The RHS Chemical Control Protocol: Timing is Everything
The RHS emphasizes that amateur gardeners often waste years spraying knotweed at the wrong time of year.
Spraying knotweed in May or June causes top leaves to wither, but the plant's sap is rising, meaning minimal chemical penetrates the subterranean rhizome system. Within weeks, deformed dwarf shoots re-emerge.
The optimal application window is late summer (August, September, and early October):
• Flowering Window: During late summer, knotweed flowers and naturally shifts its physiology to translocate photosynthetic carbohydrates downward to store energy in the crown for winter.
• Translocation Absorption: Applying a professional-strength glyphosate formulation during this window pulls the herbicide deep into the root core, killing rhizome nodes up to 3 metres below ground.
| Method | Best Application Time | Effectiveness | Risk to Surrounding Plants |
|---|---|---|---|
| Foliar Glyphosate Spray | August to October (Flowering) | High (Requires 2–3 seasons) | Risk of spray drift onto lawn/borders |
| Direct Stem Injection | Late August to September | Very High (Precision delivery) | Zero drift risk (Safe near prize plants) |
| Repeated Weakening (Digging) | Spring through Autumn | Low (Very labour intensive) | High risk of spreading rhizomes |
| Root Barrier Membrane | During groundwork/landscaping | High (Physical containment) | Prevents lateral boundary creep |
Disposal Rules & Statutory Waste Liabilities
The RHS provides unequivocal guidance regarding the disposal of knotweed plant debris:
• Domestic Council Bins: It is strictly illegal to place Japanese knotweed stems, roots, or soil in your council green waste garden bin or black wheelie bin.
• On-Site Drying and Burning: If you cut knotweed stems, the RHS recommends allowing them to dry thoroughly on a clean tarp until brittle, and then burning them in an incinerator or bonfire where local council bylaws permit.
• Licensed Landfill: Any soil excavated containing knotweed roots must be transported by a licensed hazardous waste carrier to an authorized landfill.