Norwich
Knotweed is taking over the northeast and it’s become a major concern to ecologists and landowners because it destroys wildlife habitat, not to mention property values. Once the plant has a foothold, it’s almost impossible to control.
The Upper Valley Land Trust and the Hanover Conservation Commission will host a free workshop on the mechanical, cultural, biological and chemical weapons available against knotweed at the Montshire Museum on Thursday, Jan. 24, from 6:30-8 p.m. For more information, call 603-643-6626.
Forest ecologist John Roe, vice president of stewardship and strategic initiatives for the Upper Valley Land Trust, discussed the ongoing battle against knotweed in a Q&A. The exchange has been edited for length and clarity.
Question: What makes knotweed such a menace in our area?
Answer: Its preferred habitat is forest edge and disturbed areas, which just about defines the Upper Valley. Humans obviously create disturbed soil in many areas, including rural roads. But disturbed areas also include natural soil disturbance, such as in riparian areas subject to flooding. This is one of the reasons most streams in our area have populations of knotweed. Once established, it is spread easily downstream. It can also tolerate a very wide range of soil types and structures and fairly deep shade, so it can expand by its extensive, quick-growing root structure into a variety of other habitats.
This plant easily outcompetes other species completely, and it uses chemicals as well as shade to do that, so that all native plants are removed from the area and natural ecosystem function and habitats are lost.
Without rapid response to control it, the infestation quickly gets to a point where it is almost impossible to remove.
Q: Why is it so difficult to control?
A: To start with, two-thirds of the plant is below ground, and if one cuts the above ground vegetation, it just encourages the wide spreading rhizomes (root-like structures) to send up new sprouts. Those roots can extend 65 feet from the plant and extend up to 10 feet deep into the soil. It also grows very quickly, forming a 10- to 13-foot high dense canopy each year. On a good summer day the stems and leaves can grow up to 8 inches in one day.
Finally, nearly any piece of the stem or root larger than about half an inch can readily root and form a new plant. Thus, floodwaters or human transport of soil for construction can easily move the plant great distances.
Q: Where did the species originate?
A: Knotweed originates in eastern Asia (China, Japan and Korea). It has now spread around the world and is considered a major problem worldwide, except in highly arid locations.
It was planted in the U.S. in the late 1800s, and by 1930 it was already recognized as a likely pest. Initially it was planted for horticultural reasons because of its beautiful white flowers, but then it was encouraged for erosion control, as it easily took to disturbed areas.
The irony is that erosion actually can increase in areas of knotweed. While the plant has an extremely dense root system, it almost always creates an extensive monoculture devoid of any native plants. Resistance from erosion normally comes from a wide variety of roots, not just a single type from one plant.
Q: How else does it spread?
A: Although it was once thought to spread only through rhizomes, we know now that seeds also play a major role. In addition, pieces of it as small as half an inch can root and grow; therefore, mowing is now considered a spreading mechanism, rather than a controlling technique.
Q: How can you recognize knotweed?
A: The easiest comes from an inaccurate but descriptive name for it: Japanese bamboo.
It is not a bamboo at all, but it has hollow segmented stems, grows in tall visible clumps, and grows quickly, which are all characteristics of bamboo. However, the leaves are large and roundish with a point, unlike bamboo’s narrow long leaflets.
Q: Is knotweed toxic to humans, like wild parsnip or poison ivy? How about to animals?
A: It is clearly not toxic to humans or animals by touch, and as far as is known, is not toxic to either if eaten. However, I don’t know of any studies that looked at a situation where animals eat at a large amount. In general, that’s not a good idea with many invasive plants, because they often are high in various chemicals.
Knotweed is no exception — it uses chemicals to reduce other plants growing beneath it, and its leaves contain lots of tannin and oxalic acid. The latter is what makes rhubarb toxic.
Q: What are some of the methods landowners can use to eradicate knotweed from their property?
A: Eradicate is a strong word when it comes to knotweed. It is clearly the goal, but usually just control is more likely. Infestations can affect property values because it is so aggressive, hard to beat, and enters even the smallest cracks to damage cement walls and walkways, pipes and septic systems. If you see a plant growing through asphalt on a warm summer day, it is most likely knotweed. Many herbicides have been found ineffective, but a few injectable forms are showing promise. But techniques for control and eradication are evolving.