Can knotweed grow through concrete?
Some companies state that Japanese knotweed can cause damage to concrete Tarmac and other hard surfaces.
Japanese knotweed (Fallopia japonica)
It is a highly invasive plant species known for its ability to grow through various surfaces, including concrete. The plant has a robust root system and can penetrate cracks and crevices in concrete, asphalt, and other hard surfaces.
The rhizomes
(underground stem) The plant can grow to a depth of 3 meters and extend up to 7 meters horizontally. Moreover, it can grow through small gaps and cracks in the concrete. However, it’s important to note that concrete is rugged and durable. Suppose the concrete has been laid correctly, i.e., compacted, vibrated, and reinforced. Japanese knotweed will grow around it, not through it.
Part of the issue with Japanese knotweed is that Rhizomes from this plant can spread seven meters horizontally. As seen in the photo above, the roots have spread under the surface of the concrete to the edge. Consequently, if the knotweed is cut, the plant will grow and swell, which may cause cracking to the surrounding hard surface. Therefore, removing the concrete slabs is the best way to resolve this problem. Let the Japanese knotweed grow, and then chemically treat it.
Digging out the plant
The trouble with digging out Japanese knotweed is that if the plant is close to a building, say within one meter; the rhizomes could potentially spread under the foundations, making it impossible to remove the Japanese knotweed without undermining the building and its foundations. The environment agency prefers that Japanese knotweed is always treated to prevent the spread of the plant off-site to other locations. Some companies will always recommend mechanical removal as this process is more lucrative.
Note that it is not illegal to grow Japanese knotweed in your garden as long as it does not spread to a neighbouring property.