Invasive knotweed is a large, perennial invasive shrub native to Asia. There are three similar species of invasive knotweed frequently encountered in the Adirondacks including Japanese knotweed (Reynoutria japonica), giant knotweed (Reynoutria sachalinensis), and a hybrid of the two known as Bohemian knotweed (Reynoutria × bohemica). While the species are unique, they are very similar in appearance and impact.
Knotweed is a large, densely growing herbaceous shrub. At peak growth, an established plant may exceed 12 feet in height.
Stems are stout and cane like, resembling a bamboo species. They are hollow and green, and are sometimes covered with red spots or blotches. Raised ridges (nodes) are found along the length of each stem.
Leaf shape is variable by species (see above) but is typically heart- or spade- shaped. Leaf size can differ signifcantly between species and sites, with some leaves exceeding 12 inches in length.
Knotweed produces large clusters of small white flowers at the ends of its branches. Plants typically bloom August to September.
Knotweed spreads primarily through the expansion of an extensive underground root system. The lateral roots (rhizomes) of knotweed are fast growing and aggressive. They can extend 30+ feet horizontally from where plants are visible above ground. Rhizomes produce new plant shoots, expanding the size of the knotweed infestations. Rhizomes and stems are very durable, sometimes emerging through asphalt or concrete.
Knotweed can also be easily spread through the fragmentation of its roots or aboveground stems. If the plant parts are broken apart -- such as by a mower along a roadside or a flood near a river -- each piece of knotweed can produce rootlets and establish a new infestation.
Knotweed presents multiple ecological, environmental and societal impacts to landowners, natural resource practitioners and transportation professionals. For example, dense infestations of knotweed exclude native plant species, reducing biodiversity in natural systems and limiting habitat for native animals. When established along a roadside or driveway, tall knotweed plants can impede line of sight or block road signs creating hazards for motorists. The root system of knotweed does not grow as deeply as native plants in the same habitat, which leads to increased erosion when knotweed is established along rivers and streams. Bottom line -- this plant is bad news.
Invasive knotweed is one of the most difficult invasive species to control. There is no single, quick fix to manage knotweed on your property. In many cases, a variety of techniques will be required and will need to be repeated for several years. Large or established patches of knotweed may require some level of maintenance management in perpetuity.
With the right tools and techniques, you can manage knotweed on your own property. To learn more, check out our Landowners Guide to Knotweed Control.