Farmer Minoru Nanri has struggled with damage from alligator weed. Although the parts above the water die back in winter, its roots and stems survive underwater and resume growing in spring. March 19 in Kase, Saga City. (©Sankei/Mayuko Ichii)
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Saga City is launching a trial using kushinsai, or water spinach, to slow the spread of alligator weed, a designated invasive alien aquatic plant that has proliferated in agricultural waterways and become a problem across Japan. Water spinach is a popular leafy green, often used in stir-fried dishes across Southeast Asia and parts of East Asia.
The first project of its kind in the country, the experiment aims to curb the weed's growth by having water spinach absorb the nutrients it needs to thrive. Other local governments struggling to eradicate the plant are watching with interest.
The experiment brings together two unlikely rivals: a notoriously resilient aquatic weed and a familiar Chinese vegetable that may help starve it of nutrients. Whether water spinach can slow the spread of alligator weed is now the question Saga City hopes to answer.
Farmers Sound the Alarm
Native to South America, alligator weed now inhabits lakes, rivers, and waterways across Japan. It is notorious for its vigorous growth and regenerative ability. Even after being cut back, it can spread from tiny fragments of the stem. The plant can obstruct crop growth and harvesting and threaten ecosystems by crowding out native species.
In Saga City, its spread has been especially severe in the southern districts, including Kase, where farmers are raising the alarm.
"No matter how much we cut it back, if even a small piece of stem or root is left behind, it grows back thick the next year. I've never seen an aquatic weed like it," said 67-year-old farmer Minoru Nanri, who grows rice, wheat, and other crops.
According to Nanri, the weed began spreading in the area about five years ago. Since then, it has invaded fields, making them difficult to use, and clogged intake pumps that supply water. Nanri has tried removing it with heavy machinery and by hand, but destroying the roots beneath the water is difficult, and the plant returns every year.
If heavy rain raises water levels and washes the weed into rice paddies, it can wrap around rice plants, making machine harvesting impossible.
"We have to keep it out of the paddies at all costs," Nanri said. "Otherwise, we won't be able to farm them."
According to Saga City, the area affected by alligator weed has more than doubled, growing from about 50,000 square meters (538,000 square feet ) in fiscal 2019 to roughly 138,000 square meters in fiscal 2025. Removal costs have also kept rising. In fiscal 2026, the city expects to spend about ¥150 million (about $954,000)—twice as much as the previous year.
But even with the extra spending, eradication remains difficult. After consulting research institutions, the city turned its attention to an unlikely candidate: water spinach.
Absorbing the Nutrients
Since 2004, Ena Agricultural Senior High School in Ena, Gifu Prefecture, has worked with the city and other partners to purify water in a local dam reservoir using water spinach. The reservoir had suffered major outbreaks of blue-green algae, which cause murky water and foul odors. But when water spinach is grown hydroponically, it helps suppress the algae.
According to Vice Principal Ayumu Mizuno, water spinach is especially good at absorbing nutrients such as nitrogen and phosphorus from water. By doing so, it helps curb the growth of phytoplankton, which causes blue-green algae blooms. Preventing the water from becoming overly nutrient-rich is believed to be key to controlling the algae.
"By using something environmentally friendly, we can protect ecosystems and the environment," Mizuno said. "I think it is worth trying as a way to suppress the spread of alligator weed as well."
Similar water-quality improvement projects using water spinach have been carried out elsewhere, including in rivers in Nagoya and a regulating pond in Koshigaya, Saitama Prefecture. After harvesting, the water spinach is used for food and dietary education.
Saga City now hopes to adapt that approach and develop a new weed-control method. The idea is to slow alligator weed's growth by letting water spinach absorb the nutrients first. The city will study how much nitrogen and phosphorus, both essential for aquatic plant growth, the water spinach can remove from the water. It aims to develop the method into a formal control program from fiscal 2029 onward.
Spread to 30 Prefectures
According to the National Institute for Environmental Studies in Tsukuba, Ibaraki Prefecture, alligator weed was first confirmed to have taken root in Japan in Amagasaki, Hyogo Prefecture, in 1989. It has since spread to around 30 prefectures, leaving local governments struggling to contain it.
In Shiga Prefecture, the plant has been found in and around Lake Biwa since 2004. A prefectural official involved in removal efforts described the painstaking work required.
"We regularly patrol lakeshores, rivers, and waterways. If we see signs of rapid spread, we move quickly to contain it," the official said. "When we remove it by machine, we always follow up by hand, carefully clearing away any fragments left behind."
With some municipalities spending hundreds of millions of yen a year on removal, calls for national support are growing. Relevant ministries are now working on measures, including developing eradication techniques and assessing the ecological risks of herbicide use.
Can water spinach become a lifeline for residents and local governments struggling with this relentless weed?
"We won't know whether it works until we try," said the Saga City official in charge of the trial. "But we want to test it as one possible option."
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(Read the article in Japanese.)
Author: Mayuko Ichii, The Sankei Shimbun
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