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Help to Identify Japanese knotweed

Once seen, Japanese knotweed is a hard plant to forget!  Depending on the season, it is usually very tall (up to 3m), produces bamboo-like canes in distinctive and rather dense clumps and spreads very rapidly.  Overall, these features give the plant an appearance that is distinctive and easy to recognise. But it can also be difficult to spot e.g. in winter or where there has been some previous management.
 
Here we try to give a few clues (a mini-guide if you like) to the complexities of Japanese knotweed identification under a wide range of conditions. But if in doubt, you can always contact one of our invasive weed PCA members who should be happy to help identify the plant via your photos or perhaps a site visit. 
 
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March to April identification

Spring is when new season Japanese Knotweed stems are generated from the plant’s underground rhizome system, using the energy stored within to fuel rapid growth.

Sitting at ground level, red ‘bud like’ shoots a few millimetres high will first appear then, as the weather warms, these buds erupt and start to grow as stems, at a rate of up to 12-18cm per day in mature plants. Emergent stems can have the appearance of asparagus-like spears when around 10-20cm high, going on to develop a more distinctive appearance as branches and leaves unfurl.

Japanese Knotweed Identification - Spring growth - PCA

May to July identification

By early summer, the plant’s stems should be reaching full height, which in mature Knotweed plants is typically 2.5 - 3m high and sometimes higher (especially Giant knotweed).

The free-standing ‘canes’ have purple/pink speckling towards the base and may be largely concealed by a semi-dense canopy of foliage above and at the sides of a stand; the leaves are spade (or shovel)-like in shape with a very distinctive flat base. The branches grow from nodes on the plant’s stems in an alternate pattern giving a mild zig-zag shape to the branches. The leaf stalks (petioles) grow from the stems in the same alternate pattern. 

Japanese Knotweed Identification - Summer growth - PCA

August to September identification

In late summer, typically in the month of August, mature Knotweed plants will produce bunched, drooping clusters of tiny white flowers. The flowers are short-lived, the petals falling off the plant 4-5 weeks after emerging leaving spindly infertile seed heads on the plant through autumn and winter.

 

Japanese Knotweed Identification - Autumn growth - PCA

October to November identification

In the autumn, the Knotweed prepares for winter, re-absorbing nutrients back into the plant’s underground rhizome system.

This turns the leaves and stems yellow then brown as the above ground parts of the plant die off. The brown leaves fall from stems/canes which in turn become a buff-brown colour. The brown stems typically stay standing as they are semi-rigid/fibrous.

Japanese Knotweed Identification - Oct to Nov growth

December-February identification

This is, perhaps, the hardest time of year to identify Japanese knotweed, but only if the stems have been blown down by strong winds or cut and cleared.  

Otherwise, the dead canes are themselves a good diagnostic aid for the presence of knotweed e.g. the height, colour and the pattern of branching plus the spindly desiccating flower stems.  Where stems have been blown down or cut and cleared you may be able to find the cut or wind snapped base of the stems at ground level. These will appear as tell-tale hollow stem bases.

Japanese Knotweed Identification - Winter growth - PCA

Mis-identification of Knotweed with other plants

Other plants are commonly mistaken for Japanese Knotweed.  A few examples include Common bindweed, Dogwood, Dock, Himalayan honeysuckle, Lilac and Russian Vine.

Japanese Knotweed is a free-standing plant so discounting climbers such as Common bindweed or Russian Vine should be easy once you know what to look for.

As always, if in doubt please don’t hesitate to contact one of our local PCA members for help.

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Difficult Knotweed identification circumstances

There are circumstances which may prevent Japanese Knotweed (even if it has been present for a long time) from being seen/identified. Below, we list a few examples that might help you (and/or your surveyor) avoid a false-negative survey.

1) Time of year. 

Japanese Knotweed will be visible in the winter by its brown desiccated stems, but if these have been cut and cleared before a survey, visual evidence of knotweed could be hard to find. Until new stems emerge in late-spring and have grown high enough to be seen over any surrounding vegetation it will remain difficult to spot.

2) Previous management.  

This could have been recently or some time ago by the current or previous property owner or a hired contractor. Treatment works can remove the visible identifiable above ground presence of the plant, but the invisible underground rhizome may persist in the soil. Normally, if thorough chemical treatments are applied by e.g. an accredited PCA contractor, any rhizome should be moribund for at least the period of any guarantee.  But poor or inconsistent treatment can result in only short-term rhizome dormancy so that the above ground growth may return.

3) Bonsai knotweed etc.  

Under certain circumstances Japanese knotweed can produce growths which are deformed.  So-called ‘bonsai’ knotweed is stunted stem and leaf growths growing in sporadic clumps to only a few centimetres (max.) above soil/ground level making it difficult to spot amongst other vegetation. Some of the normal growth characteristics may still be discernible but in miniature; typically, the leaf shape as well as being miniature may also be elongated. The usual trigger for bonsai growth is previous herbicide treatment but it can also be caused by pollutants in the soils such as hydrocarbons.

4) Concealment 

Due to the stigma associated with Japanese knotweed it is not uncommon to see attempts to conceal its presence. Sometimes the concealment may have been the result of innocent garden clearance and tidying, other times deliberate e.g. cutting and clearing stems, cultivating soil beds and covering areas with tarpaulin, landscape fabrics, aggregates, or ornamental bark chippings/mulch and so on. Where there is a suspicion of rhizome presence that cannot be verified by surface inspection, further investigation is advised/recommended.

Removal & Treatment of Knotweed

Homeowner guidance towards the treatment & removal of Japanese Knotweed.

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Legislation & the Law

Are you breaking the law? As a controlled species, find out more about the legislation around Knotweed

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More about Japanese Knotweed

Looking to find out more about Japanese Knotweed.  Visit our Knotweed hub page

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Think you need some help?

Thinking you might need a little bit of help and guidance?  If you, you can always contact one of our local & qualified Japanese Knotweed members who in most cases, will be happy to assist with advice over the phone, or if need be, to organise a survey.  Simply use the search tool below to find a local PCA knotweed specialist near you. 

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