Although rather attractive, Japanese knotweed (Fallopia japonica) is a real thug as it spreads rapidly. In winter the plant dies back beneath ground but by early summer the bamboo-like stems shoot to over 2.1m (7ft), suppressing all other growth. Eradication requires steely determination as it is very hard to remove by hand or with chemicals.
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).
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.
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.
When tackling Japanese knotweed, cultural control methods pose some problems;
Weedkillers for gardeners (Adobe Acrobat pdf document outlining weedkillers available to gardeners; see sections 4 and 5)
Chemicals: using a sprayerChemicals: using safely and effectivelyChemicals: using spot and broad-scale weedkillers
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.
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.
Bamboo controlBindweedEnvironment Agency: Japanese knotweedGiant hogweedGround elderHimalayan balsamInvasive non-native speciesJapanese Knotweed AllianceRagwortRHS video: weed control
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Label on 30/04/2014
I have just been told I have Japaneese Knotweed in my back garden, I have plans to build a single story extension on part of the area where the knotweed is, work is due to start in 5 weeks time. Do I need to put the building work off and what are my options to eradicate the problem?
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