Shocking sight” of the South American devil “Nagaetsunogeitou,” the earth’s worst “invasive plant,” multiplying massively in Japan.
Regenerating quickly even after being pulled out, the plant expands its habitat with its astonishing reproductive power, and its agricultural products are severely damaged. ......
A major problem around the world
Huge swarms of waterweeds cover the surface of the water with their amazing reproductive power. This is the “Nagaetsunogeitou,” the worst invasive plant on earth, which has been spreading across Japan this summer. The photo shows a swarm of the invasive plant that has transformed the landscape of the Shintone River flowing through Kawachi Town, Ibaraki Prefecture. A man in his fifties who runs a rice farm nearby laments, “There is nothing more we can do.
There is nothing more we can do. They proliferate even in the rice paddies, and even if you pull them out, they quickly regenerate from their roots or parts of their stems. …… We have nets on canal taps to prevent the influx, but some of the finer weeds slip through. Herbicides cannot be used after rice planting, and I am really worried that it will cause serious damage to the rice we harvest.
The perennial grass is native to South America and has the English name “alligator weed” because of its devastating effect on the aquatic and terrestrial environment. In Japan, it was confirmed in Hyogo Prefecture in 1989, and since then its habitat has expanded to various areas, and it is now distributed in 30 prefectures, mainly in the Tohoku and southern regions. It is designated as a Specified Invasive Alien Species.
If there is a node only a few millimeters in diameter, the roots and shoots regenerate and double in size in about 40 days. The shredded parts are washed into the water, where they reproduce repeatedly and rapidly expand their habitat. Not only do fish and other aquatic organisms become unable to live in the water because of the obstruction to water passage, but the fact that they can also grow on land is a major problem around the world because of the tremendous impact on the agricultural and stockbreeding industries.
In the Shin-Tone River and its vicinity, Ibaraki Prefecture conducted a large-scale extermination eight years ago, but the number of invasive alien species has further increased, and is scheduled to be exterminated again this fiscal year at a cost of approximately 25 million yen. Hidemi Marui of the Ecology Research Institute, who is familiar with invasive alien species, says that the large proliferation has something to do with global warming.
As is true of all tropical aquatic plants, the rate of increase increases as environmental conditions improve, such as hotter days. To get rid of them, we need to spray them with herbicides and cover them with light-shielding sheets to prevent photosynthesis. However, there are no examples of successful extermination outside of a controlled, enclosed environment. One of the factors allowing invasive alien species to invade is that excessive development of the riparian environment by humans has reduced the number of native species that should serve as a bulwark.
The “South American devil” is encroaching on Japan’s ecology at a pace far beyond anything we can imagine.
From the September 26, 2025 issue of FRIDAY
Reporting, writing, and photography： Masayoshi Katayama (Journalist)
