Japanese Knotweed: My Part in Its Downfall

When we bought the house (and for years afterwards), our yard was only a couple of cars on blocks and a bathtub shrine away from Redneck Nirvana. We focused first on the interior, with necessary attention to Things That Were Blowing Up Right Now. Later, we moved on to Things That Could Cause a Freak Accident and slightly more leisurely points beyond. During this time, we mowed the scrubby grass, cut down the worst of the monster weeds, and planted a few gift plants. It’s all we had time for.

I wish I were kidding. This was our yard.

One of the gift plants was called “fleeceflower,” and it looked really pretty. It was vigorous and attractive, and I didn’t have to do a thing to it. It had pink veining on the leaves, and fluffy flowers in late summer.

What’s not to love?
Photo credit: Wikipedia/Ancatdubh43

But pretty is as pretty does, and in this case, pretty does Dallas. It’s a very busy plant, which is a nice way of saying it’s an evil, invasive bully from hell. When it first started its conquest of our garden, I was pleased – but when it knocked out the ditch lilies and lily of the valley, I started to be concerned. When it started heaving up large rocks, I freaked. And researched.  And despaired. 

But I knocked it back in one season, and three years later, there’s no sign of it. It wasn’t fun, and it wasn’t environmentally friendly, but here’s how I did it.

1. Commitment. With Japanese Knotweed, you have to want it. I had a mantra: “I am smarter and meaner than you, plant, and I am going to win!” Hold on to your mantra, because fleeceflower will make you feel dumb and helpless if you let it.
2. Glyphosate (Round-Up). If this is a midway stop in your research, I can imagine your laughter. Round-Up will not kill it, no. But it’s a good start on weakening it. Spray the bastards with Round-Up to cover every leaf surface, top and bottom. You’re going to want the bigger “tank” size.
3. WAIT. Seriously. Wait. Round-Up requires the plant to keep going while it gets down into the roots. Wait a couple of weeks. Cackle maniacally from time to time, because the plant has no idea what’s coming for it.
4. Cut it down. Knotweed can regenerate from clippings, so put down a tarp and cut the stalks off about a foot from the ground. Gather EVERYTHING in the tarp and dry it out, then burn it. Do not compost it. Do not trash it. Do not leave any pieces behind. Do not fail to completely clean your shoes and tools before leaving the area. Destroy it.
5. Round-Up. What?? Yes, more Round-Up. Once you cut it down, you’ll have the gaping maws of the sheared canes exposed. Stick the Round-Up nozzle right down their throats and spray with abandon. Renew your commitment by imagining them gagging and choking.
6. Wait. Give it another couple of weeks. Yes, this area looks like hell. War is hell and you are at war with this plant.
7. Round-Up. Really? Yes, because the thing will most likely put up some sprouts. Look yards away for sprouts – I found some that vined under the porch and came out where it thought it was safe. Coat the leaves on the new growth, top and bottom.
8. Wait. A couple of weeks, yes. And while waiting, imagine the plant in its death throes. Hate it a lot! You have reason.
9. Dig! Get your tarp out again, and start digging. The roots will be unbelievable – woody, thick, gnarled, vicious. You’ll need a saw. Dig. Cut. Dig. Cut. Tease out the smaller roots. Follow every big root you can find and get it out of there. Gather everything on the tarp; you’re going to burn it or encase it in nuclear waste. (Burning is more practical.) I also had a heap of dirt that I screened and supplemented with clean fill. When I was doing this, I had a trench over three feet deep. I insisted the Kev spend some time outside so the neighbors knew he was well. It looked as if we'd had some sort of extremely serious argument with illegal outcomes.
10. Keep hating it!
11. Drill. You are not going to get all the roots. I followed one down to a meter below grade, and then it took a turn straight down into the bowels of the earth. Spray the cut ends with…you guessed it! Let dry for a few days, and then take your drill and riddle the stinkers with holes. Cover each one completely with stump removing powder.
12. Fill. Fill in with uncontaminated dirt back up to grade, compacting as you go. Get bog standard dirt – anything with lots of organic matter or fertilizers will encourage it to come back, and it needs no encouragement.
13. Cover. Get a big, nasty, impermeable tarp and cover the area. Go at least five feet beyond the kill zone. Weigh it down with big old rocks, and then cover the whole thing with landscape rocks or mulch. You won’t be growing anything here for a couple of years. I kept container plants on top.
14. Patrol. By now, you know the shoots and leaves and even the roots of your enemy. Check beyond the edges of the tarp for signs of life. If the tarp heaves up somewhere from new sprouts, peel it back, spray, and re-cover. Repeat at least once in a couple of weeks, then cut it back, and re-tarp. Stay alert!
15. Wait. And patrol. For a couple of years. You want at least a year to pass between the last sprout and the next step. Keep in mind it can resprout from rhizomes over ten years later.
16. Resume your life (sort of). Get some of the fancy-pants landscape fabric – the stuff that lasts 25 years with the rubberized backing – and replace the tarp. Cut holes to plant stuff through. Don’t spend a lot of money on plants for this area, ‘cuz you may have to move them. Keep an eye out for scary leaves. Gardening will never be the same, but your alert level can step down a bit.

I am aware that some of these steps may be superfluous or not supported by research (particularly the bits about hating), but hey: success.

If you face knotweed, good luck, soldier!

Somewhat related: Note that Round-Up is not necessarily the be-all and end-all in herbicides. Other products contain similar levels of glyphosate. When buying glyphosate products of any brand, you may want to select ones that do NOT claim to show overnight results. Glyphosate works by spreading through the plant from the leaves. If the leaves are shriveled by an extra ingredient added to reward a lack of patience, the product is not going to work as well. Minus five points for consumerism.