Japanese knotweed – Reynoutria japonica

Japanese knotweed is a fast-growing perennial herb in the family Polygonaceae. It forms dense stands which may reach up to 3 m tall. In its native habitat in Japan, it usually grows only 0,3 to 1,5 m in height.

Japanese knotweed thrives in a wide range of habitats. It requires light to grow and, therefore, it favors sunny habitats such as riversides, landfills and along roads and railways. It also grows in semi-shaded places such as forest clearings and edges, but rarely in forest. It also thrives on clayey and very barren habitats, and it is not demanding on soil acidity.

The stems of Japanese knotweed are slightly woody, hollow and 'bamboo-like'. Usually, the lower parts are branchless and at the top slightly branched. The stems are pale green in colour and often have reddish to reddish-brown spots. The above-ground part is annual and new stems grow from the rhizome every year. In Finland, Japanese knotweed can grow to 3 meters, while the close relative giant knotweed (Fallopia sachalinensis, also called Sakhalin knotweed Reynoutria sachalinensis) grows even tallerer, 4 to 5 metres in height. Thus, tall knotweeds produce plenty of litter.

The leaves of the Japanese knotweed are 10 to 15 cm long, almost triangular in shape, obtuse at the base, pointed- and long-tipped. They can be distinguished from giant knotweeds on the basis of the size of the leaves, which on the latter may grow even more than 40 cm long. Bohemian knotweed (Fallopia x bohemica, also called hybrid knotweed) is a hybrid of Japanese knotweed and giant knotweed and its appearance is an intermediate form of these two: the leaves are acuminate at the tip and less cordate at base than the leaves of giant knotweed. Most certainly the species of knotweeds can be distinguished from each other by the hairs underside the leaves: the leaves of Japanese knotweed are usually glabrous, while at the mid-vein on underside of leaves, the giant knotweed has 1 mm long white hair and the hair of the Bohemian knotweed are slightly shorter, 0,5 mm long.

In Finland, Japanese knotweed blooms in September - October. The inflorescences are erect cymose panicles with small white to pink petals. The species is dioecious, but mainly pistillate plants have been imported to Europe and North America. Therefore, the propagation of the species is almost solely vegetative. Even tiny pieces of ground stems are able to grow a new plant, which increases the risk of spreading along with transfering soil. The root system reaches down to 3 m deep, and horizontally, ground stems may grow 5 to 6 m long.

Japanese knotweed is native of eastern Asia: Japan, the island of Sakhalin, Korea, China, Taiwan and Vietnam. In its native range, it grows on riversides, peatland forests, roadsides, forest edges and oak forests. 

In the Netherlands, Japanese knotweed was introduced in 1823 as an ornamental. It soon became a common garden plant in many European countries. In Germany, Japanese knotweed was also used as a fodder plant and for controlling erosion. In the mid-1800s, the species spread into the wild and in the late 1800s, it became established in many European countries, including England, France, Germany, the Netherlands, Poland, Scotland and Wales.

Today, it is widely established in Europe, North America, Asia, Australia and New Zealand. In recent decades, Japanese knotweed has spread rapidly in Europe. Typically, it spreads as fragments of rhizome in soil which is used as backfill in parks, yards and road construction. It also spreads along riversides with garden waste.

So far, Japanese knotweed and giant knotweed are mainly present in southern Finland, and only locally here and there. The occurences are concentrated in urban and other populated areas. Most commonly, they grow along roadsides and as a “weed” in newly planted sites, which indicate to the contamination of the soil used. It is not known that knotweed has propagated from seeds in Finland. Instead, they are accidentally transfered to new sites as fragments of rhizome in garden waste and transported soil. Both species are sometimes found fugitives as large stands, originating from garden waste.

Invasive alien species included in the national list (National list of alien species)

Japanese knotweed is included in the national list of invasive alien species. Invasive alien species may not be released into the wild, imported into Finland from outside the EU or from other EU countries, transported, transferred, bred or cultivated, disposed of, passed on, sold, marketed, or otherwise handed over.

Tall knotweeds are strong competitors and there are few other species that can grow in the stands they form. They spread well vegetatively and compete with synanthropes in the vicinity of inhabited areas. Knotweeds also occur to some extent in protected areas, e.g., groves, and in the vicinity of endangered forest, meadow and limestone rock species.

