The number of reports about the Japanese knotweed has increased considerably in recent years, according to an analysis based on figures from Waarneming.nl. The rapid growth and easy spread of the plant are major problems in the Netherlands. The plant is very difficult to control and can cause a lot of damage to foundations, roads and sewers.
Compared to previous years, the number of reports nationally increased significantly. This year alone (until the end of May) there were almost one and a half times as many reports as in the first months of last year.
Chris van Dijk, plant expert and researcher at Wageningen University, says that the plant has been widespread in the Netherlands for some time. He says it is good that more is being reported. The more people know about the problem plant, the better its spread can be contained.
The Japanese knotweed can be recognized by its fresh green leaves. In the spring they have a red vein. The stem is green with red dots and hollow. In autumn, from August to October, the knotweed has white flowers.
The plant causes damage by growing up through existing cracks in concrete, masonry or asphalt. This can make the cracks bigger. The surrounding soil can also be pushed up. The Japanese knotweed is an invasive exotic and is already rampantly displacing other plants that originally grow here. The species was imported from Japan in 1823.
Invasive exotics are often spread unconsciously by humans
“The Japanese knotweed is only spread by humans, animals do not contribute to it,” says plant expert Van Dijk. Often the spread occurs accidentally. “A lot of digging is done in the Dutch soil. If there is a small piece of root in the excavated soil and this soil is used elsewhere, the plant is so spread out.”
According to Van Dijk, it helps if people recognize the invasive exotic species earlier, so that they know better what to do with it. For example, mowing the plant sometimes only leads to further spreading. It is also important that the plant waste is not thrown with the garden waste but with the residual waste, to prevent the plant from spreading further.
Dennis Geraerts of pest control company Geraerts-Someren says that despite careful removal, the plant often returns. “Exhausting the plant is the only way to get rid of it. You have to keep busy to eradicate the plant completely.”