Japanese knotweed, Reynoutria japonica (synomyns: Fallopia japonica and Polygonum cuspidatum) is the most widespread form of knotweed in the UK. Stems form a zig-zag growth pattern, with one stem shoot per node. The leaves are fairly smooth, mid-green in colour, with a characteristic straight top edge, giving the leaf a shield or shovel-type shape.
Knotweed flowers are small creamy-white and form in loose clusters (panicles) in late summer or early autumn. All Japanese knotweed plants growing in the UK are female and therefore do not produce viable seeds. Download our Japanese knotweed identification guide, here (2.3Mb).
FREE Japanese Knotweed Identification
What does Japanese knotweed look like?
|1. Japanese Knotweed Flowers||Creamy white coloured panicles of flowers (late summer)|
|2. Japanese Knotweed Stems||Zig-zag growth pattern, green with red/purple flecks|
|3. Japanese Knotweed Leaves||Lush green shovel-shaped leaves with a pointed tip|
|Height||Up to 3m tall|
|Seeds||Not viable in the UK as we have no male plant. Small winged, heart-shaped seeds|
Japanese knotweed in spring
The fastest Japanese knotweed growth is during the spring. New shoots that emerge are red/purple and can look like asparagus spears. The leaves are normally rolled up and dark green or red in colour. In late spring, canes can reach up to 3 metres (10 feet) high. The pictures below show Japanese knotweed in spring.
Japanese knotweed in summer
During the summer the knotweed leaves are green and heart/shovel shaped and can be 20cm across. In late summer early autumn small clusters of white flowers will appear. The stems are mostly hollow and bamboo like and the general growth habit has a distinctive zigzag appearance. The photos below show what Japanese knotweed typically looks like in summer.
Japanese knotweed in autumn
In Autumn the dense covering of leaves will remain, however, they start to turn yellow and wilt as we move into September and October. The knotweed plants are still about 2-3 metres tall and the hollow stems start to turn brown. See our images below to identify Japanese knotweed in Autumn.
Japanese knotweed in winter
During late autumn and the beginning of winter the knotweed canes die off and the weed becomes dormant. The leaves turn yellow, then brown and fall off. The canes are hollow, dark brown and brittle and they collapse upon one another. If the area hasn't been treated, often previous year's decomposition can be seen underneath. See the images below to identify Japanese knotweed in winter.
Japanese knotweed leaves
Japanese knotweed leaves are shovel shaped (some people think they look heart shaped) with a point at the tip and staggered on the stem (one stem per node), creating a zig-zag stem growth pattern. They're a luscious green colour and grow up to 200mm long. See the images below for easy identification of the Japanese knotweed leaf.
Japanese knotweed flowers
Elongated clusters of creamy white flowers may appear towards the end of August and early September. The clusters grow to approximately 0.5cm wide and up to 10cm long. The leaves will still be apparent and along with the flowers, it will create a dense foliage. See our images below to identify knotweed flowers.
Japanese knotweed rhizome
Japanese knotweed rhizomes are the underground part of the weed and are actually considered to be underground stems. If it's fresh, it will snap easily like snapping a carrot. The outside is dark brown and the inside is orange/yellow in colour. The Japanese knotweed rhizome system can grow to depths of 2 metres and can extend up to 7 metres horizontally from the plant. It's the knotweed rhizome that spread the plant by vegetative means. As little as 0.7g of viable rhizome can give rise to a new plant. See the images below to assist in identifying knotweed rhizomes.
Japanese knotweed stems
Japanese knotweed stems grow to 2-3 metres tall. They're similar to bamboo with nodes and purple speckles and the leaves shoot out from the nodes in a zig zag pattern. The inside of the stem is hollow. At the mature stage, the stems are hollow and not woody and can be snapped easily to show their hollowness. However, in the winter the stems become brittle, as can be seen from the images below.
Japanese Knotweed Identification Video
We show you how to identify Japanese knotweed by showing you its key identifying features and how it can sometimes be confused with other common plants.
