Chemical Control of Japanese Knotweed: What Works (and What Doesn’t)

Japanese knotweed’s aggressive rhizome system makes it one of the toughest invasive plants to eradicate. Soil-applied residuals like dichlobenil (Casoron G4) and sodium chlorate only suppress growth—complete kill is unlikely. Foliar-applied glyphosate translocates to roots, offering better results, but timing and technique are critical.

Effective Glyphosate Strategy

1. Winter prep: Cut old stems for access.
2. First spray (May): When regrowth reaches ~3 ft (90 cm). This is the optimal window—taller plants (7+ ft at flowering) are impractical to spray thoroughly.
3. Second spray (mid-summer): Target regrowth.
4. Fall check (Sept): Spray again if new growth appears before dieback.
5. Spring follow-up: Treat distinctive bushy, small-leaved regrowth (20–36 in) immediately—it looks different from mature knotweed but is equally invasive.

Additional Options

• Roundup Tree Stump & Rootkiller: Label recommends application to cut canes.
• Professional contractors: Access stronger herbicides that may cut eradication time in half (typically 3–4 seasons with glyphosate alone).

Critical Precautions

• Glyphosate kills desirable plants on contact—shield them carefully.
• Treated soil remains “controlled waste”; manage to prevent re-spread.
• Persistence is key: expect multi-year commitment.

Chemical control can work, but it demands a disciplined, multi-season approach. For large infestations, professional help is often the most reliable path.