Roij Scholten provides advice on everything related to soil. He will also be doing this when Zuidasdok starts work on the embankment alongside the A10 this year in order to insert the roof of the new Brittenpassage and partly move the southerly railway platform A10 Zuid heading to Utrecht at the same time. But, as was discovered during a specialist inspection in the summer of 2022, an uninvited guest has set up home in that embankment: Asian knotweed. This is a stubborn, fast-growing exotic variety of plant. ‘It’s also one that actually used to be available for sale at garden centres.’
Asian or Japanese knotweed?
Asian knotweed is a collective term sometimes used to describe three different varieties of plant: Japanese knotweed, giant knotweed and Bohemian knotweed, a hybrid between the two. Knotweed is not native to the Netherlands, but comes from Japan, China, Taiwan and Korea. In the middle of the 19th century, the plant was introduced to the Netherlands for research purposes. From the middle of the 20th century, the plant really began to take hold, when it was spread in public areas via garden centres, green waste, soil transport and mowing.
Damage
On our work site in Arnold Schönberglaan, the Japanese variety of this stubborn exotic plant has been discovered – it is growing in the embankment along the A10, right between the roof sections that are ready and waiting for the additional station passageway in Parnassusweg. ‘All they need to grow is a few millimetres of stem or root – this plant really is invasive and can damage foundations and infrastructure’, says Scholten. ‘It’s the last thing you want on a building site. It’s also a very busy place, with lots of soil movement and the related risk of further spread. If you plan to get rid of the plant, you need to be sure that you get every single piece.’
Rooting it out
Whenever a project in Zuidas involves digging up and moving soil, checks are carried out for the presence of knotweed. In this case, a visual inspection was enough to identify the plant. However, there are other methods of detection, including DNA testing and soil sampling. But if you want to know really quickly whether there is knotweed in the soil, another faithful four-legged friend can assist: specially-trained sniffer dogs can identify knotweed from the smell of the roots.
Removal
In the Netherlands, knotweed has no natural enemy, but there is one in East Asia: the Japanese knotweed jumping louse. The City of Amsterdam experimented with it in Piet Kranenbergpad back in 2020. However, according to Scholten, the method turned out to be more suitable for management than removal. ‘In any case, the whole process takes patience and we don’t have the time.’ For an immediate result, you can opt to dig up and remove the soil where the knotweed has been found. A processing company can then clean the soil by heating it up or wrapping it in a barrier to suffocate the plant. The heating process consumes quite a lot of energy and is not very sustainable. ‘Following thermal treatment, the soil is also ‘lifeless’, making it less suitable to be reused elsewhere’, adds Scholten.
Other factors at play
Zuidasdok make serious efforts to reuse any soil, but in tackling the site alongside the A10, there are other factors at play. ‘In order to maintain the stability of the embankment, it made sense to keep most of the soil in place’, points out Scholten. ‘We only dug up and removed the topsoil. We enriched the remaining soil with CleaRoot granulate before covering it with an airtight barrier. CleaRoot nourishes the microbes in the soil that then consume the oxygen. This causes the plant roots to die.’ In order to give the microbes sufficient time, the barrier will remain in the ground until excavation work starts for the tunnel.
Freezing
Knotweed is widespread in Amsterdam. All kinds of techniques are being tried to get the better of the plant, including electrocuting the knotweed or sieving the soil to make it possible to separate the pieces of plant by hand. ‘Within Zuidasdok, we’re also always on the lookout for better techniques’, says Scholten. ‘For example, on our work site between the tracks to the east of Beethovenstraat, where it’s been discovered growing in various locations.’ Do we have plans to dig it up and apply a barrier in this case? ‘No, and we won’t be applying heat treatment either. This time, we’ll probably try the opposite: freezing the soil so that the plant cells become irreparably damaged. And then persist with that until they perish.’
Interested in seeing some green areas that we really are proud of? If so, take our special Green Walk (Ommetje Groen).
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