What is Japanese knotweed?
The plant, also known as reynoutria japonica, is native to Japan, China, and Korea, although the species has fully established itself in various environments in North America and Europe too. It is graded as a pest and invasive in several countries.
New leaves of Japanese knotweed are dark red and one to four centimeters long. Young leaves are green with dark red veins and shaped like a heart. Fully grown leaves are generally around 12 centimeters in length.
Mature plants can be up to three metres tall with solid thickets. The stems look similar to bamboo, but feature rings and purple speckles.
Japanese knotweed plants are cherished by some beekeepers as a vital source of nectar for honeybees, during a season when not many flowers are blossoming. It provides a monofloral honey, called bambo honey, cultivated by northern US beekeepers, and is similar to a mild-flavoured version of buckwheat honey.
The young stems are edible as a spring vegetable, and taste similar to rhubarb. In some areas, using Japanese knotweed as a source of food has been a way of controlling knotweed populations from spreading to sensitive wetland areas.
The plant has also been used to treat various illnesses in traditional Chinese and Japanese medicine, although there is no evidential proof from clinical studies of its medical proficiency.
Why is Japanese knotweed a problem for gardeners?
The plant builds thick, dense colonies which overwhelm any other herbaceous species and is considered by the World Conservation Union to be one the world’s 100 worst invasive species.
It is a frequent coloniser of temperate ecosystems, roadsides and waste places. It can withstand a very wide range of soil types, pH and salinity, and its rhizomes can tolerate temperatures of -35C.
Japanese knotweed can grow to seven metres horizontally and three metres deep, and this structure makes it extremely tough to remove by excavation. It is also stubborn to cut as it resprouts from the roots.
Japanese knotweed overgrows other vegetation, buildings, and other structures. It encourages fire and damages paved surfaces.