The most common things that people report to us when they are concerned that they might be affected by Japanese knotweed are as follows:

• Japanese knotweed shoots are small, purple and seem to suddenly appear from the ground
• Clumps of dead, hollow stems
• Early Japanese knotweed canes and stems are bamboo-like
• Japanese knotweed leaves are shield-, shovel- or heart-shaped
• Incredibly rapid rates of growth
• Encroachment of invasive plant vegetation from neighbouring property
• Damage caused to built structures from plants growing through them

The above are early indicators that you should look out for if you are worried that knotweed might affect you. Any of these concerns should be reported to a professional to give further advice. However, your first stop should be our Japanese knotweed identification page, which has detailed information on identifying Japanese knotweed to help you determine whether or not the invasive plant affecting your property might be knotweed.

The above are early indicators that you should look out for if you are worried that you might be affected by knotweed. Any of these concerns should be reported to a professional so that further advice can be given. However, your first stop should be our Japanese knotweed identification page, which has lots of detailed information to help you determine whether or not the invasive weed affecting your property might be knotweed.

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.

The small shoots are quite distinctive if you are familiar with them. However, many other plants produce similar shoots (the red colour resulting from protective pigments called anthocyanins) and this can cause alarm to 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 the appearance of knotweed 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 cause complications for the most effective treatments.

• New Japanese knotweed leaves are dark red and 1 to 4cm long
• Young leaves are green and rolled up with dark red veins
• 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

• 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 carried out lead to stems that are much thinner and shorter than mature stems.
• Mature growth forms dense thickets 2m to 3m tall.
• Mature stems are hollow and not at all woody (they can be snapped with relative ease to show their hollowness)

• Japanese knotweed flowers bloom in late summer (late August/September)
• Knotweed has small creamy white flowers that are around 0.5cm wide
• Knotweed flowers form clustered ‘spikes’ of flowers amongst the foliage
• Spike lengths are approximately 10cm

New shoots start to appear in early Spring, around March to April. The rate of growth of Japanese knotweed 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 (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 Spring

• New shoots 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

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 purpose of the stems is to 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.

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

Key Points of Japanese Knotweed Identification in Summer

• The hollow stems grow to a maximum height of ~2-3m
• Stems 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 that the stems emerge from form dense clumps
• Japanese knotweed flowers appear in the late summer (August/September)

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. The flowers and seeds 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 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

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.

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, some knotweed shoots can persist, or new shoots emerge, throughout the winter months

• 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

• The treatment of Japanese knotweed with herbicides can cause a number of distinctive effects on stem growth
• 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 its related species. The most common of these hybrids is that of Japanese knotweed and giant knotweed.

The hybrid knotweed then has the ability to spread by seed, which Japanese knotweed was lacking 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.

Japanese knotweed also produces hybrid seed from the pollen of the closely related and common garden ornamental plant, Russian vine, which is also known as ‘mile-a-minute’ and Bukhara fleece flower. Luckily, this seed very rarely develops into viable plants, which is probably due to it being particularly sensitive to the relatively mild and wet winters we experience in the UK and possibly because the seedlings appear quite fragile.

The Russian vine hybrid is known as railway-yard knotweed, which was named after Ann Conolly of Leicester University who, along with John Bailey (who registered the name as a birthday surprise for Ann), was a pioneer researcher on UK knotweed.

Giant knotweed is native to the island of Sakhalin which is just north of Japan but is actually part of Russia. As with Japanese knotweed it was brought to Europe to be grown in botanical gardens. It has also escaped into the wild where it is spreading. It is similar to Japanese knotweed in many respects but is larger, growing over 4m high and having leaves around 20-40cm long. Giant knotweed also has flowers that are greener in colour and leaves that are more rounded at the base than Japanese knotweed. The leaves also have scattered hairs (trichomes) on the undersides.

The hybrid plant grows slightly larger than Japanese knotweed and has slightly larger leaves but is smaller than giant knotweed. The leaves also have a pointed tip and a slightly rounded base, which makes the leaves appear more heart-shaped than its Japanese knotweed parent.

The table below details some of the more obvious leaf features that can be used as a rough guide to identify whether a plant is hybrid knotweed or Japanese knotweed.

Japanese knotweed leaf

• 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

Hybrid knotweed leaf

• darker green
• crinkled edges
• lobes at base forming heart-shaped leaf
• mature leaves are larger than those of Japanese knotweed
• short trichomes (hairs) on the undersides of the leaves

Other forms of knotweed that are found in some areas of the UK include a dwarf variety and Himalayan knotweed. These plants can be differentiated from Japanese knotweed by dwarf knotweed only growing to approximately 1m in height and Himalayan knotweed having long, slender leaves and can grow up to 2m in height. A number of other closely related species that can often be confused with Japanese knotweed include some bistorts, water peppers and other Persicaria species.

You can read more about these on our Plants that are Mistaken for Japanese knotweed page.