Introduction

Timing is everything when it comes to controlling Japanese Knotweed. Treat it too early, and the herbicide may be ineffective. Treat it too late, and the plant’s roots may already be preparing for winter dormancy — meaning you’ve missed the optimal window.

In this 2025 treatment calendar, we explain the best times of year to treat Japanese Knotweed using herbicide, excavation, and other professional methods. If you’re serious about removing knotweed efficiently and legally, knowing the seasonal timing is essential.

Why Timing Matters

Japanese Knotweed’s lifecycle follows a predictable seasonal pattern. Herbicides are most effective when the plant is actively drawing nutrients down into its roots, which happens late in the growing season. Other techniques, such as excavation or stem injection, may be more flexible but still benefit from seasonal awareness.

Knotweed Treatment Calendar (UK, 2025)

Month Stage of Growth Recommended Treatment
March–April Early shoots emerging Site survey, ID, planning only
May–June Rapid vertical growth Light herbicide (optional), stem injection begins
July–August Full leaf cover Main herbicide treatments begin
September–October Nutrient drawdown Peak treatment window (most effective)
November–February Dormant underground Excavation, root barrier installation

🟢 Spring (March–May)

What’s Happening:

  • Shoots emerge (red-purple, asparagus-like)

  • Rapid above-ground growth

  • Rhizomes still relatively shallow

Best Action:

  • Site survey to map infestation

  • Photographic monitoring

  • Planning a formal treatment schedule

  • Avoid cutting or disturbing the plant

📌 Note: Herbicide is NOT effective yet — the plant isn’t drawing nutrients down into the roots.

🌿 Summer (June–August)

What’s Happening:

  • Full growth phase — tall stems, dense foliage

  • High photosynthesis, but limited downward translocation

Best Action:

  • Stem injection or targeted spraying (June–early July)

  • Start herbicide treatment plan under PCA-approved supervision

  • Monitor extent for treatment adjustments

📌 Avoid mowing or DIY cutting — this can cause the plant to spread through broken rhizomes.

🍂 Autumn (September–October)

What’s Happening:

  • Plant begins to transfer energy and nutrients back to roots

  • Leaves may yellow, flowers appear (cream-coloured)

Best Action:

  • Apply glyphosate-based herbicide — most effective time

  • Follow-up stem injections for persistent areas

  • Cut-back and clean-up as part of monitored treatment

✅ This is the PEAK treatment window for chemical control.

❄️ Winter (November–February)

What’s Happening:

  • Stems die back, plant goes dormant

  • Rhizomes remain active below ground

Best Action:

  • Excavation work can be safely performed

  • Root barrier systems installed to prevent lateral spread

  • Good time for planning next season’s strategy

📌 Do not assume the plant is dead — it will regrow in spring.

Which Method Should You Use, and When?

Treatment Method Best Time to Use Notes
Herbicide Spray Late Summer to Early Autumn Most effective when plant draws down
Stem Injection Summer (June–Aug) Good for hard-to-reach or small areas
Excavation Winter or early Spring Requires space and licensed disposal
On-site Burial Winter Only with root barrier installation

Common Mistakes to Avoid

  • ❌ Treating too early in spring

  • ❌ Using DIY weed killers without guidance

  • ❌ Disturbing the soil or roots in summer

  • ❌ Assuming winter die-back means it’s gone

Pro Tip: Combine Treatments Over Seasons

Many of our clients see the best results when we combine:

  • Summer herbicide application

  • Autumn herbicide application

  • Winter monitoring or excavation (if needed)

This phased approach ensures both surface and subsurface control.

Frequently Asked Questions

Q: Can you treat Japanese Knotweed in winter?

A: Yes — but not with herbicide. Winter is ideal for excavation or installing root barriers.

Q: What’s the best month to apply herbicide?

A: September is typically the best month, as the plant is drawing energy (and herbicide) into its rhizomes.

Q: Can I treat it myself with weed killer?

A: DIY treatments rarely work long-term and may cause legal issues if knotweed spreads. Always use a licensed contractor.

Need Help Timing Your Treatment?

At Japanese Knotweed Elimination Ltd, we offer:

  • Year-round surveys and advice

  • Seasonal treatment plans tailored to your property

  • Long-term guarantees accepted by mortgage lenders

  • Safe, Environment Agency-compliant methods

📞 Call Today: 07582 869979

🌐 Visit: www.knotweed-wales.com
📅 Book: Free seasonal treatment consultation