Why is Knotweed a problem?
Japanese knotweed is an invasive weed that was introduced into the U.S. from Eastern Asia as an ornamental on estates in the late 1800’s.
Knotweed forms impenetrable strands which choke out and displace entire ecosystems. Knotweed features broad leaves with pointed tips and flat or heart-shaped bases, and large, hollow stems which give it a resemblance to bamboo. An extensive network of coarse, root-like structures called rhizomes allow knotweed to out-compete native plants, even choking out deep-rooted willow trees.
Knotweed can produce seeds, but it is rare for the seeds to germinate. The most common method of spreading is by means of stem sections or rhizomes. Even a small stem section can root and spread the plant.
How do you get rid of nasty knotweed?
- The best way to remove knotweed is by excavating the entire rhizome system (root structure). This means going as deep as the rhizomes exist. However, even a small missed rhizome can cause regeneration of the infestation. Rhizome networks can extend 65 feet laterally and up to 25 feet deep and are capable of growing through concrete, making this plant a serious threat to infrastructure, especially roads and building foundations.
- Frequent mowing. However, this carries the risk of spreading the plant through fragments. Any mowed knotweed should be dried out on site first and then burned. (Knotweed is classified as a “controlled waste” and cannot be disposed of by adding to household waste under Environmental Protection Act 1990.)
- Herbicide treatments. In fact, one can hire a company to come in and inject the plants (Alice Publow has the details on this).
- Use heavy tarps to smother the plants. This involves covering the knotweed area with tarps in the Spring. Periodically, the plants will rise up under the tarps and will need to be trampled down. The LLWIA is trying this approach on an area of knotweed at the Preserve. However, we have been told by one group that has tried this, that even using this method for three years might not be enough to eradicate the plant.
Japanese knotweed is one of the world’s most invasive plants. It has been nicknamed the “Godzilla weed”. It has wreaked havoc on even more landscapes than that of the fictional monster!