Alien invasion, or what bad things knotweed can do

There are 3 species of knotweed found in Poland: the Japanese knotweed (Japanese knotweed)(Reynoutria japonica), the Sakhalin knotweed (Reynoutria sachalinesis) and the Czech knotweed (intermediate)(Reynoutria x bohemica), which is a hybrid of the first two. All 3, according to the Law on Alien Species and the Executive Order [1, 2], are considered invasive alien species(IGO), posing a threat to Poland, spreading widely.

Knotweed – characteristics

The knotweed is a tall, strongly spreading perennial plant native to southern Asia. Its original range included Japan, Korea, Taiwan, as well as northern China. It appeared in Europe in 1825. and spontaneously spread in the environment. Japanese knotweed and Sakhalin knotweed found their way to Poland through targeted shipments and trade in exotic plants. Initially they were readily cultivated in botanical gardens, arboreta and home gardens.

In the case of Japanese knotweed, the first references to wild sites (outside cultivation) come from the second half of the 19th century. from Greater Poland (Gniezno), Lower Silesia (Wroclaw), Upper Silesia and the Baltic coast. In subsequent years, the number of known sites increased – from 3 before 1900, through 63 in 1950, to more than 3,000 in 2000. Japanese knotweed is the most widely distributed species of this group in Poland. It is now common throughout the country and, unfortunately, is successfully colonizing more tracts of land. Their uncontrolled spread in the wild threatens biodiversity, as they displace native species through their expansiveness.

Expansion and threat to biodiversity

The knotweed grows extremely fast (up to 10 cm a day), reaching heights of 2-3 m, and, forming dense rows, shades and prevents the development of other species in its vicinity. Not only that – the thick and slowly decomposing layer of autumn-fallen leaves and stems prevents other plants from germinating and developing, thus occupying the exclusive colonized area at a rapid pace. Knotweed stems are covered with rust-colored spots and resemble decorative bamboo shoots, hence they were initially popular and planted as an ornamental plant.

Currently, it mostly overgrows the banks of rivers and streams, roadside and railroad embankments, and other ruderal habitats. Its rhizomes reach up to 3 meters deep into the ground, laterally spreading up to 7-8 meters, passing under asphalt roads and building foundations. Interestingly, the entire plant can regrow from a small, even centimeter-long rhizome containing a single node. Knotweed shoots can pierce asphalt, as well as destroy the foundations of houses or concrete buildings. In the UK, for example, its mere presence can significantly reduce the value of a property and effectively prevent you from getting a mortgage. In Poland, so far there is no detailed analysis of the economic losses resulting from the spread of these species, yet they damage foundations, walls of buildings and drainage canals, as well as the surfaces of roads, sidewalks or parking lots.

Compact, single-species patches of knotweed often occupy vast areas and soon cause changes in the structure and functioning of riverine ecosystems, especially riparian ecosystems, where they significantly reduce the biodiversity of natural and semi-natural habitats. They are a very serious problem, especially in river valleys, where, by overgrowing the banks of watercourses, they contribute to their erosion and changes in water flow, as well as undermine flood defenses and hydraulic engineering structures. The lagging dead matter left behind by their above- and below-ground parts impedes water flow. Shoots, rhizomes and whole clumps of plants settle on the branches of uprooted trees, which is particularly dangerous during periods of high water and can cause local flooding or flooding.

Difficulties in the control of knotweed

To all these expansive features of knotweed, one must add the extreme difficulty of getting rid of it from an environment it has already managed to control. These plants respond poorly to chemicals, and the herbicides used to control them do not produce the expected results. It is advisable to remove knotweed before the flowering period and destroy its stems mechanically afterwards, without digging, which could further spread the rhizomes. Once the weed has been removed from the garden or plot, care should be taken to dispose of it properly. You also need to remember to regularly clean up the area from which it was removed by cutting out regrowing shoots. It is also important not to move the soil in which the knotweed grew to other sites, as this will cause it to spread further.

Obligation to report the presence of knotweed

The Law on Alien Species [1] indicates that anyone who notices knotweed in the environment should immediately report this fact to the mayor of the municipality, the mayor or the mayor of the city with jurisdiction over the place where the plant was found. This can be done in writing on paper or electronically via the municipality’s inbox or by e-mail. Some municipalities have developed special notification forms for this purpose. For example, in the Municipality of Krakow, notifications can be sent to the address of the Environmental Management Department at [email protected] using a form available on the BIP website of the City of Krakow.

