Knotweed Project

The Town has embarked on an effort to control invasive Japanese knotweed along Mill Pond Road and around the Town Hall. Ellen Snyder, Durham Land Stewardship Coordinator, is working with Doug Cygan, Invasive Species Coordinator with NH Department of Agriculture, Markets, & Food, on implementing the best control methods. Japanese knotweed (Reynoutria japonica) is an aggressive and highly invasive herbaceous to somewhat woody perennial originating from eastern Asia (Japan, Korea, China and Taiwan). One of the plant characteristics is the noticeably jointed stems, much like bamboo. It was first brought to the United States in the late 1800s for ornamental and horticultural purposes. It quickly became popular in the nursery trade and has been planted in landscapes throughout North America. It was also planted for erosion control and as a forage crop; little did they know at the time how damaging these practices would be.

Invasive characteristics of Japanese knotweed:

Fast growing, ~ 8” per day
Large woody rhizomes that penetrate the ground up to 10’ deep and laterally can exceed 40’
Allelopathic properties (chemical compounds that are released by certain plants to eliminate vegetative competition) allowing it to displace native vegetation
Forms dense clonal communities
Regenerates from rhizome (root) fragments as small as ½” in length
Rhizomes can remain dormant for up to 20-years
Cut or mowed stem fragments can regenerate from nodes
Outcompetes native species and reduces or eliminates native plant diversity
Grows through concrete and pavement causing issues with infrastructure, utilities, drainage, septic systems, walls, and foundations

The most effective control method is herbicide application by a NH licensed applicator. Japanese knotweed is unlike most plants in that the flow of nutrients/carbohydrates is in one direction, moving upward during the growing season until flowering and then the process reverses to deliver the nutrients/carbs back down to the rhizome system for overwintering. Therefore, chemical application is most effective just after flowering up until the first killing frost (September – November). To increase effectiveness of the treatment, the stems can be cut to the ground in June. The re-growth will be reduced by the time of treatment in the fall. We’ve done this at two sites—behind the town hall and at the College Brook bridge. These piles will be left to dry out to ensure they are non-viable before being taken to the town transfer station for burning. Doug Cygan will be treating the plants in the fall and other invasive plants along Mill Pond Road (not on the Milne Sanctuary) as well. Ellen is mapping knotweed populations throughout Durham. If you know of a stand, please email ellensnyder1@gmail.com. Thanks to all those who have submitted sites thus far.