Invasive species in the United States is a problem that costs somewhere in the range of $138 billion annually to control — with everything from zebra mussels coating our lake floors to kudzu choking out our southern forests, and New Hampshire is no exception. As it stands right now, there are 32 plant species that the New Hampshire Invasive Species Committee considers to be invasive. One of these plants is a dense growing shrub commonly known as Japanese knotweed.
Japanese knotweed was introduced to Europe and the eastern United States as an ornamental plant from Asia some time in the early 1800s. Its original attraction as an ornamental in Europe was its dangling strips of small white flowers that produced its seeds and the deep green of its large, heart-shaped leaves. Another reason for its introduction to the United States was that it is an excellent plant to help control erosion, which is why it can be found in dense thickets along roadsides and stream beds. It produces a series of underground, root-like stems called rhizomes that will produce their own roots and eventually form new plants. This underground system of connecting rhizomes and roots helps to bind the soil together and prevent erosion. However, this same property of the plant makes it extremely difficult to eradicate.
The reason why Japanese knotweed is such a problem in New Hampshire is that it has no natural predator, allowing the weed to grow unchecked. The weed will form dense thickets that will eventually crowd out natural plant life and completely change the habitat it has been placed in. According to Douglas Cygan, the chair of the New Hampshire Invasive Species Committee, that is the main problem with Japanese knotweed and invasive plants like it. Japanese knotweed can outcompete the slower growing native plants and in some cases create monolithic stands (only one plant covering a large area), reducing plant diversity in the area.
You have to be careful about how you remove Japanese knotweed, as well. Maliciously mowing through a growth of the weed will not help your problem and will most likely make it worse. Japanese knotweed can produce asexually from a single fragment of the plant. One node on the stem can produce the shoots and roots of a whole new plant. In fact, Cygan believes that mowing and improper removal of the plant is the number one cause of its spread for this reason.
So, to rid yourself of this menace you must be vigilant. Dig deep around the base of the plant and try to remove all of the rhizomes running underneath the ground. Once you have removed the plant and as much of its root system as you can, place it in a plastic bag (to help prevent its spread) and bring it to the dump. Herbicides can be used, as well, but be careful and try to use these only as a last resort. As a substitute to herbicides, salt water has been proven to be effective against Japanese knotweed while having a minimal impact on our environment.