Japanese knotweed identification

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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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. 

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.

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.