Identifying Japanese knotweed

Japanese knotweed is easiest to identify in summer, by its shield-shaped, flat-based leaves growing in a zig-zag along hollow, bamboo-like stems speckled reddish-purple. It can be recognised all year, though, from red spring shoots, creamy-white late-summer flowers, standing brown winter canes and a rhizome that is orange inside.

The plant and its origins

Japanese knotweed (Reynoutria japonica, formerly Fallopia japonica and also known as Polygonum cuspidatum) is a tall herbaceous perennial, native to Japan, Taiwan and northern China, that often forms dense thickets. In the UK it is common on waste land, railways, roadsides and riverbanks. The RICS professional standard used for mortgage valuations treats all four invasive UK knotweeds (Japanese, dwarf, giant and the Bohemian hybrid) as "Japanese knotweed" for practical purposes.

Introduced as an ornamental, it won a gold medal in Utrecht in 1847 as the year's most interesting new ornamental plant. Kew received it from Philipp von Siebold's nursery in Leiden on 9 August 1850, and a British nursery is first recorded selling it in 1854. The first recorded escape from cultivation in Britain was in Cardiff in 1886, and by 1996 it had been recorded in 55% of the 10 km squares in the British Isles mapping scheme.

Leaves and stems

The leaves are shield- or spade-shaped and broadly oval, usually 10 to 15 cm long, with a flat base. Some websites call them heart-shaped, but the flat base distinguishes them from giant knotweed leaves, which have a heart-shaped, lobed base. They alternate up the stem in a zig-zag, one to a node and never in opposite pairs, and feel tough, with a smooth underside.

The stems resemble bamboo: hollow, with conspicuous nodes and reddish-purple speckling. The Property Care Association (PCA) describes mature canes as typically 10 to 25 mm across and fibrous rather than woody, branching alternately in a mild zig-zag. Mature plants often arch at the top, and RICS notes that stands can grow to over 2 m in spring and summer.

Through the year

Spring shoots

Growth begins at ground level as rounded pink or red buds a few millimetres high, followed by reddish-purple shoots that can look like asparagus spears at 10 to 20 cm tall. The first leaves are pinkish-red, later turning green. Shoots usually appear around March and April, although the PCA notes that timing varies with altitude, latitude, exposure and weather.

Summer growth and flowers

The stems form a dense stand and reach full height in late summer, when clusters of small creamy-white flowers appear towards the tips, usually from August. The PCA describes these drooping clusters (panicles) as falling after four to five weeks, leaving spindly flower stems through autumn and winter.

Autumn die-back

In October and November the leaves and stems turn yellow, then brown, as the plant draws nutrients back into the rhizome. The leaves fall, and the stems turn buff-brown but usually stay upright.

Winter canes

Last year's dead canes often persist, showing the plant's height, alternate branching and old flower stems, and a cut or snapped base reveals the hollow centre. The RHS notes that stems can also collapse into dry brown sections on the ground, but are more likely to stay upright in sheltered spots. The PCA says identification is hardest from December to February only where the canes have been blown down or cut and cleared.

The rhizome

Below ground, knotweed grows from rhizomes, or underground stems. These are thick and woody, like tree roots, with a dark brown outer layer, but bright orange inside when cut or snapped. A rhizome crown sits at the base of the plant, and the RHS says rhizomes often extend 1 m or more down.

Stunted and concealed growth

After herbicide treatment, or sometimes because of pollutants in the soil, knotweed can regrow only a few centimetres tall with small, elongated leaves. The PCA warns that this "bonsai" regrowth is easy to miss, and that stands are often concealed: cut down, or hidden under tarpaulin, sheeting, gravel or bark.

RICS adds that rhizome can lie dormant in the soil for decades, that some herbicide treatments can induce dormancy, and that even a competent inspection can miss knotweed, for example where a stand is small, the visit is outside the growing season, or growth has been cut down or cleared.

Giant, hybrid and dwarf knotweed

Giant knotweed has no red flecking on its stems and is relatively uncommon in the UK. The hybrid sits between the two in size and leaf shape, with some red flecking. Both are genetically diverse and, unlike Japanese knotweed, can set seed. The shorter dwarf form is still invasive, the RHS points out.

How it spreads

The entire British population of Japanese knotweed appears to be a single female clone. A DNA study of 150 samples from Land's End to Shetland found no genetic variation, and no male-fertile plants are known in Britain, so it produces no seed here and spreads instead from fragments of rhizome and stem.

RICS says sections of rhizome under 1 cm or 1 g can regrow, and the RHS and Welsh Government both say fragments under 1 cm can produce new plants. The widely repeated 0.7 g figure comes from greenhouse trials by Brock and Wade in 1992. A 2021 study found that 0.5 g fragments regenerated and survived provided they had at least one node, while those without a node never did. Far from disproving the 0.7 g figure, this suggests that even smaller pieces can regrow if they include a node.

Confirming identification

If you can see several of these features, or are buying or selling a property where knotweed may be present, get a specialist's view. Even where knotweed is having little impact, RICS tells surveyors to recommend a specialist inspection to clients other than lenders, particularly where it is growing within 3 m of a boundary.

Photographs are a sensible first step: many PCA contractors offer free identification from photos, and the RHS advises sending photos rather than plant samples. Smartphone apps can help, but RICS says they are not definitive and their results may need checking. Once you have ruled out the common look-alikes, the photo check below gives an automated, indicative result.

If it does look like knotweed, avoid digging or moving soil until a specialist has seen it and can advise on treatment and removal. If you need a survey, we can put you in touch with up to three independent specialists who cover your area.