Japanese Knotweed Identification & Information

Japanese knotweed has spread with vigour throughout the United Kingdom in a relatively short amount of time. This is somewhat due to the wide variety of conditions in which the plant can survive. Some of its native habitats in Japan include riverine gravels, highway margins and well-fertilised pastureland. However, in areas of volcanic activity, the plant can thrive in soil with pH levels of less than 4 and is often the first plant life to grow after an eruption. In mountainous areas, its hardiness allows it to withstand months of frost, surviving temperatures of -35°C. Its sheer robustness makes Japanese knotweed eradication notoriously difficult.

Japanese knotweed is native to Eastern Asia. In Japan, it is found on the islands of Hokkaido, Honshu, Kyushu and Shikoku. Its range also extends to Northern China, the island of Taiwan (formerly Formosa) and also on the mainland of Korea.

It is disputed as to when the plant was first brought to the West and to the United Kingdom.  The plant was first registered as Reynoutria japonica in 1777 by a Dutchman named Houttuyn. Its introduction to the West is likely to have begun circa 1823 when Philippe von Siebold is said to have brought the plant to his nursery in Leiden, Holland.  Von Siebold sent the flower to the Royal Botanical Gardens Kew in August 1850, when it was introduced to the gardens.  It was planted at the Royal Botanical Gardens Edinburgh in 1854.  The spread of the plant and its rhizome across the country can be attributed to a number of factors:  

• The commercial sale of the plant, beginning in the 19th century, as at the time it was regarded as a fashionable ornamental plant. 

• Travelling along watercourses

• Transportation by and on vehicles such as cars, trains and lorries.  Some areas of vehicles, such as tyres, are susceptible to picking up pieces of the plant and rhizome

• Fly-tipping

• Urban development: the transportation of soil and materials for the construction of roads and buildings distributed the rhizome around many modern urban areas.

The following describes the growth process of Japanese knotweed:

The buds around the crown and the rhizome's many nodes thrust shoots at such speed so as to smother other species present, using energy from a mature rhizome's huge reserve of stored food. Shoot extension can exceed 8cm daily. As the stems develop, broad leaves are produced in an alternating pattern on the stems. This arrangement allows the leaves to be placed where they can receive the maximum sunlight to fuel the photosynthetic factory. Biochemical switches do not allow all the nodes to produce shoots, so that maximum profit of energy receipts over expenditure are achieved by not wasting it on shoots which will be shaded by neighbours.

With all leaves fully expanded, the phloem system increasingly carries the starches, made in the leaves, back to the rhizomes, both to fuel current extension, and to provide the stored energy for explosive growth the following spring.

Plant rhizome systems may extend at several metres per year. Shoots develop from the tips and nodes, capable of breaking the surface as new stems, from depths of greater than one metre. In the late summer, fully-grown stems produce sprays of flowers, which, in the case of the UK clone, are all female from female plants. Where plants are pollinated, which is by bees and other insects, the pollen-parent will usually be the related Giant Knotweed, or male-fertile specimens of hybrid origin. Whether or not pollination is achieved, the knotweed flowers are a good source of late nectar for many flying insects. Few seeds are produced from these hybrid unions, and seedling survival is uncommon.

In Japan, and other countries where pure-strain seed does develop, high volumes of seed are produced about two weeks after flowering. Germination and seedling survival rates are low, and seeds are dormant when shed. This dormancy is broken by a combination of environmental factors, which, when achieved, give the best chance of establishment for the fragile and frost-tender seedlings. Seed dormancy can be prolonged, with germination not occurring for more than one year.

All remaining moisture is withdrawn from the stems as autumn progresses, and all above ground parts die back to a forest of brown, persistent stems. In preparation for the winter dormancy stage, the rhizomes and the clusters of dormant buds and rhizomatous material reside at the base of each dead stem. These structures, from which the rhizomes grow, are known as crowns.

In the absence of seeds, dispersal is achieved by the transport of portions of rhizome to fresh sites. Water flow and human activity are the main vectors. So tenacious is the rhizome that portions weighing no more than 0.7g residing 300mm below the surface can give rise to new plants. Even internodal tissue, not furnished with growth-bud primordia, can develop and initiate shoots. Stem material is also very able to make roots and shoots if detached from the parent plant. 

Japanese knotweed is a hardy, herbaceous, rhizomatous perennial.  Plants are fully dioecious, with all individuals of the UK clone being functionally female. 

Rhizomes measure up to 10cm in diameter, bearing nodes at 1-2cm spacings, and extend generally up to 7 metres from the parent plant (though distances of 20 metres have been recorded).  The rhizome penetrates downward to a depth of 2 metres or more. 

Stems can reach 3 metres high and are stout, hollow and bamboo-like with erect bases that eventually branch.  Stems are sometimes red-brown, but often green and are glabrous to adpressed-pilose, with thinly membranous sheath. 

Leaves are broadly ovate with the bases truncated and the tips abruptly cuspidate or acuminate.  The leaves measure 5-12cm wide and 5-15cm long, petioles 1-3cm long and stipules 8-15mm long. 

The female flower is greenish-white and drooping.  Flowers are 2.5-3mm across in dense, branched, axillary panicles, which are 5-9cm long.