Japanese knotweed

There are 6 principal identifiers of Japanese knotweed:

• Leaf: mid-green, shield-shaped (pointed tip with a straight edge along the top). Leaves can vary in size.
• Petiole (leaf stem): distinctive zig-zag, due to leaves growing alternately along the stem. Often pink/purple in colour.

Figure 1: Japanese knotweed leaf and petiole

• Stem: bamboo-like, hollow, jointed. Green, often with a pink/purple speckle.
• Crown: a knotty mass in the ground at the foot of stems on mature knotweed plants, often partially visible above ground. The most regenerative part of the plant. Thick, fibrous roots and rhizomes grow from the crowns.

Figure 2: Japanese knotweed stems and exposed crown (glove indicates scale)

• Rhizome: an underground stem system that can spread several metres in any direction. In mature plants, rhizomes grow from the crowns with many pushing up to the surface as new shoots. Internally rhizome is orange, rather like a carrot, though can be bleached white when very young. Wispy, fibrous roots grow from the surface of mature rhizomes.

Figure 3: Japanese knotweed rhizome (cross-section)

• Flower: small, white, 5-petalled, appearing in clusters in late August. Persist until October/November.

Figure 4: Japanese knotweed flower

Different types of knotweed in the UK

When you refer to knotweed in the UK it is usually Japanese knotweed (Fallopia japonica). This is the most common species of knotweed in the UK but it is not the only one. Below are brief descriptions of the other knotweeds found in the UK.

Note: When 'knotweed' and 'Japanese knotweed' are referred to throughout this section the inclusion of the species below should be inferred.

Giant knotweed
(Fallopia sachalinensis)

This originates from Sakhalin Island, Korea, northern Japan, and the Kurile Islands. It was introduced to Britain sometime during the late 1860s but is less widely distributed than Japanese knotweed. It is a much taller plant (up to 4-5 metres tall), with much larger, elongated heart-shaped leaves. It possesses the same zig-zag on the stems, due to the alternate placing of the leaves, but the bamboo-like stems are more greenish-brown in colour. New growth tends to begin later in the spring than other knotweeds, though flowers still appear late summer or early autumn. The flowers themselves are greenish-white in colour and produced in dense clusters. Rhizomes are often more creamy in colour than the orange of Japanese knotweed.

Figure 5: Giant knotweed

Bohemica
(Fallopia x bohemica)

Otherwise known as bohemian knotweed, hybrid knotweed or the somewhat long-winded Fallopia japonica var. japonica x Fallopia sachalinensis, this is a hybrid formed by the pollination of Japanese knotweed by Giant knotweed. While most such hybrids are short-lived or have little long-term effect on their local environment, bohemica is proving to be highly successful and occurrences of this plant are increasing across Europe and the UK. Not only is it on the rise, but it has proved to be much more resistant to herbicides than other knotweeds.

Bohemica was only recognised as a separate species in 1983, but it is clear that it had been flourishing for at least a century prior to this. It grows to a similar height to Japanese knotweed, though stems are often thicker and leaves are larger, darker green, more crinkly in texture (especially around the edges) and heart-shaped instead of shield-shaped. Rhizomes often have less colour internally than Japanese knotweed and mature bohemica plants have been discovered with an absence of crown. Flowers appear in late summer, are creamy-white and are produced in densely-packed clusters.

Figure 6: Bohemica

Himalayan knotweed
(Persicaria wallichii)

This originates from the Himalayan region of Asia. It is one of the least common knotweeds found in the UK, and does not particularly resemble such knotweeds as Japanese, Giant, Bohemica or Dwarf. It grows to an average height of 1.8-2 metres and the stems are quite different to other knotweeds. They are not particularly bamboo-like and although they zig-zag this is caused by node points where groups of leaf stems branch from rather than simple alternate leaves. Brown sheaths can be found at these node points and the stems are covered in visible white hairs. Leaves are variable but tend to be long and thin, tapering to a point, and slightly crinkled along the edges. Flowers are small, white or pale pink with yellow centres and appear in upright loose clusters on the ends of the stems in mid-summer.

Figure 7: Himalayan knotweed

Dwarf Japanese knotweed
(Fallopia japonica var. 'Compacta')

This is a much smaller version of Japanese knotweed, growing no more than 70-100cm in height on average. Leaves and flowers have some distinct differences from standard Japanese knotweed, with the leaves being darker green and more variable in shape. The leaves have a leathery texture and are usually concave, with crinkled edges curling upwards. Flowers appear late summer in tightly packed upright spikes, and are usually pale pink or white, maturing to a dark pink or red.

Figure 8: Dwarf knotweed

Lesser knotweed
(Persicaria campanulata)

This originates from North India and south-west China. It is not particularly widespread in the UK, and bears little resemblance to the main knotweed species. It grows up to 60-90cm tall and has pink-green, tubular, crooked stems that are often free of any leaves on the bottom two-thirds. Leaves are variable, but long and pointed. They grow at approximately 90° to the stems, are dark to mid-green and have a distinctive 'herring-bone' pattern on them. The undersides of the leaves are lighter in colour and covered in white hairs, giving them a felt-like appearance. Flowers are tiny, bell-shaped and appear in mid-summer. They are white or pale pink in colour and are produced loosely along short, upright spikes.

Figure 9: Lesser knotweed

Conollyana
(Fallopia x conollyana)

This is the rarest of knotweeds in the UK. It is another hybrid species, produced by the pollination of Japanese knotweed by a non-knotweed: Russian vine (Fallopia baldschuanica). It tends to be short-lived in the UK, though has been recorded more commonly in Europe. It has an elongated, pointed leaf with a straight back edge.

Figure 10: Conollyana