In dense knotweed stands, the growing conditions change completely and, for example, the seeds of woody perennials cannot germinate and the natural development of vegetation in the area slows down or alters completely. Especially in bright sites, Japanese knotweed can form a completely dominant vegetation, while in shady sites it is a slightly weaker competitor to other plants.

Along rivers and streams, knotweed vegetation can increase erosion and soil drift into waterways, as the vegetation completely withers during the winter and does not bind the soil during winter and spring rainfall. The diversion of loose soil into waterways can pose a threat e.g. to spawning grounds for trout.

According to the DAISIE database funded by the European Commission, Fallopia japonica is included in the list of 100 worst invasive alien species in Europe. It has been classified as an invasive alien species even in Estonia, Sweden, Norway and Denmark. The spread of the plant into the wild was prohibited in the United Kingdom in 1981.

Knotweeds have proven to be extremely difficult to eradicate. Mere weeding or cutting of stems is not enough, as knotweeds easily generate new growth. It is usually impossible to eradicate knotweeds by digging them out, because they create new growth from the tiniest rhizome fragments remaining in the soil. Eradication of small-scale stands may succeed if the stand is first cut down completely and then covered with black plastic for 3 to 4 years. The following method has proven successful in controlling knotweed stands: the stems are cut close to the base and pesticide is sprayed into the hollow stems. This is repeated a few times per year until the stand vanishes. The cut stems can be left to decompose on top of the stand.

An alternative method of controlling large-scale stands is to bring sheep to graze in the area. In this case, it is important to remove all dry Japanese knotweed stems from the previous year, because they are razor sharp for grazing animals.

Pesticides may have to be used in tricky sites. These may not be used at all in groundwater areas, or in the vicinity of wells or surface water, or close to children’s playgrounds. New emerging shoots can be removed by pulling them up manually. They are then disposed of by burning or composting.

Because the species is legislated as a nationally invasive alien species, its import, breeding, sale and other possession as well as release into the environment is prohibited.

Do not buy or breed the invasive alien species. Also, it is prohibited to order a Japanese knotweed from online stores. The species must not be passed on. Eradicate invasive alien species from your garden and treat garden waste so that germinating seeds or plant parts cannot survive. Make sure that the species does not spread with the soil either. In your own garden, it is worth to prefer domestic or poorly spreading plants.

Professional operators have the duty to prevent Japanese knotweeds, as well as its seeds and plant parts, from spreading outside the area under their control for example in compost, soil and other plants.

Alberternst, B. & Böhmer, H. J. 2011. NOBANIS – Invasive Alien Species Fact Sheet. Fallopia japonica.

CABI. Invasive Species Compendium. Fallopia japonica (Japanese knotweed).

Handling of moderate amounts of plant waste. 2021. Prevention and information. Handling of waste. Moderate amounts of waste. Vieraslajit.fi.

YouTube. Video. 25.6.2013. Invasive Plants Can Suffocate Trouts.

DAISIE database. Japanese knotweed.

Terve askel luontoon -hankkeen vieraslajiopas. (pdf). Guidebook. 2019. Allergia-, Iho- ja Astmaliitto ja WWF Suomi. (in Finnish). 

Halme, M. 2018. Kasvaako pihallasi japanintatar? Kasvi saattaa tunkeutua jopa talon perustuksiin seitsenmetrisillä juurillaan. Meilläkotona. Julkaistu: 19.4.2018. (in Finnish).

Huusela-Veistola, E. ym. 2020. Ehdotus kansallisesti haitallisten vieraslajien hallintasuunnitelmaksi. 138 sivua. Valtioneuvoston kanslia. Valtioneuvoston selvitys- ja tutkimustoiminnan julkaisusarja 32/2020. (in Finnish).

Ryttäri, T. & Teeriaho, J. 2020. Kurtturuusun, komealupiinin ja isojen tatarlajien levinneisyys Suomessa.. Teoksessa: Huusela-Veistola, E. ym. 2020. Ehdotus kansallisesti haitallisten vieraslajien hallintasuunnitelmaksi. s. 67-80. Valtioneuvoston kanslia. Valtioneuvoston selvitys- ja tutkimustoiminnan julkaisusarja 32/2020. (in Finnish).