Giant knotweed – Reynoutria sachalinensis

Knotweeds are perennial herbaceous plants in the family Polygoneae. Giant knotweed ((Fallopia sachalinensis or Reynoutria sachalinensis), also known as Sakhalin knotweed), Japanese knotweed (Fallopia japonica) and their hybrid, Bohemian knotweed (Fallopia x bohemica) are native to eastern Asia. From there they have been introduced as ornamentals into gardens around the world. The common features of Asian knotweeds are large size and erect growth. Rhizome is perennial, but the above-ground parts are annual. All species grow very rapidly and stems can grow up to several metres tall over a summer. Thus, tall knotweeds produce plenty of litter.

The species have been popular garden perennials because they are hardy, fast-growing and spread easily. However, the same characteristics have made them very difficult and harmful weeds. In Finland, Asian knotweeds only spread vegetatively and do not produce seeds. Due to the rhizome, the plants are able to quickly form extensive dense and high stands. The thick ground stems can grow a metre per year and reach up to 20 metres in length and two metres deep. The plant is able to penetrate even asphalt. Even a tiny fragment of rhizome quickly produces new growth, due to which knotweeds spread to new places of growth very easily, i.e. along with garden depris or soil.