Five easy ways to identify Japanese Knotweed

Japanese Knotweed | 20th June 2024

You can identify Japanese knotweed at any time of year, provided you know what to look for. Though, in the summer, Japanese knotweed’s distinctive leaves, stems, and flowers are vibrant. This makes it much easier to recognise knotweed. In this article, we explain the main Japanese Knotweed identifiers for you to look for.

 

The five top knotweed identification tips

Below we explain what you should look out for when you are trying to identify Japanese knotweed. However, don’t worry if you are still not sure, below is a link to our free ‘ID My Weed’ service.

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You can use this for Knotweed and the identification of other problematic invasive plants. Simply email us a photo and your details, and we’ll take a look for you.

1. Spade shaped leaves

The spade shaped leaves of Japanese knotweed (Reynoutria japonica)

The characteristic spade shaped leaves of Japanese knotweed (Reynoutria japonica)

Japanese knotweed leaves are described by some as heart-shaped, but that’s the shape of Giant knotweed leaves. The leaves of Japanese knotweed have a flat base where the stem joins the leaf, making them look more like a spade (on a playing card), or shield shaped. The leaves grow alternately along the stem, which is a characteristic feature that is useful to note. In the summer, Japanese knotweed’s vibrant green leaves make it more noticeable. When you see this leaf shape, combined with the other aspects listed, you can be more confident that you have identified knotweed.

2. Zigzag stems

Zigzag stem of japanese knotweed

The distinctive zigzag stem of Japanese knotweed.

One of the most distinctive features of Japanese knotweed is the zigzag lateral stem structure. The stems grow in an understated zigzag pattern that becomes more visible in mature plants. These horizontal, or lateral, stems protrude from a main stem that is hollow and can reach up to 3 metres in height in the summer. Recognising this zigzag pattern can help you identify Japanese knotweed more easily. The tall, main stems of the knotweed are green, with purple speckles that add to their distinctive appearance.

3. Orange-tinged roots

Orange tinged knotweed rhizome with Japanese knotweed roots emerging

Orange/yellow healthy Japanese knotweed rhizome with roots emerging

If you discover what appear to be thick, orange-tinged roots in the ground, these may in fact be Japanese knotweed rhizomes. The rhizomes of Japanese knotweed are another key identifier, but they are actually underground stems that spread laterally from the plant. The knotweed roots are small, quite thin and white, and extend from the thick rhizomes that have an orange or yellow colour when cut. Rhizomes snap easily and are the most troublesome and invasive part of the plant. A new plant can grow from just a tiny fragment that has been spread or left in the ground.

Concerned that you have Japanese knotweed?

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Japanese knotweed roots, rhizomes and other plant material are considered controlled waste, as such it can only be disposed of at a licensed landfill by professionals registered as licensed carriers.

4. Creamy white flowers

Japanese knotweed clusters of creamy white flowers

The creamy white flowers of Japanese knotweed typically emerge towards the end of summer.

In the summer, the plant forms large swathes of leafy vegetation, which can reach up to 3 metres in height. Then, in later summer, typically the end of August or early September, Japanese knotweed flowers start to appear, and can continue into autumn. Tiny, creamy/white flowers emerge from flower spikes that are around 10cm in length, producing quite an attractive display. However, this is also a seasonal characteristic that again makes it easier to spot Japanese knotweed in the summer. As Japanese knotweed is a perennial plant, it dies back in winter. However, sometimes, the crispy, dead flowers will still be present on bamboo-like canes (mains stems).

5. Dead winter canes

Knotweed canes in the winter

Japanese knotweed canes become brown and brittle in the winter months.

Although the plant dies back in the winter months, the dead canes are another feature to look for. Autumn Japanese knotweed canes become brown, and later brittle, but retain the characteristic zigzag stem patterns. The decomposing material they produce often traps litter, which can attract rodents if not cleared. However, the hollow stems and the distinctive pattern can help you recognise Japanese knotweed in winter. Then, when new shoots appear in the spring, it’s from the knotweed crown where from where the canes emerged in previous seasons.

Habit as a Japanese knotweed identifier

Japanese knotweed is a perennial plant that dies back in winter, but there are telltale signs to look for all year round. In the summer, Japanese knotweed is certainly at its most prominent, and its famous for its growth rate during these months. However, even in the winter you can look out for knotweed crowns, broken canes or intact canes displaying the distinct zigzag pattern. Then, at the start of a new knotweed growing season, in mid to late spring, early Japanese knotweed shoots emerge that often look look asparagus-like in appearance. Later these form into shoots with  with rolled back leaves that are a reddish purple colour. The more vigorous spring growth, pictured in the main image of this article, often looks asparagus-like in appearance.

new growth emerging from a knotweed crown

New Japanese knotweed growth emerging from a crown with old canes present.

Need Help Identifying Japanese Knotweed?

If you’re still unsure about recognising Japanese knotweed, PBA Solutions offers a free Japanese knotweed identification service. Simply email a photo of the plant along with your contact details, and we’ll take a look for you.

For more information on how PBA Solutions can help you, or your community, in identifying and removing Japanese knotweed and other invasive weeds, call and talk to a member of our team on 0203 174 2187 or 01202 816134.

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All pictures © 2024 PBA Solutions

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