• Japanese knotweed can be identified by its creamy white flowers, around 0.5cm wide.
• Dense clusters of flowers are formed on thin spikes approximately 10cm long.
• Japanese knotweed flowers in late summer and early autumn, usually in August and September. The flowers are small and creamy white.
• A Japanese knotweed plant with flowers indicates a more advanced infestation that will likely prove more challenging to eradicate.

• New Japanese knotweed leaves are dark red and 1 to 4cm long.
• Young Japanese knotweed leaves are green and rolled up with dark red veins.
• Japanese knotweed leaves and hybrids are shield-shaped, shovel-shaped, or heart-shaped.
• Mature Japanese knotweed leaves are green and heart-shaped but flattened at the base (a bit like a shield) and are usually around 12cm long.

• Rhizomes are underground, fleshy shoots used for energy storage and to spread.
• Sections range in diameter from a few millimetres to approximately 20 cm.
• The exterior is dark brown; the inside is orange to yellow.
• They are easy to snap by hand, similar to a carrot.
• Rhizomes have buds along their length, from which new shoots can emerge.
• Networks of rhizomes often spread a few metres outwards from the crowns.
• Rhizomes are typically found within the top metre of soil but can reach depths of up to 3 metres.
• In its native range, ‘rings’ of knotweed can form from the natural die-back of crowns from the centre of large clumps of growth.

Japanese knotweed spreads through an underground system of rhizomes. These store energy and produce buds that can grow into new shoots. The plant can lie dormant underground for years, which makes it difficult to remove fully.

Rhizomes usually spread a few metres from the crown, though they can go further in the right conditions. They are commonly found within the top metre of soil but are sometimes recorded at depths of up to three metres. In its native range, rhizomes gradually form rings of new crowns – shoots appear at the outer edge of these rings, as the older crowns in the centre die back. These new outer groups can eventually break away, forming separate populations of plants.

• Knotweed stems are ‘bamboo-like’, with obvious nodes/rings and purple speckles.
• Leaves shoot from the stem nodes alternately, which creates a ‘zig-zag’ pattern at the top.
• Immature growth from small crowns, fragmented or disturbed rhizomes, and areas where mowing or other methods to reduce plants’ vigour have been used can lead to stems that are much thinner and shorter than unaffected stems.
• Mature growth forms dense thickets 2m to 3m tall.
• Mature Japanese knotweed has hollow stems that are not at all woody and can be easily snapped.

• Japanese knotweed crown buds form in early spring.
• The buds are round at the base of old stems and are 1-3cm wide.
• The crown buds are bright pink/red.
• New Japanese knotweed stems grow annually and develop from the crown buds.

The following describes the early growth of Japanese knotweed:

• Clumps of dead, hollow stems.
• Japanese knotweed canes and stems are bamboo-like.
• Japanese knotweed shoots are small and purple and appear suddenly from the ground.
• Incredibly rapid rates of growth.
• Invasive encroachment of new shoots from rhizomes spreading from neighbouring property.
• Damage to built structures from shoots appearing in cracks or growing through the structures [7].

• Young stems vary in appearance depending on the maturity of the stand and any previous control measures.
• From mature, healthy crowns, new shoots emerge in early spring from thumb-sized buds that are dark red to pink.
• These rapidly growing shoots resemble thick asparagus spears until leaves begin to unroll at around 30cm in height.
• Shoots from underground rhizome buds are smaller than crown buds and emerge a few metres away from the crowns. They appear dark red or purple with tightly rolled leaves.
• Herbicide treatment can cause deformities in new shoots, such as micro-leaves, twisted stems, or ‘bonsai’ growth.
• Digging out crowns can lead to smaller-than-usual stem development in the following season.
• Due to the variability in early growth, especially after ineffective treatments, professional identification is often recommended.

• New shoots appear in early spring (March to April).
• Shoots emerging from crowns are fleshy and ‘asparagus’-like.
• Shoots from rhizome buds are dark red or purple with tightly rolled leaves.
• Growth is rapid, with shoots reaching a metre or more within weeks.
• Unfolding leaves often display two pale green stripes that may persist into summer.

In spring, shoots can appear a few metres from the main crowns as rhizomes spread underground. This can be the first sign that knotweed is growing beyond the visible stand.

Early shoots are easy to spot if you know what to look for. Ones from rhizomes are often dark red or purple, with tightly rolled leaves. These colours, along with the shape, make them stand out from most other spring plants. Pairs of pale stripes can form on some new leaves from the bleaching of pigments on the edges of the leaves as they unroll. Sometimes, these stripes can persist on large leaves into early summer.

• Japanese knotweed is in full leaf during this period, making identification easier.
• The hollow stems grow to a maximum height of ~2-3m.
• Stems are green with red/purple speckles.
• They have clearly visible nodes between stem sections, which makes them resemble bamboo.
• Japanese knotweed leaves form an alternate zig-zag pattern along the stems in the summer.
• The crowns from which the stems emerge form dense clumps.
• Japanese knotweed’s creamy white flowers appear in late summer (August/September).

From May to September is when Japanese knotweed reveals itself in all its glory.

The stems can reach over 3 metres, forming a dense canopy of lush, green leaves. The stems provide temporary scaffolding to support the chemical ‘factories’ in the leaves, where they can capture as much light as possible to power food production by fixing CO2 from the air into carbohydrates stored in Japanese knotweed’s rhizomes. As such, no energy is wasted in producing the stems, which are hollow, unlike the permanent, solid stems of woody perennials.

The thicker stems on mature knotweed stands can be 5cm in diameter. Rings, or nodes, around the stems resemble bamboo canes, but unlike bamboo, Japanese knotweed stems can be snapped easily by hand. The purple speckling on the stems also makes them easily identifiable from bamboo.

