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Fallopia japonica

Nova espécie: 

Perennial herb with aerial stems up to 3-m tall and large leaves. The flowers are whitish, tiny, and are gathered in groups that often appear to be “protruding”, above the vegetative part of the plant.

Scientific name: Reynoutria japonica Houtt. = Fallopia japonica (Houtt.) Ronse Decraene.

Common name: Japanese knotweed

Family: Polygonaceae.

Status in Portugal: Invasive species (listed in Decree-Law no. 92/2019, 10th July).

Risk Assessment Score: 17 | Value obtained according to a protocol adapted from the Australian Weed Risk Assessment (Pheloung et al. 1999), by Morais et al. (2017), according to which values above 13 mean that the species has risk of having invasive behavior in the Portuguese territory | Updated on 30/09/2017.

Synonymy: Fallopia  japonica (Houtt.) Ronse Decraene var. japonica, Reynoutria japonica var. compacta (Hook.f.) Moldenke, Reynoutria japonica var. hastata (Nakai ex Ui) Honda, Reynoutria japonica var. spectabilis (Noter) Moldenke, Reynoutria japonica var. terminalis (Honda) Kitag., Reynoutria japonica var. uzenensis Hond, Pleuropterus cuspidatus (Siebold & Zucc) H. Gross, Polygonum cuspidatum Siebold & Zucc. , Polygonum reynoutria Makino, Polygonum sieboldii Reinw. ex de Vries, Polygonum zuccarinii Small, Tiniaria cuspidata (Houtt.) Hedb., Tiniaria japonica (Houtt.) Hedberg.

Last update: 11/06/2020

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Avistamentos actuais da espécie: 
Family: 
Appearence: 

How to recognize it

Perennial, rhizomatous herb with annual aerial stems up to 3 m tall.

Folhas: ovate, truncated at the base, 5-14 x 3-13 cm, petiolate, glandular.

Flowers: unisexual, whitish, gathered in fascicles of 2-5 flowers, in turn gathered in panicles, glandular; 5 tepals persistent in fruiting, the 3 outer winged.

Fruits: dry, trigonal, black, 4 x 2 mm.

Flowering: from July to September.

Similar species

Due to its larger leaves, unisexual flowers and not being a vine, it distinguishes itself from other Fallopia (or Bylderdikia) present in Portugal.

Characteristics that aid invasion

It has underground organs that accumulate reserves (rhizomes), whose richness in starch reaches 50% of dry weight and which are buried in the soil up to 3 meters deep. Reproduction occurs both sexually (seeds) and via vegetative propagation, being one of the main forms of dispersion associated with the transport of soils contaminated with rhizomes. It regenerates quickly after cutting or other aggression (insects, herbivores, human action), regenerating leaves and stems from the reserves accumulated in the rhizomes. It presents a great rapidity of growth of Organs vegetative organs, reaching large dimensions and being able to “smother” the competing flora due to the dense foliage that produces a strong shading. It has a good adaptation of the flowering and fruiting periods to the temperate climate, taking advantage of most of the summer to accumulate reserve substances.

It has allelopathic substances that produce necrosis in the roots of nearby plants.

Native distribution area

Asia (Japan, Korea, and China).

Distribution in Portugal

Mainland Portugal (Minho, Douro Litoral, and Beira Litoral).

For more detailed locations of this species, check the online interactive map. This map is still incomplete – we need your help! Contribute by submitting records of the location of the species where you can find it.

 

Other places where the species is invasive

Central Europe and in some regions of South America, North America, and New Zealand.

Introduction reasons

It was introduced in Europe intentionally, first for forage and honey cultivation and later as an ornamental plant.

Preferential invasion environments

It shows up and proliferates on the banks of streams, degraded and natural areas of colder areas in the North of the country.

Impacts on ecosystems

Outcompetes native vegetation, preventing its regeneration; harms the indigenous fauna (not prepared to use this plant) and can cause organic contamination in the soils due to the bad decomposition of its leaves.

Economic impacts

Decreases visibility on roads, damages river canals, reduces the ability to drain rivers and canals by encroaching on banks, hinders rail traffic, damages buildings and public works, decreases the value of pastures, causes damage to the landscape due to the effects of uniformity that arise when it completely invades an area.

 

Controlling an invasive species demands a well-planned management, which includes the determination of the invaded area, identifying the causes of invasion, assessing the impacts, defining the intervention priorities, selecting the adequate control methodologies and their application. Afterwards it is fundamental to monitor the efficiency of the methodologies and recuperation of the intervened area as to perform, whenever necessary, the follow-up control.

The control methodologies used for Reynoutria japonica include:

Preventive mesasures: as a priority, preventive measures must be adopted to prevent the establishment of this species (and other invasive species), which include, for example:

- Investing in the good state of conservation of riparian ecosystems and restoration of altered/intervened ecosystems, which will help to prevent the establishment and expansion of the invader,

- Controlling the fate of "contaminated" soils  with Reynotria in order to avoid its transfer and reuse.

