• Japanese knotweed can be identified by its creamy white flowers that are around 0.5cm wide. Dense clusters of the flowers are formed on thin spikes approximately 10cm long.
• The small creamy white flowers appear very late in the summer and early autumn, usually between late August and September.
• 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, shovel 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.

The following describes early signs of Japanese knotweed.

• Clumps of dead, hollow stems.
• Japanese knotweed canes and stems are bamboo-like.
• Japanese knotweed shoots are small and purple and seem to suddenly appear from the ground.
• Incredibly rapid rates of growth.
• Encroachment of invasive plant vegetation from neighbouring property.
• Damage is caused to built structures by plants growing through them [7].

• Young Japanese knotweed stems look very different depending on the maturity of the stand from which they emerge each spring and on what control measures might have affected their development.
• From healthy and mature crowns, new shoots appear from thumb-sized buds that are dark red to pink in colour and start showing themselves in late winter. The rapidly extending shoots first look a bit like thick asparagus spears, until the leaves unroll, which happens when the stems reach a height of around 30cm.
• When young Japanese knotweed shoots emerge, seemingly without notice, from the ground a few metres from the nearest crowns, they appear as dark red/purple shoots with tightly rolled leaves. This growth arises from rhizome buds underground, which are much smaller than the crown buds.
• If you are familiar with them, the small, red shoots are quite distinctive. However, many other plants produce similar shoots (the red colour resulting from protective pigments called anthocyanins), which can alarm the unwary. The unfurling shoots of peonies are one example of a plant often reported to be knotweed in early spring.
• New shoots can appear deformed if Japanese knotweed has previously been treated with herbicides. Micro-leaves and micro-stems often develop when glyphosate is used, producing what’s often referred to as ‘bonsai’ knotweed. Other herbicides can cause twisting and curling of stems and leaves. If crowns have been removed by digging, this can sometimes cause smaller than usual stems to develop.
• The variability in knotweed’s appearance often means getting professional help in identifying Japanese knotweed is advisable. This particularly applies if there is evidence of previous ineffective treatment, as this can complicate the most effective treatments.

Key Points of Japanese Knotweed Identification in Spring

• New shoots appear in early spring and are fleshy and ‘asparagus ’-like when they appear from crowns.
• New Japanese knotweed leaves from rhizome buds are dark red/purple in colour and are often rolled up.

New shoots start to appear in early spring, around March to April. Japanese knotweed’s rate of growth in spring is rapid. The fleshy, asparagus-like shoots from mature crowns can reach a height of a metre or more within just a few weeks.

If you are interested in foraging, now is the time to collect knotweed stems for eating. If you can be sure your actions won’t spread it, you might like to try this Japanese knotweed crumble recipe!

The sudden appearance of the small, dark red/purple shoots from rhizome buds can be alarming, particularly as they can seem to arise from nowhere. The tightly rolled leaves and the colour of the shoots make them particularly distinctive at this time of year.

The way the leaves unfold often results in a pair of almost parallel pale green stripes on the leaves that can persist well into the summer.

Key Points of Japanese knotweed Identification in 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.
• In the summer, Japanese knotweed leaves form an alternate zig-zag pattern along the stems.
• 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 heights of more than 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, does its late flowering. Few plants produce dense stands of 3m tall stems that flower profusely in September.

Key Points of Japanese knotweed Identification in Autumn

• 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.

Key Points of Japanese knotweed Identification in Winter

• The leaves fall, and the shoots die back to leave dead, straw-coloured, hollow stalks which look a lot like bamboo stems.
• Following a mild autumn with no frosts, knotweed shoots can persist throughout the winter months – sometimes new shoots can emerge.

Once the leaves have fallen, all that remains are the dead stems, which can persist for a few years. However, as they are quite fragile and brittle, they are usually toppled by the weather or are pushed over as new stems emerge the following spring.

As most other non-woody vegetation dies back, knotweed stems, particularly where they appear as dense stands, can be easier to spot in the winter. In particularly mild winters, and in areas where plants are shielded from the weather, some knotweed stems have been seen in a relatively healthy, green state as late in the season as February.

• Sections can be from a few mm to ~20cm in diameter.
• The exterior of Japanese knotweed roots is dark brown, and the inside is orange/yellow.
• They are easy to snap (like a carrot).

Japanese knotweed is a rhizomatous plant, which means that it relies on a fleshy network of underground shoots to store energy and spread through the ground. The rhizomes in the ground form a bank of buds from which new shoots can emerge. In this way, Japanese knotweed rhizomes (roots) can be considered analogous to a seed bank in the soil Bud- and seed-banks are types of reproduction strategies that plants use to survive unfavourable growth or germination conditions. Both rhizome and seed survival strategies allow some plants to remain dormant in the ground for many years – decades in some cases.

Japanese knotweed produces particularly large rhizome networks in the ground, which often spread a few metres outwards from the crowns. In the right conditions, the spread can be much further.

Japanese knotweed roots are also renowned for the depth in the ground they can penetrate, which is often a metre and sometimes much deeper (3m is often quoted in guidance).

In its native Japan, knotweed spreads by extending rhizomes causing rings of crowns to appear after decades of growth. This happens as above-ground shoots appear on the edge of the extending growth and the older, central, crowns die back. These ‘ramets’ of stems on the edge of the rings eventually separate from the parent growth, forming new ‘daughter’ populations of knotweed plants. This is a common form of reproduction and spread in rhizomatous 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 snapped with relative ease.

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

• 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 was unable to do during its early introduction due to the absence of any male plants in the United Kingdom.

This is a very important distinction because Japanese knotweed plants usually only reproduce from cut pieces of stem and rhizome, whereas hybrid knotweed plants can also, potentially, reproduce from seed, which can 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 in order 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
• Plants could be reintroduced back onto the site after treatment from windblown seed from hybrids in the surrounding area.

• 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 that results from seed resulting from the cross-fertilisation of Russian vine and Japanese knotweed – which readily hybridise but the seeds they produce almost never 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