Like a character from a 1950s era horror film, this plant can be poisoned, chopped up, and even burned, but still refuses to die. Polygonum cuspidatum, also known as Japanese knotweed, is so hardy that it can live in any area that has a temperate climate, in nearly any type of soil. Japanese Knotweed eradication is a constant battle between this green colonizer and those who must control it.
As its name indicates, this invasive species was originally from Asia, but is not really a newcomer. Japanese Knotweed Expert Ltd told us that the the nectar contained in its flowers is useful to beekeepers, as well as being a source of various herbal remedies. The stalks may be even be eaten as a vegetable, but are not widely popular. It is not an unattractive plant, but can spread extremely rapidly once established in the soil.
The secret to its survival is hidden underground. The plant forms rhizomes, commonly called creeping root stocks, which have the capability of generating new growth even when broken into very small pieces. The roots often extend nearly twenty-five feet beneath the soil, and easily survive sub-zero temperatures. The wrong soil pH or excess salinity do not seem to bother it, and growth explodes during good weather.
Once established, it favors propagation via shoots rather than pollination. The bulk of its biomass lies beneath the soil, and any effort to destroy it usually fails unless every fragment is completely destroyed. In an effort to prevent this weed from gaining a foothold, some regions favor methods more closely resembling disease control than plant management.
The ongoing eradication effort requires a multilateral approach. Unlike many weeds, cutting it down only stimulates the underground rhizomes, and even composting does not kill the remnants. Multiple cuttings may weaken plants a little, but in severe cases excavation and removal of the topsoil may be recommended. That extreme measure is neither practical nor possible for many homeowners, who rely primarily on herbicides.
Although an established underground rhizome system can actually take years to die, herbicides are currently the most effective weapon. While most plants will quickly shrivel from an application to their above ground shoots, killing this green invader requires poisoning the roots as well. Extreme caution when using herbicides is necessary to prevent them from reaching water supplies, or from killing unintended plant and animal species.
Once treated, all plant fragments must be dug up and allowed to dry thoroughly, and then burned on site if local regulations allow it. Any tools or machinery used in the process should be totally free of any plant material before leaving the area. Due to these complications, some countries are attempting to develop safer biological controls, but in most parts of the world, knotweed has few natural enemies.
The current solutions are not an easy fix, but left to their natural patterns these plants actually decrease property values by damaging pavement, retaining walls, and even home foundations. Left alone, this plant crowds out beneficial native plant and insect species. Japanese Knotweed eradication is currently the focus of international research aimed at finding controls that are both practical and environmentally safe.