Physical control

Hand pulling: where the plant is already established in  small, very localized invasions, possible control methods include rooting out rhizomes. However, for this method to be effective all fragments must be removed, which may not be easy. Rhizomes can be found in the soil up to a depth of 3 m, making their removal very laborious, time-consuming and expensive and requiring labor with suitable material (sieves, etc.). All extracted fragments, once removed, must be completely destroyed (e.g., allowed to dry out and then burned) or removed to a safe place (where they cannot give rise to a new plant). These actions may not be very effective, because the plant has regeneration mechanisms and the resulting fragments can convert into propagules that contribute to increasing the extent of the invasion. All tools used in the interventions must be carefully cleaned so as not to transport fragments to other places where they will originate new outbreaks of invasion - fragments with a few cm give rise to a new plant. In order to be effective, it must be carried out every 15 days throughout the growing season, for at least 2 years.

Cutting and shading: in small invaded areas, geotextiles have already been used successfully to prevent regeneration after cutting. This method consists of covering the soil with a geotextile screen (available on the market to protect slopes against erosion, or to prevent weeds from growing in gardens) and an additional 40cm layer of soil (free of invasive species). Since the rhizomes can spread underground, it is recommended to cover up to a radius of 3m away from the edge of the area where the plants were detected. This eliminates all existing vegetation, including individuals of Reynoutria japonica. The associated cost is high and requires native species to be placed, to occupy the area and compete with any invasive shoots immediately after the intervention. Successive, very frequent and repeated cuts can also be made, as soon as the plant grows back, but only when you have a safe destination for the resulting biomass and always taking great care in cleaning the tools used.

Intensive grazing: with domestic animals, at least for 5 years. This method is hardly applicable in river areas, where the movement of animals can damage the canals, and along communication routes.

Chemical control

This species is resistant to almost all herbicides. One of the few active ingredients successfully used is glyphosate, although due to its toxicity to aquatic invertebrates and other organisms its use is not recommended in situations close to water, where food is grown, and other sensitive areas. Its use is justified in the treatment of highly serious cases and commercial products approved for use in these situations should always be used, which still needs to be updated in Portugal (more information can be consulted in the SIFITO system). As a practical example of treatment with glyphosate, used successfully in other countries, one can refer:

– 1st phase: spraying the leaves 15 days after the appearance of the stems with a dose of Roundup of 6 l/Ha during the first hours of the morning, or late in the afternoon;

– 2nd phase: repeat spraying two months after the first application to destroy the shoots of buds that were not affected, or were not affected enough, accompanied by a previous excavation of the soil to a depth of 50 cm;

– 3rd phase: two months after the second herbicide application, perform a new mechanical excavation on the remains to improve the action of glyphosate on the rhizomes.

Or alternatively:

- 1st phase: cut after the first spring "surge" (as a guideline, it refers to May/June but may vary depending on the region); plant vitality is reduced; the height of the next regrowth is reduced (≤1.30 m). Cut again;

- 2nd phase: spraying with herbicide (0.7% glyphosate) (September-October); the plants are in the post-flowering stage and near the end of the growing season;

- 3rd phase: restoration by planting a high density of species (riparian if it is in a riparian environment).

Biological control

Insects: After several agents studied, an insect (the psila Aphalara itadori Shinji), originally from Japan, was released in Britain (in 2010) and in the Netherlands (in 2020) in a program of biological control against Reynoutria japonica. However, to date, and even though the insect has survived, its establishment in Britain and the increase in populations has fallen far short of expectations, not resulting in substantial control of the target species. In the Netherlands, the establishment, although very preliminary, appears to be more positive.

Fungi: to use certain phytotoxic compounds of fungal origin, and the University of Montana has isolated 25 different phytotoxins that could be used as natural phytocides against this plant..

Visit the webpage How to Control for additional and more detailed information about the correct application of these methodologies.

 

Flora Digital de Portugal (2014) Reynoutria japonica. Disponível: http://jb.utad.pt/especie/reynoutria_japonica [Retrieved 15/09/2015].

Marchante H, Morais M, Freitas H, Marchante E (2014) Guia Prático para a Identificação de Plantas invasoras">Invasoras em Portugal. Imprensa da Universidade de Coimbra, Coimbra, pp. 130.

Morais MC, Marchante E, Marchante H (2017) Big troubles are already here: risk assessment protocol shows high risk of many alien plants present in Portugal. Journal for Nature Conservation 35: 1–12

Pheloung PC, Williams PA, Halloy SR (1999) A weed risk assessment model for use as a biosecurity tool evaluating plant introductions. Journal of Environmental Management. 57: 239-251.

Sanz-Elorza M, Sánchez EDD, Vesperina ES (2004) Atlas de las plantas alóctonas invasoras en España. Dirección General para la Biodiversidade, Madrid, 254pp.

DAISIE European invasive">Invasive Alien species">Species Gateway (2012) . Fallopia japonica. Disponí­vel: http://www.europe-aliens.org/pdf/Fallopia_japonica.pdf [Retrieved 11/09/2017].