Japanese knotweed appears to grow through tarmac and paving, but what it actually does is exploit existing cracks, joints and edges rather than bore through sound surfacing. Its vigorous shoots push up wherever there is a weakness, widening it as they go. Our knotweed FAQs address the common worry about hard surfaces, and this page explains what is really happening and how a stand affecting a drive or path is dealt with.
Can Japanese knotweed grow through tarmac?
Japanese knotweed does not drill through solid, intact tarmac, but it readily exploits existing cracks, joints and edges, and its strong shoots can lift and widen those defects until it emerges through the surface. A driveway that already has a hairline crack or a poorly sealed edge gives the plant its opening. So the shoots you see coming through tarmac have found a weakness rather than created one, which is why sound, well-laid surfacing over ground with no rhizome beneath it is not at risk in the same way.
Why does it exploit cracks and joints?
It exploits cracks and joints because the shoots grow fast and forcefully from the rhizome's stored reserves, following the path of least resistance upward, so any gap in tarmac, paving or a kerb line becomes a route to the light. The plant does not need much of an opening: a joint between slabs or a crack at an edge is enough. This opportunistic habit is the same reason it enters defective drains and gaps around structures, taking advantage of existing weaknesses rather than breaching sound material.
How is knotweed under tarmac dealt with?
Knotweed under tarmac is dealt with by treating or removing the rhizome, which is the source of the shoots, rather than simply resurfacing over it, since new tarmac laid over a live rhizome will be exploited again at its weak points. Depending on the site, that means a herbicide programme to exhaust the rhizome or excavation of the affected ground, followed by reinstatement of the surface. A survey establishes how far the rhizome extends under the hard surfacing, which decides the method.
Will resurfacing solve it?
Resurfacing alone will not solve it, because a fresh layer of tarmac over an untreated rhizome leaves the plant alive below, and it will find the joints and edges of the new surface just as it did the old. The lasting fix is to deal with the rhizome first, by treatment or excavation, then reinstate the surface. Laying a root barrier membrane can help contain a stand where full removal is impractical, and a professional remediation combines the ground works with the resurfacing so the problem does not simply return.
Common questions
- Does knotweed coming through my drive mean structural damage?
- It means the plant has found a crack or joint in the surfacing, not that it has damaged sound structure. It exploits existing weaknesses, so the fix is to treat the rhizome and reinstate the surface.
- Can I just spray the shoots on my driveway?
- Spraying the visible shoots does not exhaust the rhizome beneath, so they return. A professional herbicide programme or excavation of the affected ground is what deals with the source under the tarmac.
- Will a root barrier stop it coming through tarmac?
- A root barrier membrane can help contain a stand and protect a surface where full removal is impractical, but the rhizome still needs dealing with. A survey decides whether a barrier, treatment or excavation suits the site.