Another feature that helps with the identification of knotweed is the zig-zag pattern of alternate leaves and side stems, particularly at the tops of the stems. So, too, its late flowering. Few plants produce dense stands of 3m tall stems that flower profusely in September.

• Flowers and small (5mm) seed cases fall from stem bracts.
• The knotweed leaves turn yellow and begin to fall – yellowing can begin at leaf margins and gradually spread.
• Stems redden and shed leaves.

Japanese knotweed flowers are often still present in early October at the start of autumn. Some of these will have developed into seeds (achenes – from the pollen of Russian vine, as there are no male Japanese knotweed plants in the UK).

The small creamy white flowers and seeds are arranged in conspicuous panicles. They are shed by the end of October. Japanese knotweed leaves start to turn yellow and then brown about this time. However, if there have been no frosts, leaves can remain on some plants into December, particularly if they are protected from winds. The stems also start to die and become brittle, turning a red-brown or straw colour.

• Leaves fall, and shoots die back, leaving hollow, straw-coloured stems.
• Stems resemble bamboo but are brittle and snap easily.
• Dead stems can remain standing for years, but often collapse due to weather or spring regrowth.
• In mild autumns without frost, some shoots may persist into winter.
• Occasionally, new shoots can emerge during winter in sheltered conditions.
• Dense stands are more visible in winter as surrounding vegetation dies back.
• In sheltered areas, some stems may remain green into February.

In winter, Japanese knotweed enters dormancy. The leaves fall, and the shoots die back, leaving hollow, straw-coloured stems resembling bamboo. These dead stems are brittle and can remain standing for several years, although they are often brought down by weather or pushed aside by new shoots in spring.

Some knotweed shoots may persist through the winter if the preceding autumn has been mild and frost-free. In rare cases, new shoots may even emerge during winter, particularly in sheltered spots. Green stems have occasionally been observed in such areas as early as February.

As surrounding vegetation dies back, dense knotweed stands often become more visible in the landscape, making winter a useful time for identification.

• Japanese knotweed treatment with herbicides can cause several distinctive effects on stem growth[8].
• Herbicides containing glyphosate (e.g., Roundup) can result in short clumps of growth with tiny leaves – often referred to as ‘Bonsai’ growth.
• Herbicides containing synthetic auxins, such as aminopyralid, can cause marked stem twisting.

Japanese knotweed can also hybridise with related species. The most common hybrids are Japanese knotweed and Giant knotweed. The hybrids can spread by seed, which Japanese knotweed could not do during its early introduction, due to the absence of male plants in the United Kingdom.

This distinction is important because Japanese knotweed plants usually only reproduce from cut pieces of stem and rhizome. In contrast, hybrid knotweed plants can reproduce from seed and remain dormant in the soil over successive growing seasons as a ‘seed bank’.

Although Japanese knotweed hybrids more commonly reproduce vegetatively from rhizome fragments, the potential risk of viable seed being present from hybrids can mean that to remove hybrid knotweed from a site:

• Herbicide treatments might be required over longer periods;
• Pre-emergent herbicides might need to be incorporated into the treatment programme; and/or
• After treatment for windblown seed from hybrids in the surrounding area, plants could be reintroduced onto the treated site.

• Lighter green
• Smooth edges
• Flat base forming shield-shaped leaf
• Mature leaves are smaller than those of hybrid knotweed
• No trichomes (hairs) on the undersides of the leaves

• Mature leaves are slightly larger.
• Bohemian knotweed leaves are a darker shade of green.
• Leaves have slightly crinkled edges.
• Lobes at the base of the leaves make them appear heart-shaped.
• On the undersides of the leaves, there are trichomes (hairs), which can be of variable length.

• Generally, plants are very similar to Bohemian knotweed but everything about them is much larger
• Larger leaves (20cm to 40cm long)
• Taller plants (up to approximately 4m tall)
• Leaves have large lobes at their base, making them heart-shaped
• Long trichomes (hairs) on the undersides of the leaves
• Flowers have a light green hue

• Plants are shorter (around 1m tall)
• Leaves are long and slender
• Leaves have evenly-spaced, parallel veins
• The undersides of the leaves are paler than the topsides
• The midrib on the topside of the leaves is distinct and is a pale green/pink
• Fused stipules wrap around the nodes, forming sheaths of thin membranes that cover the rings on the stems where the leaves branch off

• Plants are generally shorter, growing to approximately 1.5m in height
• The leaves are similar to those of Himalayan knotweed, having parallel veins and an elongated shape, but they are generally not as slender
• Flowers form clusters of bell-shaped individual flowers that are pink and large (approximately 1cm in width)

• This is a dwarf variety of Japanese knotweed
• Compared to Japanese knotweed, it has smaller, rounder-shaped leaves (approximately 5-10cm in diameter)
• Plants are shorter than Japanese knotweed (less than 1m tall)
• Flowers have a pink hue

• This is a variegated variety of knotweed, whereby mottled patches of pigment are missing from the leaves and stems, making them look like they’ve been spattered with white paint

• This is a variety of Japanese knotweed that has bright pink/red flowers
• The leaves can be a slightly darker shade compared to Japanese knotweed.

• This is a very rare species arising from seed produced from the cross-fertilisation of Russian vine and Japanese knotweed – which readily hybridise, but the seeds from which rarely develop into seedlings
• Mature plants form dense bushes
• Leaves are small and thin, appearing much like those of Russian vine

• Slightly shorter stems (approximately 1.5-2m tall)
• Long clusters of bright magenta flowers hang down from slender stems
• There is a pale midrib on the upper sides of the leaves, a bit like Himalayan knotweed but the leaves are shaped more like Bohemian knotweed