Killing knotweed
Knotweed is exceptionally hard to kill. A botanical James Bond, knotweed is stubborn, elusive, and resilient. If you think mowing it or burning it will work, you’re wrong. If you think that vinegar-based weed killer you whipped up will work, you’re wrong. Save your time and condiments and read the following article. We describe several methods that have proven successful. Depending on the age and the severity of your situation, some of these methods might work.
Digging out knotweed
This method is only practical with new and relatively limited populations. If you dig out knotweed, remember that rhizomes can be large, leggy and run deep. So dig generously around affected areas to ensure you remove the entire rhizome. Also, make sure that all the material removed is destroyed before disposing of it. We suggest solarizing or smothering the area where you removed the knotweed to ensure destruction. Check the area every month for signs of new plants. If you see new sprouts, repeat the process or expand your efforts.
Smothering knotweed
If you have lots of knotweed, this method can be labor intensive. It’s no picnic, but can be a very effective method if you’re keen to avoid chemical herbicides.
Here’s a plan:
Step 1: Let the knotweed to grow in early Spring but then cut the stalks very close to the ground in early June. This forces the plant’s rhizome to channel its stored energy into regrowing the stalk (as opposed to strengthening itself).
Step 2: Safely dispose of all cut stems. If you cannot remove them, then gather stems on an impervious surface (thick tarp, flat pavement, etc.) to completely dry to a brown, brittle state, at which point they are less viable.
Step 3: Add a generous layer of mulch or other similar substance over the area cut. This is a cushioning layer. Do not use the cut knotweed stalks.
Step 4: Cover area with a thick dark colored tarp or heavy black plastic sheeting. Extend this covering beyond the limit of the area affected. Generously overlap sheeting to prevent any sunlight from penetrating. You don’t want any new stalks navigating their way to daylight.
Step 5: Secure the sheeting (edges, corners, overlaps, middle) with weights (large rocks, sandbags, other heavy objects). Repair any holes or tears that develop.
Step 6: Monitor the site monthly, especially during growing season. Typically, after five years, the area can be cleared and reclaimed.
Poisoning knotweed
DISCLAIMER: We are providing information, not recommendations. Herbacide is serious stuff. Do your research. Be safe!
Some herbicide treatments have been successful when carefully applied and well executed. Herbicides with reported success include glyphosate and triclopyr. Depending on where you live, chemical control may require a licensed applicator or permit.
Important: thoroughly research all federal, state, and local laws and restrictions before pursuing chemical treatment. Follow all application rates and adhere to all site restrictions including required setbacks from sensitive environments like wetlands and water.
Timing: Herbicidal control is most successful within five years of knotweed first emerging, and it is most effectively delivered at the end of knotweed’s annual lifecycle. Specifically, after the plant has flowered but before it has died back (roughly September – November). This precise timing greatly improves the efficacy of this method and reduces its potential negative impact on pollinators. Applying herbicide earlier than this has little to no effect on knotweed. Why? Because, knotweed rhizomes deliver energy up through stalks to leaves during the growing season, then reverse and suck energy back from leaves, through stalks, and into rhizomes before dying back. This process powers up rhizomes so they can overwinter and wreak havoc the following Spring. Herbicide is applied to leaves so this is the moment to hit them!
We’ve devoted an entire section to herbicidal treatment. Read about it HERE.