Japanese Knotweed is One Tough (But Edible) Weed
Helpful Products from Gardens Alive!
Heavy Weight Weed Barrier Mat
Weed-Aside™ Herbicidal Soap
Iron X!™ Selective Weed Killer for Lawns
Q. I've got a quarter acre of Japanese knotweed. Is there any good use for the ten foot tall stalks besides making tomato trellises? Digging it up by the roots is good exercise, but I fear I'll never get anything else done, as they just keep popping up. How do I assure total elimination?
---Kaki in Tunkhannock, PA
A. Move to one of the few remaining continents where this plant has not yet appeared. Or another planet. And don't take any garden soil with you. In other words, forget 'total elimination'; the most you can hope for with this monster is temporary control. (But as the Duct Tape Council reminds us, "all solutions are temporary.")
Q. I have a bad case of Japanese knotweed. Any suggestions on eradication other than explosives?
---Scott in Stafford Va.
A. Sorry to negate such a wonderful Elmer Fudd fantasy, but explosives would just spread the rhizomes around faster.
Q. We have Japanese Knotweed taking over the stream banks that run through our property. It seems to like wet areas. It has been suggested to cut the stalks and paint the cut ends with Round-Up, which I really don't want to do. Can you suggest another solution? This weed is tough and just keeps coming back year after year.
---Keith in Bryn Mawr, PA
A. And it will keep doing so. Every article on this invasive giant carries the same warning; once the plant gets established, it will take years to remove, whether the tactic is repeated cutting or the use of toxic chemical herbicides. So let's try and do this without killing millions of frogs and toads with that Roundup.
Also known as monkeyweed, donkey rhubarb and Japanese, American, and Mexican bamboo (although it is neither a bamboo nor a rhubarb), this escaped Asian ornamental especially likes to colonize the areas along stream banks, where removal becomes ecologically as well as physically problematic, as it does do a good job of holding the soil in place and preventing erosion.
If it appeared next to my stream, I'd first try spraying the plants repeatedly with salt water; a tactic reported as having good results in Wikipedia's article on the topic, which also has good photos of the monster. Spraying towards land and using a spreader to keep the salt on the plants should hopefully minimize the saline effects on the stream. (And anything's better than using Roundup in an aquatic environment.) Whatever you do, don't chop it down and let the pieces fall into the water; the experts warn that those pieces will root downstream.
Now, as with 'running bamboo', repeated cutting of the new growth will weaken the plant. It's the rope-a-dope method of invasive eradication; you force the roots to use energy to grow shoots, then cut them down before the leaves can photosynthesize. I'm thinking the best way to achieve this would be to first reduce the size of tall stalks with pruners or a machete, pick up all the pieces, and then use a bagging lawnmower with a very sharp blade to cut the plants down low. Carefully empty the mower bag into a super thick trash bag afterwards and let it sit in the sun for a week before you put it out at the curb for pickup.
Do this 'cook and trash' routine with any cut portions; the ability of this plant to re-root is legendary. Don't even think about putting it in your compost pile!
Repeat this cutting weekly for as long as you can and then try a tactic suggested by landscaper David Beaulieu at "About dot com" and try and smother the patch by covering it with overlapping pieces of old carpeting and heavy tarps. He warns that new sprouts can punch their way through solid objects (they've been known to impale concrete and undermine foundations), so you need a smothering media that is thick and heavy but that also has some 'give'. He says to keep a sharp eye on the area and then just step on any new shoots you find trying to poke their way through. At the very least, it should make satisfying sounds…
And, no, it's not ALL bad.
The new shoots are edible, and described as tasting like a combination of rhubarb, bamboo shoots and asparagus. Most sources suggest boiling or soaking them first; and here's a link to master forager Steve "Wildman" Brill's detailed suggestions for consuming this abundant resource.
And the roots contain extremely large amounts of resveratrol, the anti-aging, anti-dementia nutrient famously found in red wine. In his excellent book, "Wild Urban Plants of the Northeast", Harvard University researcher Peter Del Tredici says that knotweed root contains such high concentrations of resveratrol that it's become the main commercial source of this powerful phytonutrient. And the leaves of a related species, giant knotweed, are the active ingredient in "Regalia", a new organic control for a number of different plant diseases.
So maybe consider farming that quarter-acre.
Otherwise, be ready to recognize this pest as soon as it shows up—which is often in a load of 'fill dirt' that held tiny traces of the rhizomes. The new shoots look a bit like bamboo or asparagus and are a chartreuse/red. The leaves are four inches long, triangular and pointy. Pretty greenish-white flowers appear in late summer, followed by 'winged' seed pods with lots of viable little dark seeds inside. The plants turn brown over winter, can reach a height of ten feet, and the rhizomes can go an astounding nine feet deep.
God help us all.