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How To Identify Japanese Knotweed?

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What's Japanese knotweed?

Japanese knotweed, also known as Reynoutria japonica syn. Fallopia japonica and Polygonum cuspidatum is a plant that is not native to the UK and has become invasive.

It was brought to the country during Victorian times. Initially, it was valued for its beauty, strength, and ability to regenerate.

However, it is now widely regarded as a nuisance species that outcompetes native plants and causes damage to man-made structures.

The images on the right display the plant’s most distinct characteristics. These visuals will assist you in identifying Japanese knotweed throughout the year.

How do you identify Japanese knotweed?

It’s important to consider the time of year when you’re checking, as the appearance of Japanese knotweed changes with the seasons. The easiest time to identify it is during the spring and summer months.

Here are some key traits to look for when identifying Japanese knotweed:

  • Red shoots that emerge in spring, resembling asparagus.
  • Leaves that are shield or shovel-shaped.
  • Stems that resemble bamboo canes with purple speckles.
  • Small, cream-coloured flowers that develop towards the end of summer.

In the autumn, the leaves will turn yellow and drop as winter approaches. If left unattended, Japanese knotweed can grow up to two or three meters. The stems will darken to a brown colour before the plant becomes dormant in winter.

How To Identify Japanese Knotweed
In the autumn, the leaves will turn yellow and drop as winter approaches. If left unattended, Japanese knotweed can grow up to two or three meters.

What are the Early Signs of Japanese Knotweed?

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

  1. Japanese knotweed shoots are small, purple, and seem to suddenly appear from the ground.
  2. Clumps of dead, hollow stems.
  3. Early Japanese knotweed canes and stems resemble bamboo.
  4. Japanese knotweed leaves are shield-, shovel-, or heart-shaped.
  5. Incredibly rapid rates of growth.
  6. Encroachment of invasive plant vegetation from neighbouring property.
  7. Damage caused to build structures from plants growing through them.

If you are worried that knotweed might affect you, these early indicators should be looked out for. If you have any of these concerns, it is advised to report them to a professional for further advice.

What does Knotweed look like in Autumn?

Japanese knotweed is a perennial plant, meaning it dies back in the fall and comes back in the spring. As the weather gets cooler and light levels decrease in October and November, the leaves of Japanese knotweed start to turn yellow and then brown.

Eventually, they drop to the ground just like leaves from a deciduous tree.

The stems of the plant also lose their green appearance and gradually become brown and brittle. After a few frosts, Japanese knotweed completely dies back, leaving behind brown and brittle canes.

In the autumn, there are key ways to identify Japanese knotweed. The leaves turn yellow and develop brown spots. You can also see clusters of old flowers, sometimes with heart-shaped seed pods. Additionally, the stems of the plant change from green to a rich red/brown colour.

How do you identify Japanese knotweed in Winter?

In the spring and summer months, it is relatively easy to identify Japanese knotweed once you understand its characteristics.

However, surveyors are expected to be able to identify knotweed throughout all seasons, even though it becomes more challenging to spot this problematic weed in the winter. Some things to look out for are:

  • Brown, brittle canes that remain standing.
  • Distinctive crowns in the ground.
  • Scorched areas of grass and bare patches of earth.
  • Smaller plants with a distinctive zigzag stem distribution.

There has been a growing number of cases where surveyors are being sued for professional negligence because they missed Japanese knotweed during surveys. We are here to help ensure that knotweed can be correctly identified regardless of the time of year.

When-is-the-best-time-to-spot-Japanese-knotweed
In the spring and summer months, it is relatively easy to identify Japanese knotweed once you understand its characteristics.

How to Identify Japanese Knotweed in Spring?

New shoots begin to emerge in early spring, usually between March and April. During this season, the growth rate of Japanese knotweed is quite rapid.

In just a matter of weeks, the mature crowns produce fleshy shoots that resemble asparagus and can reach a height of one meter or more.

The sudden appearance of small shoots, which are dark red or purple, may be somewhat alarming as they seem to appear out of nowhere. These shoots have tightly rolled leaves and their distinctive colour makes them easily recognizable during this time of year.

