What’s the Deal with Japanese Knotweed?
Japanese knotweed (Reynoutria japonica) is a tenacious perennial that spreads through rhizomes—those underground roots that can run up to 3 m deep and 7 m wide. Even tiny fragments left behind can grow into full-blown colonies. It’s one of the worst invasive species out there (The Sun, Wikipedia).
The Full-Scale Removal Game Plan
1. Identify and Inspect
- Japanese knotweed usually shows up with bamboo-like hollow stems and heart-shaped or shield-shaped leaves (The Times).
- In King County (that’s Seattle’s turf), controlling knotweed is highly recommended—especially near waterways and residential areas—and it requires long-term commitment (nwcb.wa.gov).
2. Integrated Pest Management (IPM)
Seattle’s best approach? Do not rely on just one method. King County recommends using IPM—pick the combination of techniques that fit your property, budget, and goals. And commit to it for several years (nwcb.wa.gov).
3. Mechanical Methods (Manual Work)
- Digging it out? Talk about tough: the root network is massive, and any leftover piece can re-sprout. Unless you’re developing the land, it’s a high-risk and labor-intensive method (nwcb.wa.gov).
- Frequent cutting: King County suggests cutting stems twice a month from April through August, then once a month until frost—for at least 3–5 years. Keep it under 6 inches tall to starve the rhizomes (nwcb.wa.gov).
- Smothering: cut, dry, and cover with thick landscape fabric or clear plastic (some say clear plastic works like a greenhouse to cook them), leave sealed for years. Again, long-term patience is key (New Hampshire Department of Agriculture).
4. Chemical Assault (Herbicide)
- Foliar herbicide: Apply glyphosate or imazapyr from late July to early October when the plant is actively growing (King County).
- Stem injection: This is a sharper tool—inject glyphosate directly into the hollow cane, letting it go straight to the root system. It’s precise and reduces collateral damage (mdc.mo.gov).
- Bonus: King County lends out knotweed injectors for free from July to October—reach out and see if you can borrow one (Noxious Weeds Blog).
5. Protect and Restore
- After treatment, bring in native plants and stabilize soil—especially along creeks—to rebuild habitat and keep knotweed from creeping back (Mountains To Sound Greenway Trust).
What Seattle Folks and Gardeners Say
- One gardening veteran puts it bluntly: “You can’t cut it, you can’t mow it, nothing… even a tiny piece can start a new plant.” They suggest burning dried stems—but be careful, this isn’t legal everywhere. (Reddit)
- Another forum user says the best results came from cutting it down every time it reached 10 cm tall—approximately every two weeks—for two years, while prepping for stem injection (UBC Botanical Garden Forums).
Step-by-Step Plan
- Survey your patch: mark where the knotweed is, how big it is.
- Start cutting: twice a month during spring/summer, then monthly until first frost.
- Plan for herbicide: late summer to fall, choose foliar spray or get your hands on a stem injector from the county.
- Smother if needed: after cutting, layer with thick fabric/plastic if you’re not going herbicide-heavy.
- Pile of follow-up: keep chipping away year after year—knotweed doesn’t quit, and neither do you.
- Restore: once it’s under control, plant native vegetation and let nature bounce back.
Why Seattle Like This Plan
- It’s comprehensive—you’re starving it, poisoning it, smothering it, outlasting it.
- It fits local programs—injector loan programs exist, and restoration projects are grant-funded (nwcb.wa.gov).
- It’s tough-love real—you stay at it for years, no quick fixes.
Final Thought
Mr. Lee—or, Seattle’s landscape warrior—removing Japanese knotweed is a marathon, not a sprint. You’ve got local support, chemical and mechanical strategies ready to roll, and a community that knows the fight is for the long haul. With patience, persistence, and a sprinkle of that Seattle grit, you can win this battle and probably grow some native beauties in its place.
