Japanese knotweed grows fast, which is part of what makes it a problem to leave. At its spring and early-summer peak the canes can extend by up to around 10 centimetres a day, taking a stand from bare ground to 2 to 3 metres in a matter of weeks. That speed drives its spread and its reputation, and our knotweed FAQs put it alongside the other questions a homeowner asks first when a stand appears near a property.
How fast does Japanese knotweed grow?
At its peak in late spring and early summer Japanese knotweed can grow by up to around 10 centimetres a day, one of the fastest growth rates of any plant in the UK. The red and purple shoots that emerge from around March climb quickly, unrolling into hollow canes that reach the full 2 to 3 metres by early summer. This surge is fuelled by the reserves stored in the rhizome over winter, which is why a stand re-establishes its full height each year rather than starting small.
What drives the growth?
The growth is driven by the rhizome, the underground stem system that stores carbohydrate reserves through winter and releases them into fast spring growth. Warm weather, moisture and good light accelerate it, so a sheltered, sunny stand grows fastest. Because the energy comes from below ground, cutting the canes only prompts the rhizome to push up more, which is one reason surface cutting fails and treatment has to reach the underground system.
How quickly does it spread?
Above ground the canes reach full height in a season, while below ground the rhizome extends steadily year on year, so a stand widens and can send shoots several metres from its centre over time. Most spread in the UK is not by seed but by rhizome and by fragments carried in moved soil or garden waste, where a fingernail-sized piece can start a new stand. This is why disturbing knotweed without a plan, or fly-tipping affected soil, spreads it so readily.
What does the growth rate mean for a property?
For a property the growth rate means a small stand does not stay small, so leaving it season on season lets it widen towards boundaries, paving and outbuildings and makes eventual removal larger and dearer. It rarely damages sound modern foundations, but it exploits existing cracks and gaps and can affect a sale or mortgage. Catching a stand early and arranging a survey keeps the problem, and the cost of dealing with it, contained.
Common questions
- Does it grow all year round?
- No. Growth is fastest from spring through summer, slows in autumn as the plant dies back, and stops over winter when the canes die and the rhizome lies dormant. The 10 centimetres a day figure is the peak, not the average.
- Will it grow back if I cut it?
- Yes, and often more vigorously, because cutting removes the top growth but leaves the rhizome and its reserves. The plant simply pushes up fresh canes, which is why treatment targets the rhizome rather than the canes.
- How quickly can it reach my house?
- It depends on where the stand sits and how long it is left, but a stand can widen and send rhizome several metres over a few years. It exploits existing gaps rather than breaking sound structure, and a survey assesses the risk.