Japanese Knotweed

What is Japanese Knotweed?

Japanese Knotweed is a perennial plant brought into the US as an ornamental garden plant. It is the most invasive plant/weed in Britain. We sure don’t need that weed in Lane County, Oregon

What does Japanese Knotweed look like?

Japanese Knotweed grows in the form of a bamboo like stem with green oval shaped leaves with a distinctive zig zag pattern, in early spring shoots will grow from the stem which will be red or purple in color.

Leaf topology of Japanese Knotweed

These stems grow at a great speed, reaching a height of 5 feet by May and almost 10 feet by July. In late summer to early autumn flowers will appear on the stems, which will be creamy-white and will grow in clusters.

Stem of Japanese Knotweed

The stems will die down in late autumn to early winter, when they will turn a dark brown and the leaves and flowers will be shed. The dead stems can remain for up to three years before decomposing

How is it spread?

Japanese Knotweed is spread easily by rhizomes (parts of the root) and a piece as small as 0.8 grams is all that is needed to allow the plant to spread. The plant commonly grows during transportation, or by being mowed or flailed as this causes particles to blow around. Japanese Knotweed will grow wherever it lands; it has even been known to grow through floorboards and into living rooms!

Root of Japanese Knotweed

Please ensure these basic rules are followed if you find knotweed on your property;

DO NOT waste time. If you have Japanese Knotweed on your property, treat it immediately. Do not allow it to become established.
DO NOT prune Japanese Knotweed.
DO NOT include with normal household waste.
DO NOT contaminate refuse containers with knotweed material.
DO NOT compost knotweed.
DO NOT flail, mow or cut Japanese Knotweed, as this will cause it to spread. Flailing of the dead stems after herbicide treatment and complete dieback or in the winter period is acceptable. The plant is herbaceous and the aerial parts of the weed will be killed off in the winter by frosts.
DO NOT spread Japanese Knotweed stems and crowns.
DO NOT spread soil contaminated with Japanese Knotweed rhizomes (root system). Any soil that is obtained from ground within 22 feet of a plant could contain rhizomes. The rhizome is highly regenerative and will readily grow into new plants.
DO NOT chip or shred Japanese Knotweed material. Mechanical chippers do not kill Japanese Knotweed. If you spread any chipped or shredded material Japanese Knotweed can re-grow from minute pieces of the plant

Herbicidal Control

Japanese Knotweed is a weed problem that can be successfully treated with herbicides, although it may take several applications to eradicate this weed

There are several herbicides available for use on Japanese Knotweed, and the choice of herbicide will be dictated by the situation the weed is growing in.

TORDON 22K – Picloram This product acts through foliage and the roots, has a residual activity for up to two years and does not kill grasses. The disadvantage with this product is that it cannot be used near watercourses or under tress that you wish to retain, as it can be absorbed into the root system and will kill mature trees.

ROUNDUP Pro – Glyphosate This acts through the foliage and is trans-located through the root system. This product will kill all vegetation including grasses, and has the added benefit that it can be applied in or near watercourses. The product cannot be absorbed via root systems and it is broken down on contact with soil. Depending on the time of the first application you may need to spray two to three times to ensure a good kill.

GARLON/TIMBREL – Tryclopyr This is a selective herbicide, rapidly absorbed through foliage and roots. Grasses not affected at normal dosage rates. Again, the use of these products is not permitted near watercourses.

It is essential that you comply with the recommendations on the product labels.

If you need any help with the removal of this weed please contact our Watershed Coordinator.

Credits for the above: whitehorsedc.gov.uk, wreford.co.uk

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