After Knotweed Treatment: What to Expect Month by Month

You have had your property surveyed, chosen a PCA-accredited specialist, and signed up for a herbicide treatment programme. Now what?

The waiting period between the first treatment and the final all-clear can feel uncertain. Knotweed does not disappear overnight, and seeing new growth in the months after treatment can be alarming if you do not know what to expect.

This guide walks you through a typical herbicide treatment programme month by month, so you know exactly what is normal, what to watch for, and when to contact your specialist.

The Big Picture

A standard Japanese knotweed herbicide treatment programme runs for three to five years, depending on the severity of the infestation and the specialist's assessment. It typically involves:

• Active treatment phase: 2–3 years of herbicide applications
• Monitoring phase: 1–2 years of observation with spot treatment if needed
• Guarantee: A 10-year insurance-backed guarantee (IBG) is usually issued at the start

The goal is not just to kill the visible growth above ground. It is to destroy the underground rhizome network — the root system that can extend several metres from the visible plant and survive up to 20 years in the soil.

Year 1: The First Treatment Season

Spring (March–May)

The first signs of knotweed growth appear in late March or April. Red or purple asparagus-like shoots emerge from the ground, growing rapidly — sometimes several centimetres per day in warm weather.

What your specialist does: In most cases, the specialist will wait until the plant has produced a good canopy of leaves before applying herbicide. This is deliberate. The more leaf area available, the more herbicide the plant absorbs and translocates down to the rhizomes. Spraying too early, when the plant is small, is less effective.

What you should do: Do not cut, strim, or mow the knotweed. Let it grow. This feels counterintuitive, but interfering with the plant reduces the effectiveness of later treatment.

Summer (June–August)

This is typically when the first herbicide application takes place. The specialist applies a glyphosate-based herbicide — usually by stem injection for larger stands, or by foliar spray for smaller growth. Stem injection delivers the herbicide directly into the hollow stems, ensuring maximum uptake.

What happens next: Within two to four weeks, the treated leaves begin to yellow, wilt, and curl. The stems may darken. This is a sign that the herbicide is working its way through the plant and into the rhizome system.

What is normal: The plant does not die completely after one treatment. Some leaves may remain green. Some stems may still stand. This is expected. The first application weakens the rhizome network; it rarely kills it outright.

Autumn (September–November)

A second herbicide application is often carried out in early autumn, before the plant enters dormancy. As the plant prepares for winter, it naturally draws nutrients — and with them, herbicide — down into the rhizomes. This makes autumn treatment particularly effective.

After the second spray, the above-ground growth dies back as it would naturally over winter. Your specialist may cut back the dead stems at this point, or leave them until the following spring.

Winter (December–February)

The knotweed is dormant. There is nothing to see above ground and nothing for your specialist to do. The herbicide continues working in the rhizome system underground.

What you should do: Nothing. Wait for spring.

Year 2: Reduced Growth

Spring (March–May)

New shoots will appear. This is normal and expected. However, you should notice clear differences from the previous year:

• Fewer shoots — the number of emerging stems should be noticeably lower
• Weaker growth — stems are thinner and shorter
• Smaller leaves — the canopy is less dense

If the growth looks as vigorous as the previous year, contact your specialist. This may indicate that the treatment needs adjusting or that there are additional rhizome sources.

Summer–Autumn

Your specialist applies further herbicide treatments — typically one or two applications, depending on the level of regrowth. The process is the same as Year 1, but with progressively less plant material to treat.

By the end of Year 2, many infestations show an 80–90% reduction in above-ground growth compared to the original infestation.

Year 3: Monitoring Begins

Spring

You may see some very small shoots — just a few centimetres tall, with small leaves. Or you may see nothing at all. Both outcomes are normal at this stage.

What your specialist does: If any regrowth appears, they will apply spot treatments — targeted herbicide application to individual shoots rather than a broad application.

The Rest of the Year

Your specialist visits the site periodically (typically quarterly or twice yearly) to inspect for any regrowth. Each visit is documented with photographs and a written report, which is provided to you and your mortgage lender if applicable.

If no regrowth is observed throughout the monitoring period, your specialist will issue a final completion report.

Years 4–5: Extended Monitoring (If Needed)

Some infestations require a longer monitoring period, particularly if:

• The original infestation was severe
• The rhizome network was extensive
• There is a neighbouring source of knotweed that could cause re-infestation

Your specialist will advise on whether extended monitoring is needed. This does not mean the treatment has failed — it is a precaution to ensure the rhizome system is truly exhausted.

The Insurance-Backed Guarantee

One of the most common misunderstandings is about when the IBG is issued. Many homeowners assume they need to wait until treatment is complete before they receive the guarantee. In practice:

• The IBG is issued at the start of the management plan, not at the end
• It typically covers a 10-year period from the date of issue
• It covers the cost of further treatment if knotweed returns during the guarantee period
• It covers structural damage caused by knotweed during the period
• It is transferable to future owners of the property

This is why mortgage lenders accept properties with active knotweed management plans — because the IBG provides financial protection from day one.

Can You Sell During Treatment?

Yes. This is a common concern, and the answer is straightforward:

• If you have a management plan in place with an IBG from a PCA-accredited specialist, most mortgage lenders will lend against the property
• The management plan and IBG transfer to the new owner
• You should disclose the knotweed on the TA6 Property Information Form (answering "Yes" to Question 7.8 about Japanese knotweed)
• Provide the buyer's solicitor with copies of the survey report, management plan, IBG certificate, and any progress reports

Having professional treatment in place can actually make a sale easier than trying to sell without addressing the knotweed at all. See our mortgage guide for more detail.

What If Treatment Fails?

True treatment failure is rare with PCA-accredited specialists, but it can happen if:

• The rhizome network is deeper or more extensive than initially assessed
• There is an untreated source of knotweed nearby (for example, on neighbouring land)
• Ground disturbance during building works has fragmented and spread rhizome material

If your specialist reports that the treatment is not progressing as expected, they will typically:

1. Reassess the infestation and adjust the treatment plan
2. Consider alternative methods (such as excavation for persistent areas)
3. Extend the treatment period

This is where the IBG provides protection. If knotweed returns after the management plan is completed, the guarantee covers the cost of further treatment.

What You Need to Do

As a homeowner, your responsibilities during the treatment period are minimal but important:

1. Do not disturb the treated area. No digging, building, or landscaping in the treatment zone without consulting your specialist first.
2. Do not cut or strim the knotweed. Let the specialist manage it.
3. Allow access. Your specialist needs access to the site for scheduled treatments and inspections.
4. Keep the paperwork. Store your survey report, management plan, IBG certificate, and all progress reports safely. You will need them if you sell the property.
5. Report any concerns. If you notice unexpected vigorous regrowth or new knotweed growth in a previously unaffected area, contact your specialist.

Finding a Specialist

If you are considering treatment, the most important step is choosing the right specialist. Look for PCA-certified specialists near you who offer insurance-backed guarantees and have experience with residential treatment programmes.

Our guide to choosing a specialist covers what questions to ask and what red flags to watch for.