“Boiling Out” Japanese Knotweed: The Latest Method in Combatting the Invasive Weed

With its bright green leaves and white flowers, it was once seen as a beautiful plant. But now Japanese Knotweed has evolved into the mother of all weeds. The loan shark spreads extremely quickly and is harmful to other plants and biodiversity in nature reserves. But also for buildings, roads and pipes.

That is why Natuurmonumenten fights the invasive exotic species with fire and sword. On the edge of the Leggelderveld, a nature reserve near Dwingeloo in Drenthe, the Japanese knotweed is cooked.

“It is a stubborn species to combat and tackle,” says forest ranger Rudmer Veenstra RTV Drenthe. “The Japanese knotweed makes long, deep rhizomes, sometimes 1.5 to 2 meters deep. The plant can grow 10 centimeters a day and goes through foundations and asphalt. A very small piece of the plant can already ensure that the knotweed settle and grow.”

Natuurmonumenten wants to prevent this at all costs at the start of the new growing season. So the plant’s place of growth is completely excavated, to a depth of a few meters. The soil is sieved to filter out all parts of the plant. The sieved soil then enters an installation that is temporarily located at this location. The soil ends up in a drum and is heated with fire.

This ‘boiling out’ should destroy any remnants of the plant. The warm soil comes out of the machine via a caterpillar track and ends up on a large lump of steaming black ‘earth’. After this process, the clean soil can be returned to its place of origin.

Natuurmonumenten does not limit the latest Japanese knotweed control method to Leggelderveld. Soil is also transferred from a growing area in the Drents-Friese Wold National Park (near Appelscha) to the ‘boil-out installation’ at the Leggelderveld. All in the hope that this time it will be possible to deal the final blow to the plant.

Although forest ranger Veenstra is wary: “There are never any certainties. But there is a very good chance that it will not come back. So we will continue to monitor in the coming years, in the hope that after today it will be over with the knotweed here.”

Many places are eagerly awaiting the results of the trial in Drenthe. After all, almost everything has been tried in combat. From mowing and boiling the roots with boiling water, to electrocuting and injecting with poison.

In Brabant, large pigs were used to eat the invasive exotic. But the plant has not been defeated so far. The only trial that gave hope so far was the release of the Japanese psyllid, a flea that feeds on the green leaves of the knotweed. But the psyllid can’t do it alone, said biologist and researcher Suzanne Lommen previously told NOS.