If you’ve ever wondered about the gigantic white flowering, bamboo-like plant that is prevalent along waterways and roadsides in Cornish and throughout the Upper Valley, you’re looking at Japanese Knotweed (knotweed), one of the most invasive plants in New Hampshire, and one of the most difficult to manage.

Why is Japanese Knotweed a problem? Like most invasives, knotweed chokes out native plant species by completely covering the area where it’s growing, which reduces the insect/pollinator population, and in turn deprives birds of an essential food source. Knotweed also affects soil and water quality, reducing beneficial insects and thus negatively impacting the fish population. Finally, knotweed along streams prevents the natural stabilization that occurs when dying trees and other native vegetation take hold to prevent erosion.

While it is possible to control Japanese Knotweed, it can take between 3-5 years of consistent attention to see progress. The NH Department of Agriculture, Markets & Food Invasive Species Coordinator Doug Cygan gives presentations on managing knotweed and other invasives. The methods Doug suggests for knotweed are smothering for smaller knotweed patches and areas that will be restored to food production, and spot treatment with herbicides for large scale infestations. Knotweed is not prone to spreading after it dies back, but the root can stay viable for up to 20 years.

Click this LINK for a short video on Japanese Knotweed management using non-chemical methods in the home garden by Matt Tarr, UNH Cooperative Extension Wildlife Specialist.

Click this LINK for extensive information and comprehensive photos on the identification and management of Japanese Knotweed by the NH Department of Agriculture. Scroll down toward the end of the article to see photos of just how invasive this plant is—it’s been known to come up through concrete basements, and its roots can extend between 25 and 60 feet.