Japanese Knotweed
Impact – Introduced into the US from Asia as an ornamental, once sold in seed catalogs and was considered a problematic pest by the 1930’s. It is one of the most common invasive plants in the Adirondacks and spreading rapidly. In the last 10 to 20 years, JK has entrenched itself along stream banks, roadsides and woodlands. It grows extremely fast, crowding out native vegetation and altering the landscape. Native plants are rapidly out-competed. Without prompt and vigorous action, knotweed damages the scenic and recreational quality of our Adirondacks.
What is JK – Upright herbaceous perennial that can grow from 3 to 15 ft tall, have hollow bamboo-like stems creating monocultural stands. It tolerates shade, and drought and is found along roads, right of ways, old farmsteads, streams and rivers. Thriving in disturbed areas, it spreads by its extensive root system and seed dispersal. Its shallow root system is ineffective at preventing streambank erosion or stabilizing soils. New colonies are spread by seed and can sprout from plant fragments as small as ½ inch. It is sometimes spread by contractors dumping contaminated fill.
How to identify Stands are very easy to identify. Stalks are persistent through winter. Leaves grow in an alternating pattern on the stem and average 6 inches in length and 3 – 4 inches width. Leaves are heart shaped or broadly ovate (round at the base and triangular or heart shaped, pointed at the tip) to somewhat triangular. Plants sprout early in spring quickly dwarfing surrounding plants. The horizontal roots can reach lengths of 65 feet or more. Colonies are dense and thrive in shade and sun. In August, clusters of small greenish white flowers arranged in spikes near the end of the stem bloom in Aug. to Sept. turning to buckwheat-like seeds by early October. Upon cold weather growth ceases, leaves and seeds are shed and the stems take on an auburn or rusty hue.
What to do / Help / Control – First, a disposal plan for all knotweed material is essential to prevent future colonies. Re-vegetation after treatment is necessary as bare ground allows for re-invasion of knotweed. Regardless of methods, eradicating knotweed is typically a 3-5 year process. Be prepared to make follow up visits. There is no insect, pest or disease that can keep it in check. Do not mow or cut with weed trimmers as the pieces of the plant will re-spout spreading rather than controlling the plant. Do not transport knotweed fragments / cuttings. Do not take to a landfill. Do not let sit on ground where it can resprout. Burning is an acceptable disposal method.
Single young plants may be pulled by hand depending on soil condition and amount of root development. 100 % of the root /rhizomes must be removed for effective management. Not recommended for established patches.
Herbicide controls – Stem injection in May – June with glyphosate, available in garden stores as Round Up, is currently the most effective solution. Inject stems between nodes when plants are 3 ft tall with developed leaves after stems are cut to 2 inches above the ground. (See APIPP below.) As of the plants energy is stored in the extensive rhizome system, this method is effective and results in 95 % reduction. Follow up with foliar spray applications with glyphosate and triclopyr to the foliage and freshly cut stems can be effective but most will require multiple applications. Do not use foliar prays when the plant is flowering to protect pollinators during treatment. Do not use foliar sprays along stream beds, only stem injection.
Recent attempts to introduce biological controls, i.e., beetles that attack the plant in their native lands have been totally ineffective. Also, attempts to cut down and then cover it with tarps like-wise have been in effective.
Contact APIPP * who has helped landowners with loaning stem injection equipment. 518 – 576 – 2082
This is a nasty plant and a world-wide problem. Best control is not to let it get established. Our Adirondack Kudzu!
Adirondack Park Invasive Plant Program / Nature Conservancy located in Keene Valley, and a ESSLA partner.