This article has been reviewed by our Japanese knotweed expert:
Ben Lindley, Company Director at Japanese Knotweed Ltd.
Last reviewed on 30th July 2025.

All you need to know about Japanese Knotweed in Autumn

Japanese knotweed (Fallopia japonica) undergoes visible changes in autumn that can affect how it’s identified and treated. If you’re planning autumn treatment or excavation, it’s vital to understand the plant’s seasonal behaviour, its root structure, and the most effective management or removal options. In this post we look at Japanese knotweed Identification, Treatment & Removal in the Autumn season.

What Does Japanese Knotweed Look Like in Autumn?

As the temperatures drop, Japanese knotweed starts to retreat. The lush green leaves and tall stems that dominate summer landscapes begin to fade. Here’s what to look out for:

Leaf Colour: Leaves transition from green to yellow, golden brown, or even bronze.
Stems: Begin to dry and become brittle, they remain hollow inside.
Height: May remain tall, but foliage thins and dies back slowly.
Spread: Underground rhizomes continue to grow even as surface growth wanes.

Japanese knotweed turning brown in autumn Early signs of knotweed foliage changing in autumn

Treating Japanese Knotweed in Autumn

Autumn is one of the most effective times to treat Japanese knotweed with herbicide. As the plant prepares for dormancy, it draws nutrients – and herbicide – down into its extensive root system – often referred to as ‘translocation’. This makes it a prime window for its second root-focused treatment.

Two-Stage Herbicide Spray Method

Stage	Timing	Purpose
First Spray	Late Summer / Early Autumn	Weakens surface growth and begins absorption (translocation) into its rhizomes.
Second Spray	2–3 weeks after first treatment	Targets regrowth and enhances deep root herbicide uptake.

Why it works: The dual-application method takes advantage of the plant’s seasonal biology. Drawing the chemical deep into the root system during dormancy preparation significantly increases the chance of long-term suppression.

Japanese knotweed root Knotweed roots (rhizomes) are the true battleground – visible growth is just the surface.

knotweed Excavation & Physical Removal

For sites requiring rapid clearance – such as construction, property development, infrastructure projects, or property sales – excavation is often the preferred method. This involves physically removing the knotweed and contaminated soil under controlled conditions, and removing it from site (or if the site is applicable burying it in a deep barrier lined ‘cell’).

Steps in the Excavation Process – full removal:

Site assessment (survey) and excavation planning.
Removal of visible knotweed and infested soil to a set depth.
On-site or off-site disposal under Environment Agency guidelines.
Backfilling with clean soil and monitoring.

Excavator digging knotweed site Excavation in progress for Japanese knotweed removal

Knotweed coming up through paving slabs Visible signs of structures that have been damaged by untreated knotweed.

Did you know?

Japanese knotweed can regrow from a piece of rhizome smaller than your fingernail.
Proper disposal and monitoring are essential to prevent regrowth.
Also that disturbing any remaining root fragments will encourage growth of new plants.

Case study: Excavation in challenging wet conditions

Quick Reference Table: Autumn Knotweed At A Glance

Feature	Appearance in Autumn
Leaves	Shovel-shaped turning yellow or golden-brown, curled or drying at edges. Will drop off at the end of the growing season.
Stems	Woody, hollow, reddish-brown. Brittle to snap, often remaining upright.
Flowers	Small creamy clusters (early autumn).
Root Activity	High nutrient drawdown into rhizomes to prepare for winter dormancy.
Treatment	Herbicide most effective (second treatment in Autumn); excavation possible all year round.

FAQs: Japanese Knotweed in Autumn

Can Japanese knotweed be treated in autumn?

Yes, autumn is ideal for herbicide treatment. The plant naturally draws nutrients into its root system at this time, helping carry the herbicide deeper into the rhizomes.

Will Japanese knotweed die back in autumn?

Yes, the above-ground growth will begin to wither and die, but the roots remain active underground unless properly treated or removed.

Can excavation be done in autumn?

Absolutely. With the right weather and soil conditions, excavation is possible year-round, including autumn – especially when projects demand faster resolution than herbicide alone.

Is it too late to start herbicide treatment?

No. In fact, late summer and early autumn are some of the best times to begin. Even if treatment starts later, it can still weaken the plant ahead of winter.

What happens if I ignore Japanese knotweed until spring?

It will die back visibly over winter, giving a false sense of security. But the root system remains intact and will regrow vigorously in spring. Autumn action helps prevent this.