture, which provides poor erosion control compared to native stream bank vegetation, and degrades the quality of the habitat it has taken over.

Controlling Japanese Knotweed

Due to the rhizomal root systems that Japanese knotweed uses to spread, it is a very difficult invasive species to control. There are two important steps when it comes to successfully managing this invasive plant—initial control, and maintenance.

The initial control phase will take at least two seasons of work to remove the already established plots of Japanese knotweed, and there are two methods that can be used during these seasons. The first method consists of two applications of herbicide, which is most effective when first applied after late spring frosts, followed by a second application of herbicide eight weeks later. The second method is cutting the knotweed, followed by an herbicide application. Cutting alone is not an effective method of controlling Japanese knotweed, but cutting the knotweed in June and then waiting at least eight weeks before a foliar application of herbicide on the regrowth can be effective to control this invasive plant. This would put the opportune time for spray after cutting in June in the August to September time frame. Cutting before spraying helps shorten the Japanese knotweed, making it easier to obtain good foliar coverage with the herbicide, as well as redirecting the energy reserves of the rhizomes on the plant toward shoot growth rather than expanding its underground rhizome network.

Most common, broadleaf weed herbicides such as 2,4-D and triclopyr are not effective control methods against Japanese knotweed. They will cause significant foliar damage to the plant, but will have a very limited effect on the rhizomes in the root system. This limited effect on the rhizomes means that it is very likely that the Japanese knotweed will reemerge in the next growing season, and management strategies will have to be started all over again. Herbicide active ingredients like glyphosate and imazapyr have been shown to be effective in controlling Japanese knotweed. Glyphosate is a non-selective herbicide, meaning that it must be sprayed with caution to prevent it from killing or harming desirable plants nearby, but it has low toxicity to non-target organisms and no lingering soil activity, as well as having available labels cleared for use in aquatic environments. Imazapyr is another option for a viable herbicide to use against Japanese knotweed, but it has lingering soil activity and can potentially cause damage to nearby trees due to uptake from the soil.

If you have any other questions about identifying or controlling Japanese knotweed, please contact the Madison County Cooperative Extension Center at peyton_duckett@ncsu.edu, or (828) 649-2411.

Citations

Gover, A., Jackson, D. R., Wurzbacher, S., & Templeton, S. (2020, February 24). Japanese Knotweed. PennState Extension.

NC State Extension. (n.d.). Japanese Knotweed. North Carolina Extension Gardener Plant Toolbox.

USDA. (n.d.). Invasive Plants. U.S. Forest Service