The application should include such information as:

• The name and surname or name and address of the applicant;
• Contact information of the applicant, such as e-mail address or telephone number;
• The name of the observed invasive alien species posing a threat to Poland or the Union;
• The number of specimens of this species or the size of the area occupied by them (if this can be determined);
• The place and date when the presence of this species was found;
• a photograph to identify the species (optional, but such a photograph definitely increases the credibility of the submission).

Once the formal verification is completed, the information on the IGO position is forwarded by the municipality, mayor or city president to the relevant regional environmental protection directorate for inclusion in the IGO Register (portal geosewis.gov.pl). The municipality then determines the person or institution (ruler) responsible for carrying out remedial actions and informs them of the presence of knotweed on their property and the necessity to remove it from the designated site. According to the regulations, remedial measures to eliminate knotweed outside national park and nature reserve areas are carried out:

a) property manager – on property owned by the State Treasury and on property owned by a state legal entity;

(b) the ruler entity, which is a natural person or a non-state legal entity, or a non-state organizational unit – on the property it owns.

Control methods

Detailed information on knotweed, along with a description of suggested remedial actions, is presented in the study titled: “The importance of knotweed. Compendium. Knotweed control methods , which is available on the General Directorate of Environmental Protection’s projects page. Among the methods recommended for use in Poland were:

1. Mechanical methods (netting, long-term covering with mulching, repeated mowing and digging up plants with screening of soil containing rhizomes);
2. agrotechnical methods (soil cultivation – plowing and gritting, as well as mulching and sowing with a grass mixture);
3. other physical methods (use of targeted electrical pulses);
4. chemical methods (spraying of the assimilation apparatus, smearing, injection into the stem or into the rhizome);
5. Biological methods (grazing animals);
6. Mixed-mechanical – combined methods (chemical-mechanical involving spraying, smearing or injecting, followed by digging up the plants; injection into the rhizomes, followed by cutting or mowing the shoots or plowing; agrotechnical with selective chemical control).

For most of these methods, labor intensity is usually highest in the first season and gradually decreases or is significantly reduced in subsequent years. This is especially true for methods involving the use of materials to cover the area controlled by knotweed, such as netting or geotextile along with biomass. The cost and labor intensity of the treatments carried out are also significantly influenced by factors such as the reliability with which individual treatments are carried out, care not to overlook individual specimens or shoots when carrying out the work, but also the age of the site in question (in which case the growth of the underground rhizome system must be taken into account).

Taking into account the described characteristics of knotweed, the positive effects of most recommended methods will be visible only after several years, a minimum of 5. For this reason, when taking measures to completely eradicate knotweed at a given site, it is necessary to plan their repetition over a period of several years, taking into account the succession of the methods used.

Importantly, in the case of chemical methods, it must be remembered that plant protection products are considered substances that can cause water pollution (Article 16, paragraph 58(i) of the Water Law [3]), so their use may be prohibited or restricted in protective zones of water intakes and in protective areas of inland water bodies. For this reason, mechanical methods, considered the safest for the environment, are preferred, but they are effective mainly for the elimination of single individuals, clumps or in small areas.

Knotweed control costs

In Poland, there is no accurate estimate of the costs associated with the eradication of knotweed. They have been repeatedly analyzed in other European countries. The costs of removing knotweed from the sites of the 2012 London Olympics. have been estimated at £70 million. The annual cost of combating knotweed in this country was estimated at £166 million.

Photo. main: Catherine King-Dolny

In the article, I used, among others. z:

[1] Law of August 11, 2021 on alien species (Journal of Laws 2023, item 1589).

[2] Decree of the Council of Ministers of December 9, 2022 on the list of invasive alien species posing a threat to the Union and the list of invasive alien species posing a threat to Poland, remedial actions and measures aimed at restoring the natural state of ecosystems (Journal of Laws, item 2649).

[3] Law of July 20, 2017. – Water Law (Journal of Laws 2024, item 1087, as amended).

[4] Guidelines for the control of knotweed in Poland, University of Silesia in Katowice, Katowice 2015.

[5] Analysis of the degree of invasiveness of foreign species in Poland with the indication of species significantly threatening the native flora and fauna and a proposal of strategic actions for the possibility of their eradication and Analysis of the paths of unintentional introduction or spread of invasive foreign species with the development of action plans for priority paths, University of Silesia in Katowice, Katowice 2018.

[6] Invasive Alien Species – Knotweed, GDOŚ, Warsaw 2022.