How To Identify Japanese Knotweed

By Mark Montaldo on 2nd July 2019 (updated: 19th July 2019) in News

Japanese knotweed looks different throughout the year.

In order to correctly recognise Japanese knotweed it’s useful to have a reference point to better understand the various stages of this elusive plant’s life cycle.

In this guide we’ll take you through how to identify Japanese knotweed throughout the seasons. If you’ve spotted this plant on your land in any of these forms then get in touch with us (using the contact form to the right) so we can help to positively identify it and move forward with a plan.

Japanese knotweed enters several recognisable phases throughout the year, whilst treatment can take place at any time it’s typically easier to spot during the Summer as this is when the plant is most visible above ground. Knotweed often appears to ‘die back’ during the colder months, but it’s unwise to assume that your problem is simply gone. Whilst the plant enters this dormant phase above ground it is still alive and kicking under ground.

Knotweed Roots

Root systems (known as rhizomes) can burrow up to 3 metres deep into the ground with individual rhizomes growing up to 20cm thick, but this kind of growth only occurs if the plant is given the time and space to flourish. Rhizomes are dark brown on the outside and orange on the inside, you can usually break them in your hands unless they have grown into a larger clump or ‘crown’. You might well bump into a network of roots whilst you’re digging in the garden or landscaping. If you do it’s imperative that you do not distribute any of the plant matter as these rhizomes can propagate more plants from just the smallest of fragments.

Although roots can be snapped with your hands, separating them from their system could still be laborious.

Early Spring: February-March

The sooner you tackle your infestation the quicker you’ll be able to get rid of it for good, but the plant can be difficult to spot until major growth above the surface begins to take place. In early spring you might see buds appear at the base of old stems. Keep an eye out for any red/pink tips pushing through the ground, these buds grow to be around 1-3cm wide and will soon be replaced by easier to spot shoots.

Buds appear early in spring at the base of old stems, around 1-3cm wide and pink/red in colour.

Spring: March-May

Once spring is under way Japanese knotweed shoots become a lot easier to spot. Pinkish red buds soon give way to more recognisable spear-shaped shoots, which are sometimes described as ‘asparagus like’. At this stage leaves are red and rolled up, but they soon turn green as the plant grows skyward.

Small red/purple spear shaped shoots appear. Watch out for red or green leaves, with red veins.

Summer: June-August

Growth accelerates significantly during summer making the plant much more conspicuous. The hollow stems grow up to 2cm a day and the leaves now assume their most recognisable shield shape with an alternating stem pattern. This presents a good opportunity to treat the problem with a PCA-accredited glyphosate programme, professionals are able to assess your infestation and inject each plant with their specialist equipment.

Be warned that there are a variety of sub-species and cross-breeds that can easily be confused with this plant, send us photos of your potential infestations to get a positive ID before looking in to how to eradicate your knotweed problem.

By mid-summer knotweed plants are close to reaching their full height (between 1.9-3m). The stems are now less green and are rigid with more pronounced dark purple speckles. Clear nodes are visible on these stems giving them the appearance of bamboo, they can also be snapped easily in the hand.

Leaves develop distinctive shield shape with fresh green colour and flattened base, stems take on bamboo likeness.

Late Summer: Knotweed Flowers

It’s not until late summer that Japanese knotweed flowers; multiple bunches of creamy white flowers appear amidst the leaves, but the seeds that are produced from this process rarely lead to new growths.

Small creamy flowers appear with a spike length of 100mm later in summer, stems now take on a more rigid, woody form.

Autumn to Winter: September-February

As Autumn progresses, stems redden and the plant sheds seed cases. If the weed is left to its own devices the leaves and flowers will eventually begin to fade and fall back. In the middle of winter all that remains above ground is a collection of pale, dry canes with the rhizomes lying dormant beneath the surface waiting for warm weather to sprout and spread further.

Leaves fall off the stems which turn to brown and then a pale straw colour as winter progresses.

We could help you recover the costs of treating your knotweed problem, especially if you’ve been sold the land without knowledge of the plant or if it has entered your property from a neighbouring property. 

If you have any plant matter on your land that resembles these images then it’s worth taking your own photo and sending us your images using the form on the right. Write us a message, chat with us using the button on the bottom right, or give us a call on 0151 242 9050.

Mark Montaldo Knotweed Help Solicitors
Mark Montaldo

Mark Montaldo is the lead solicitor for Knotweed Help and has over 12 years of experience in the field of Civil Litigation and has advised a Parliament Select Committee on the legal issues faced by individuals dealing with Japanese knotweed on their land. Now considered one of the country's leading litigators in Japanese knotweed law he works alongside the country’s top barristers and experts. His in-depth legal experience and connections to the Japanese knotweed removal industry make him uniquely suited for handling your case.