How to Identify Japanese Knotweed
Japanese knotweed identification is not always easy, but if a potential infestation is ignored, there could be destructive and costly legal consequences. The following video provides some simple advice on what to look out for.
However, before you send us any photos, we would advise you to read through our guide on Plants That Look Like Japanese Knotweed. From our experience of receiving thousands of photos emailed to us, it is quite likely that you don’t have knotweed at all and instead have one of the plants listed on our above-linked page of ‘usual suspects’.
If you really do think you might have Japanese knotweed, then please contact us today to speak to one of our expert consultants to see how we can help.
When is the Best Time to Spot Japanese Knotweed?
The best time to spot Japanese knotweed is during mid-summer and early autumn. During spring, reddish/purple shoots appear from the ground and fat, asparagus-like ‘spears’ rapidly lengthen from bright pink ‘crown’ buds. These can grow up to 2cms a day, thus rapidly forming dense stands of bamboo-like stems that develop green heart- or shield-shaped leaves.
By early summer the mature Japanese knotweed stems are hollow with purple speckles and can reach up to 3 metres in height. The leaves alternate along each side of the stem, producing an obvious knotweed zigzag pattern.
The knotweed flowers that emerge by late summer are creamy-white in colour, and appear in lengthy cluster/spike formations. Japanese knotweed spreads mainly from its underground rhizomes/roots which lie dormant, but alive, over the winter months.
The rhizomes can spread several metres outwards from the visible, aboveground stems, and to depths of more than a metre. It is therefore very easy to accidentally fragment pieces of rhizome and spread them by disturbing the soil several metres from where the stems appear. As new growth from seeds is very rare, it is testament to the plant’s incredible invasiveness that it has spread to most parts of the UK (and many parts of western Europe and North America, for that matter) simply through the fragmentation and translocation of rhizomes in contaminated soil.
Below are some images and descriptions of the main features of Japanese knotweed that should help you identify it.
You can click on each thumbnail image to make it bigger.
What do Japanese Knotweed Leaves Look Like?
- New leaves are dark red and 1 to 4cm long
- Young leaves are green and rolled up with dark red veins
- The mature leaves are green and heart-shaped but flattened at the base (a bit like a shield) and are usually around 12cm long
Japanese Knotweed Stems
- Mature growth forms dense thickets 2m to 3m tall
- 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
- Mature stems are hollow and not at all woody (they can be snapped with relative ease to show their hollowness)
- Immature growth from small crowns, fragmented or disturbed rhizomes, and areas where mowing or other methods to reduce plants’ vigour have been carried out, are not hollow and are much thinner and shorter than mature stems
Herbicide Effects on Japanese Knotweed
- 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
What do Japanese Knotweed Flowers Look Like?
- Japanese Knotweed flowers bloom in late summer (late August/September)
- The flowers’ appearance is creamy white in colour and 0.5cm wide
- Knotweed flowers form clustered ‘spikes’ of flowers amongst the foliage
- Spike lengths are approximately 10cm
What Does Japanese Knotweed Rhizome (root) Look Like?
- Sections can be from a few mm to ~20cm in diameter
- The exterior is dark brown and the inside is orange/yellow
- They are easy to snap (like a carrot)
When do Japanese Knotweed Crown Buds Appear?
- 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 in Spring
- New shoots are fleshy and ‘asparagus’-like when they appear from crowns
- New leaves from rhizome buds are dark red/purple in colour and are often rolled up
Japanese Knotweed in Summer
- Knotweed stems grow to a maximum height of ~2-3m
- They are green with red/purple speckles
- They are hollow
- They have clearly visible nodes between stem sections, which makes them look like bamboo
- The leaves form an alternate zig-zag pattern along the stems
- The crowns that the stems emerge from form dense clumps
- Flowers appear very late in the summer (August/September)
Japanese Knotweed in Autumn
- Flowers and small (5mm) seed cases fall from stem bracts
- Leaves turn yellow and begin to fall – yellowing can begin at leaf margins and gradually spread
- Stems redden and shed leaves
Japanese Knotweed 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 Hybrids
Japanese knotweed (Reynoutria japonica / Fallopia japonica) can also hybridise with its related species. The most common of these hybrids is that of Japanese knotweed and giant knotweed (Fallopia x bohemica, also known as Reynoutria x bohemica). 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 (Fallopia baldschuanica), which is also known as ‘mile-a-minute’ and Bukhara fleeceflower. 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 (Fallopia x conollyana, also known as x Reyllopia conollyana) 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 (Fallopia sachalinensis / Reynoutria sachalinensis) is native to the island of Sakahlin 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 more green 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 (Fallopia x bohemica / Reynoutria x bohemica) 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. Other hybrids from various back crosses with Japanese knotweed and hybrid knotweed generally appear similar to Fallopia x bohemica.
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 (Fallopia japonica var. ‘compacta’, which can also be referred to as Reynoutria compacta) and Himalayan knotweed (Persicaria wallichii). 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 commonly mistaken for Japanese knotweed page.
If you are still unsure as to whether you might have an infestation of Japanese knotweed on your property, please send us a picture for a free assessment, below.
Alternatively, if you are relatively certain that you have an infestation then take a look at our Japanese knotweed removal services or our full list of knotweed treatments available. If you are a home-owner you may find our domestic Japanese knotweed services page useful. It might also be a good idea to familiarise yourself with the various legal issues associated with knotweed.
We also have a handy Japanese Knotweed Identification sheet for your reference.
Feel free to contact us today to speak with one of our expert consultants who can help with any of your Japanese knotweed identification or treatment concerns.