The seemingly innocuous alligator weed, or Alternanthera philoxeroides, poses a real problem, especially to rice farmers.

The verdant green leaves and dainty white flowers of the alligator weed are becoming a common sight in waterways across Japan.

But while the plant, whose scientific name is Alternanthera philoxeroides, may look innocuous, it is an absolute menace according to Koichi Goka, head of the Invasive Species Research Team at Japan’s National Institute for Environmental Studies (NIES).

“We first had reports of alligator weed in Japan in Hyogo prefecture in 1989 and it quickly spread from there across much of central and then southern Japan,” he told This Week in Asia. “We were soon having reports from the Tokai region as it spread to the east and now it is in Kanto and spreading to the north.”

The plant is now found in 30 of Japan’s 47 prefectures.

It was unfortunate that the plant liked the Japanese environment, Goka said, expressing concern over its effect on the agricultural sector.

“It invades paddy fields, other agricultural fields and waterways and it spreads because it thrives both in a water environment and on the land,” he said. “It spreads very easily because it can reproduce from parts that are cut off the plant, such as a piece of leaf, so cutting the plant back can actually help it to spread.”

It is, he said, “one of the most dangerous invasive plant species that we have in Japan today”.

Originally found in the Parana river basin in Brazil, Paraguay and Argentina, the plant has since proliferated throughout South America.

It has also been inadvertently transported to 30 countries around the world, including China, Australia and the US – most likely through sediment on shoes or in ship cargo, experts say.

Growing concern

Alligator weed was a serious concern for rice farmers, said Kevin Short, a naturalist and former professor of cultural anthropology at Tokyo University of Information Sciences.

“I live in Chiba prefecture and I can tell you that it is already a big problem for the farmers here because it just grows so well and so quickly,” he told This Week in Asia.

“You can often find it on the edges of roads and then it just spreads into the paddy fields.”

The plant’s ability to reproduce even from a small fragment has proved difficult to combat, despite farmers fitting mesh nets over the culverts that bring water into their fields, Short said.

“Farmers here also try to conserve water, so they recycle it from ponds, marshes and rivers and once it has run through their paddy fields, they pump it through paddy fields higher up the valleys,” he said.

“Usually aquatic plants like this are only able to spread downstream, but by pumping the water higher, they have found that the plant is spreading there as well.”

Alligator weed has also been blamed for flooding in some areas after heavy rainfall as it blocks drainage ditches, making waterways breach their banks.
In some areas, it competes with native flora for space, nutrients and access to light. Short said there was a genuine risk that alligator weed “will wipe out some domestic species”.

Japanese agriculture is already facing a crisis with fewer young people interested in making a living from the land, leaving the sector largely to a shrinking and ageing working population.

Those that remain in the low-paying sector are finding it harder to deal with the physical challenge and expense of getting rid of the weed.

The problem is so serious that the Environment Ministry gave affected communities 500 million yen (US$3.2 million) in subsidies last year to find ways to eradicate the plant.

In this year’s budget request, the ministry is seeking 1.4 billion yen solely for relief measures.

NIES was presently working on a programme to halt the growth of alligator weed, Goka said. A number of different herbicides are being tested for effectiveness while at the same time “limiting the ecological impact on native species”.

“We really cannot allow it to spread any further across Japan because the impact on crops and other plant and animal life would be extremely serious,” he said.

Source - https://www.scmp.com