Do you think you can identify Japanese knotweed?
Japanese knotweed can grow in different shapes and forms depending on the season, previous treatments and the surrounding environment. Bonsai growth is completely different from normal growth and this makes identifying Japanese knotweed a challenging prospect which requires a trained eye.
Try out our identification quiz to put your knotweed identification skills to the test!
Other Types of Knotweed
Giant knotweed (Fallopia sachalinensis)
Giant knotweed is a native of South Sakhalin, Honshu (in the north of Japan), Korea, and the Kurile Islands. It is a closely related species to Reynoutria japonica, but less widely distributed outside of the Far East. Both male and female plants have been recorded in Europe and the UK. Although similar in many respects to japonica (cane structure, distinctive ‘zig-zag’ shape of stems, similar growth habit, etc), it grows much taller (4-5 metres or 13-16 feet) and has much larger, elongated leaves. The leaves can grow to around 40cm (16 inches) long and up to 27cm (11 inches) wide.
They are pointed at the tip, somewhat crinkly in appearance and have long white hairs (trichomes) on the underside. The base of the leaves are deeply lobed, forming a heart shape. Growth generally begins later than japonica, usually mid to late spring, and leaf drop generally occurs earlier than japonica in the autumn. Rhizomes are more creamy in colour internally, rather than the distinctive orange of japonica. Creamy-white flowers appear in late summer/early autumn in dense panicles hanging off the stems. The Japanese call Giant knotweed ‘o itadori’, which, with enviable simplicity, means “big strong plant”.
Bohemica (Fallopia x bohemica)
‘Bohemica’ is a hybrid species formed by Japanese knotweed and Giant knotweed. Consequently, it is also known by the name Fallopia japonica var. japonica x Fallopia sachalinensis. For many years, ‘Bohemica’ went unrecognised as a separate species and was only formally classified in 1983. It is widely distributed, with both male and female plants recorded in the UK. It can be variable in habit and it is common to find ‘bohemica’ growing in close proximity to, or amongst, Japanese knotweed. Leaves are larger than Reynoutria japonica, up to 25cm (10 inches) long and 18cm (7 inches) wide, growing in a heart shape. Leaves are usually longer than they are wide, pointed at the tip, slightly crinkled in appearance and darker green than japonica, with short white hairs (trichomes) growing on the veins on the underside – particularly in the early part of the growing season. Veins are usually reddish purple in immature leaves. Growing habit, including stem colour and shape, is extremely similar to japonica. ‘Bohemica’ grows, on average, to a height of 2.5m-3m (8-10 feet), though taller plants up to 4m (13 feet) have been recorded. Rhizomes have a less prominent colour internally than japonica and can be bleached out completely to white. An absence of crowns has been noted during excavations. Creamy-white flowers appear in dense clusters in late summer/early autumn.
Dwarf Japanese knotweed (Fallopia japonica var. Compacta)
Also known as Polygonum reynoutria, Polygonum compactum and Polygonum pictum, ‘Compacta’ is a dwarf form of Japanese knotweed. It reaches only 1m-1.8m (40 inches) in height, and emerges later than standard japonica (usually late spring). It retains the distinctive ‘zig zag’ petiole structure, but the leaves are darker green, more variable in shape, up to 11cm (4 inches) long and up to 10cm (3.5 inches) wide. Leaves have crinkled edges, a leathery texture, reddish veins and are often curled into a concave form. Upright clusters of white or pale pink flowers appear in late summer, which often mature to dark pink or red. Both male and female plants occur in Europe and the UK, though ‘Compacta’ is rare in countries like Germany and the Czech Republic. Dwarf Japanese knotweed is still available to purchase from some nurseries in the US, where it is promoted for its ground cover properties or as a potted plant. Although smaller and less invasive than Japanese knotweed, Dwarf knotweed still retains some of the voracious growing habit of the species. It seems some lessons are slow to learn.
