Japanese knotweed is an invasive, exotic shrub that spreads quickly and can out-compete other plants in an area. Regardless of its environmental impact and control cost, it is sold as an ornamental plant for use in landscaping and flower arranging. Control of Japanese knotweed usually requires repetitious treatment with a combination of chemical and mechanical methods.
Japanese knotweed is native to eastern Asia but was exported as an ornamental plant in the late 1800s. It was first used in England, which influenced it introduction to the U.S. By the mid-20th century it was often viewed as a nuisance in the U.S., with recommendations that it be eradicated.
Native plant diversity, wildlife habitat, and wildlife food sources are lost due to knotweed. In its native habitat, knotweed is controlled by herbivores, but those organisms aren't found outside of its native range. It spreads by rhizomes and seeds and can quickly take over an area. Knotweed control is more difficult with a longer time of establishment.
As with many invasive plants, successful long-term control efforts should target the root system. Regardless of control method, multiple treatments are often required. Mechanical control involves removal by hand or mowing. Also, chemical treatments involve spraying foliage or treating a recently cut stump with a glyphosate- or triclopyr-based herbicide. Combining mechanical and chemical methods is often the most effectiveas this puts multiple pressures on the root system, according to the Kentucky Division of Forestry.
Mowing, which can take place anytime of year, results in regrowth, which gradually drains the plant of it stored carbohydrates. Mowing is only effective if it occurs several times during the growing season. Hand pulling must involve root removal so that there are fewer rhizomes from which new plants will grow. Glyphosate or triclopyr applications after mid-summer result in herbicides moving into the rhizomes to killing the root system.
Herbicides are dangerous chemicals and all recommended safety precautions must be followed for safe and effective use. Even though herbicides are developed for use against weeds, severe toxicity can occur in humans and non-target animals. Always follow label directions, and never use more than the recommended amount. In many cases, using more of an herbicide than is recommended on the label does not provide better control.