Question: I Want to Kill Japanese Knotweed by Choking It Out. What Can I Cover It With?
Reader, Ryan asked me the following question regarding how to kill Japanese knotweed (Polygonum cuspidatum) after reading my article on the subject, where I nickname it "Godzilla weed":
"We are about to challenge this 'Godzilla' in the following manner:
- Cutting all the Japanese knotweed to the ground.
- Spraying area with herbicide.
- Covering it with pond liner and digging the pond liner down about 12 feet around the perimeter.
- Covering said area with mulch.
"Do you think pond liner is strong enough and with enough give? Is this a decent strategy?"
My reply follows:
You have a good strategy for your attempt to kill Japanese knotweed. I would just caution you about a few things:
1. Most pond liners I'm familiar with are not as tough as tarps. Here's my concern. When you cut the Japanese knotweed to the ground, any sharp edges left over could puncture the pond liner, especially after the cut canes dry out and become woody.
Also, there's a longer-term concern with puncturing. As the next crop of Japanese knotweed pushes up, the pond liner will (hopefully) not allow it to poke its head through and get any sunlight. So far, so good. But this is a multi-year project. It's possible that said Japanese knotweed growth will get woody and sharp enough eventually to pierce the pond liner. Of course, if you just happen to have extra pond liner sitting around and you want to use it on this project in place of a tarp temporarily, that's OK. Just be aware you may end up eventually having to lay tarps anyhow, to "patch up" where the Japanese knotweed has poked through the liner.
In addition to tarps, people also use old carpeting to kill Japanese knotweed by choking it out. This material is tough enough that there's little possibility of its being punctured. But newspaper would be another material to rule out; while it's effective for killing grass, it's not nearly tough enough for choking out Japanese knotweed.
2. Be aware that the Japanese knotweed may send up shoots beyond the perimeter of an area one covers with a tarp or old carpeting. You'll have to keep an eye out for these and spray them. In my chat with a Japanese knotweed specialist, he mentioned a few other things to be aware of when one uses the smothering method (that is, applying a covering over the infested area to choke it out).
3. Again, I want to emphasize that your endeavor to kill Japanese knotweed could go on for years. So remain firm and vigilant against Godzilla! But the good thing about the smothering method is that, since you're covering the area with a bark mulch or similar product (applied over the tarp or old carpeting), you're retaining it as a usable space, even while you're waiting to achieve full eradication. A mulched area can be attractive; bring out some lawn ornaments and container gardens to dress it up further. Or even place a raised bed garden right over it and plant away! It need not be a wasted space in the landscape.