As the leaves unfurl, a pair of almost parallel pale green stripes often become visible, which can persist well into the summer months.

Key Points of Japanese Knotweed Identification in Spring:

  1. New shoots from crowns are fleshy and resemble asparagus.
  2. New leaves from rhizome buds are dark red or purple and are often rolled up.

What does Japanese knotweed look like in Summer?

Mature Japanese knotweed plants typically grow to a maximum height of 2-2.5m by the end of May. During this time, their leaves unfurl into a distinctive shield shape and vibrant lime green colour.

Canes often emerge from separate crowns in the ground and display a silvery green hue with purple/pink speckles.

As late summer progresses, the leaves darken to a deeper shade of green, and clusters of creamy white flowers begin to appear. These flowers are highly attractive to bees.

Smaller, immature areas of knotweed usually do not exceed a height of 1m. However, they still exhibit the same changes in leaf colour and are easily recognizable due to their unique zig-zag stem formation and shield-shaped leaves.

It is important to note that young plants may not produce flowers.

Key Summer Identification:

  • Tall stems, reaching a height of 2.5m, featuring green colour with purple speckles.
  • Vibrant green shield-shaped leaves, growing on alternate stems.
  • Clusters of small creamy white flowers.

Where does Japanese knotweed grow?

Japanese knotweed is an incredibly tough plant that can thrive in almost any environment. It has the remarkable ability to withstand extreme temperatures, moisture levels, and pH levels.

The rapid growth of Japanese knotweed, coupled with its robust rhizome system, gives it a clear advantage over other plants, allowing it to establish itself in areas where other plants would struggle.

In the past 25 years alone, we have discovered Japanese knotweed growing in a wide range of unlikely locations, including beaches, riverbeds, cliffs, and even chimneys!

Since its introduction in the 1840s, Japanese knotweed has managed to spread to every corner of the UK.

This invasive plant is most commonly found in residential gardens, roadside verges, railway embankments, and riverbanks. It has proven to be highly adaptable and can thrive in these environments.

Best time to spot a Japanese Knotweed
In high summer, Japanese knotweed is particularly distinctive, as its tall and attractive canes catch your attention.

When is the best time to spot Japanese knotweed?

Japanese knotweed is easily noticeable during late spring and summer. There are not many other species that emerge from the ground at such a rapid pace during spring, making Japanese knotweed shoots stand out in comparison to slower-growing native and ornamental plants.

In high summer, Japanese knotweed is particularly distinctive, as its tall and attractive canes catch your attention.

The nature of Japanese knotweed allows it to spread and dominate over other plants, forming large stands. If you are aware of the presence of Japanese knotweed, you can often identify it through aerial and satellite imagery.

When is the Best Time to Identify Japanese Knotweed?

The best time to spot Japanese knotweed is during mid-summer and early autumn. In spring, reddish/purple shoots emerge from the ground and fat, asparagus-like ‘spears’ quickly grow from bright pink ‘crown’ buds.

These can grow up to 2cms a day, forming dense stands of bamboo-like stems that have dark green heart- or shield-shaped leaves.

By early summer, the mature Japanese knotweed stems become hollow with purple speckles and can reach heights of up to 3 meters. The leaves alternate along each side of the stem, creating a noticeable knotweed zigzag pattern.

In late summer, creamy-white flowers appear in lengthy cluster/spike formations. Japanese knotweed mainly spreads through its underground rhizomes/roots, which remain dormant but alive during the winter months.

The rhizomes of Japanese knotweed can spread several meters outward from the visible stems above the ground, and they can reach depths of over a meter.

It is therefore easy to unintentionally fragment pieces of rhizome and spread them by disturbing the soil several meters away from where the stems appear.

Due to the rarity of new growth from seeds, it is remarkable how the incredibly invasive Japanese knotweed has spread to most parts of the UK (and many parts of western Europe and North America) simply through the fragmentation and translocation of rhizomes in contaminated soil.

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