Himalayan knotweed (Persicaria wallichii)
Himalayan knotweed is known by many names, and is referred to in some sources as Polygonum polystachyum, Polygonum wallichii, Persicaria polystachya, Reynoutria polystachya or Aconogonum polystachyum. With its slender, elongated leaves, it bears greater similarity to Giant knotweed and Lesser knotweed, to which it is closely related, and is often mistaken for Lesser knotweed (and occasionally for Himalayan balsam). Native of the Himalayan region from Afghanistan to south-west China, it is one of the least common knotweeds in the UK, though is more prevalent in the southwest of England. It grows quickly to a height of up to 1.8m (6 feet). Stems are usually green (though leaf stems can contain the distinctive knotweed pink) and have the characteristic ‘zig zag’ from node to node. Stems are hairy, and a key identifier of the plant is the brown sheaths that persist at the bases of the leaf stalks. The dark green, alternate, leathery leaves are 10-20cm long (4-8 inches), tapered to a point. Short hairs can often be found on the veins, edges and undersides. Leaf shapes can differ within the species, with leaf bases varying from tapering to the leaf stem to developing a slight heart-shaped lip. White or pale pink flowers bloom from mid summer to late autumn and occur in loose, branched clusters around 20-35cm (8-14 inches) long. Flowers are hermaphrodite (ie contain both male and female parts). Himalayan knotweed is most commonly found in moist soils and poses a significant ecological threat to riparian areas where it can survive flooding and quickly colonise scoured shores and islands when the flood waters recede.
Lesser knotweed (Persicaria campanulata)
Also referred to in some sources as Polygonum campanulatum, Polygonum campanulata or Reynoutria campanulatum, Lesser knotweed is another member of the species that is still actively being sold by garden centres and plant suppliers in both the US and the UK. A native of North India and Southwest China, this knotweed is less invasive than the others but still retains a familiar vigour of growth. Leaf size can be variable, though conforms to the same long shape. Lesser knotweed bears a casual resemblance to Himalayan balsam (Impatiens glandulifera), and its love of moist soil means it is often growing alongside this troublesome plant. Veins on the leaf can sometimes be reddish and the leaves are distinctly grooved in a pattern reminiscent of herring-bone. The undersides of the leaves are much lighter in colour and are felted by small white hairs. Persicaria campanulata grows to a height of around 60-90cm (2-3 feet) and produces flowers mid-summer that remain until the autumn. The flowers are tiny, pale pink or white, and are bell-shaped (hence the name) and produced in clusters on short spikes. Stems are usually clear of foliage for the lower two-thirds of their length and are slightly crooked due to bending at the nodes. The distinctive knotweed ‘zig-zag’ is missing, although leaves are still produced alternate on the stem. This species is less widespread in the UK, though it is more common in parts of Scotland and Northern Ireland.
Plants Mistaken for Japanese Knotweed
Two species that are not knotweeds but can sometimes be mistaken for one by the inexperienced, due to their similar leaf shapes and voracious growing habits, are:
Russian Vine (Fallopia baldschuanica)
Otherwise known as Silvervine, Fleeceflower or more commonly by the name ‘Mile a Minute’. Reynoutria japonica is known to hybridise with this vigorous climber, but the resultant seedlings rarely survive in the wild and possess none of the aggressive attributes of either of its parents. Russian vine is also known by the Latin names Polygonum baldschuanicum and Fallopia aubertii.
Hedge Bindweed (Calystegia sepium)
Also known as Greater Bindweed, Bearbind, Bellbine, Withybind, Devil’s Guts, Hedge-Bell and, most appropriately, Hell Weed. Like knotweed, it gains its strength from an extensive underground stem system and can be extremely difficult to eradicate once it has taken hold. Also, like knotweed, Bindweed can re-establish itself from root fragments. Similar in appearance, though slightly less vigorous in habit, is Field Bindweed (Convolvulus arvensis), otherwise known as Lesser Bindweed.