Japanese knotweed

What is Japanese knotweed?

Japanese knotweed is a strong-growing, clump-forming perennial, with tall, dense annual stems. Stem growth is renewed each year from the stout, deeply-penetrating rhizomes (creeping underground stems).

Appearance

In spring and summer, bamboo-like shoots grow to 2.1m (7ft) tall. Leaves are up to 14cm (5½in) in length and the creamy-white flower tassels produced in late summer and early autumn reach up to 15cm (6in).

The stems die back to ground level in winter.

back next

1

Japanese knotweed is a perennial weed, producing tall canes, up to 2.1m (7ft) in height during the summer. The canes have characteristic purple flecks, and produce branches from nodes along its length. These branches support shovel-shaped leaves.

The problem

Japanese knotweed was introduced from Japan in 1825 as an ornamental plant. The plant is not unattractive but its rapid annual growth and relentless spread, allows it to easily overwhelm other garden plants. Where established as a wayside weed, native plants are also aggressively over-run.

Although it does not produce seeds, it can sprout from very small sections of rhizomes and, under the provisions made within the Wildlife and Countryside Act 1981, it is an offence to cause Japanese knotweed to grow in the wild. Much of its spread is probably via topsoil movement or construction traffic.

Control

Non-chemical controls

When tackling Japanese knotweed, cultural control methods pose some problems;

• Digging out is possible, but due to the depth that the rhizomes can penetrate, regrowth usually occurs. This method also creates problems over disposal as Japanese knotweed is classed as 'controlled waste' under the Environmental Protection Act 1990. This requires disposal at licensed landfill sites. Alternatively, it can be destroyed on site by allowing it to dry out before burning. On no account should it be included with normal household waste
• If digging out is attempted, remove as much root as possible, then repeatedly destroy regrowth. In this way the energy reserves in the remaining underground parts will be gradually exhausted; a process which may, however, take several seasons

Biological control

• A plant sucker (psyllid) is being released in the UK as a biological control for Japanese knotweed. It is currently only being released at a handful of trial sites and is not available to gardeners. However, if successful it will be released more widely and will become widespread in Britain over the next five to ten years by natural spread

Chemical controls

Glyphosate

• Scotts Roundup Tree Stump & Rootkiller has label recommendation for controlling Japanese knotweed, instructing it to be applied to the cut canes
• Alternatively, try other tough formulations of glyphosate (e.g. Scotts Roundup Ultra 3000, Scotts Tumbleweed, Bayer Tough Rootkill, Bayer Super Strength Glyphosate or Doff Knockdown Maxi Strength Weedkiller)
• Glyphosate is usually applied to the foliage and is passed within the plant to the underground parts
• It is useful to cut away old stems during the previous winter to allow good access. The best time for spraying with glyphosate is at the flowering stage in late summer. However, it is difficult to spray at this stage, when the weed is 2.1m (7ft) or more high
• A more practical approach is to allow Japanese knotweed to grow to about 90cm (3ft), which will usually be reached in May, and spray then. There will be regrowth and consequently a second application in mid-summer is useful. Check during September and if it has grown once more, spray again before growth begins to die down in the autumn. Check again the following spring
• Avoid spray coming into contact with garden plants. Glyphosate-treated knotweed will often produce small-leaved, bushy regrowth 50-90cm (20in-3ft) in height the following spring. This is very different in appearance to the normal plant and it is essential that this regrowth is treated
• It usually takes at least three to four seasons to eradicate Japanese knotweed using glyphosate. Professional contractors, however, will have access to more powerful weedkillers that may reduce this period by half

Residual control

• The residual weedkiller Bayer Long Lasting Ground Clear containing glyphosate/flufenacet/metosulam comes in a soluble sachet. It may provide a moderate check in growth, but because of the extremely persistent rhizomes, is unlikely to eradicate the weed

Download

Weedkillers for gardeners (Adobe Acrobat pdf document outlining weedkillers available to gardeners; see sections 4 and 5)

Links

Chemicals: using a sprayer
Chemicals: using safely and effectively
Chemicals: using spot and broad-scale weedkillers

Other related species

Several species of Persicaria and Polygonum, including Persicaria lapathifolia and Persicaria maculosa can also be troublesome weeds. Where possible, attempt control at the seedling stage by hoeing or using a proprietary formulation of diquat. Once established amongst soft-stemmed plants (such as herbaceous border plants, bedding plants or vegetables) removal by hand or by hoe is the only safe approach.

Spot the difference

The plants mentioned above are from the same genus as many well-known, garden worthy plants. So identification is key before removing useful ornamental species. These are some that you may wish to grow in your garden: Persicaria.