Japanese Knotweed, Sinister Invader
Originally planted as a hedge plant due to its hardiness and ability to fill in areas quickly, Japanese Knotweed takes it’s job a little too seriously. Having only been introduced to North America in the late 19th century, this plant continues is march across the US, making its mark here in Monroe County, Indiana.
Japanese knotweed, Reynoutria japonica, is considered an invasive plant. What does that mean? Lets examine the plant’s habits to get to our answer.
Have you, dear gardener, ever battled a weed in your garden that just won’t go away? Have you had a weed take over an area, choking out other plants that you’d really rather prefer to keep? Usually you can pull it up by the roots. Does that work on japanese knotweed? Unfortunately no. It’s root system spreads by rhizomes underground sending off shoots to other locations. They can go several feet deep, and many yards away. Pulling the roots does not ensure that every bit of the plant has been pulled up, and it will simply sprout again.
Not to mention, you cannot compost the pieces you pull out. You would simply be giving it ideal growing/sprouting conditions by throwing it in a compost pile.
“No matter,” you may think, “I’ll just mow it down.” But wait, japanese knotweed actually likes that. It’s small cut up pieces can easily root themselves from just a 1/2 inch piece, potentially expanding your patch exponentially. And watch out if you’re anywhere near water. Those small pieces sprout and thrive on moist creek beds, and the small cut up parts from mowing are an ample opportunity for this plant to travel downstream and start a whole new colony.
“Well,” you may think, “let’s cover it with black plastic and try to smother it out.” Unfortunately, this doesn’t work either. The plant will simply send out shoots to another location, working its way through nearly everything in its path, including foundation walls. Or, it may ignore your cover completely and just push it out of the way. Japanese knotweed can grow over 3 inches EVERY DAY.
“Ok, enough already! I’ll try to poison it!” …This may be the most effective method, but it’s hardly one you can depend on in this situation. Herbicides often just stunt the growth without killing it completely, so reapplication must happen over and over again, with every tiny sprout, year after year.
At this point you may be frustrated and decide to just dig it all up. But hold on, you can’t just put the soil somewhere else because then you’ll just be starting another patch from any pieces contained in the soil. You must dig it up, then spread the soil out on concrete or asphalt, or bag it up in black plastic bags, until all remnants are completely dead. Honestly, at this point, I’m not even sure I’d trust it then!
The federal definition of invasive species is a non-native species whose introduction causes or may cause economic or environmental harm or harm to human health. Looking at the destruction that japanese knotweed can cause to native areas, and even building foundations and infrastructure such as roadways and sidewalks, the reason for it’s invasive label is clear.
Let’s examine a case study of a japanese knotweed patch that some friends of mine inherited in their yard when they bought their house 5 years ago. The first year they left it alone, not knowing what it was. It got pretty tall, and was an attractive plant that had pretty white flowers in the summer. But after getting it identified, they became concerned and tried to kill it. Here’s what happened:
Five years on, my friend’s japanese knotweed patch is still sprouting up and they’ve been unable to eradicate it completely. I asked her what they’d done to try to kill the patch.
Current state of my friend’s japanese knotweed patch after numerous unsuccessful attempts to kill it.
“We tried salt; we poured a 50 lb bag of salt on it. We tried the stuff called Kills All on it. We have burnt it every single season. We’ve tried digging it up. We’ve also used Roundup on it several times by pouring Roundup into the tube. We cut the stalks and poured it in, then we burned the stalks. We covered it up so that it doesn’t get sunlight and it just pushes the pond liners up. We’ve been working on it for 4 years.”
Distressingly, they are now seeing new shoots about 20 yards away. They have no idea if it was their attempt at mowing that spread the patch, or if the rhizomes have started sending off shoots.
It’s hard to express just how difficult this plant is to control and eradicate. Any silver linings, like the fact that it’s quite nutritious and the young stalks can be harvested like asparagus and cooked, is tarnished by its extremely aggressive and destructive nature. It truly lives up to its “invasive’ label.
One can understand how this plant can easily take over an area.
We aren’t the only Master Gardener organization trying to battle and inform the public about this nuisance plant. Check out great MG articles from Virginia, and New Hampshire. A thorough article on eradication comes from Cornell.
Submitted by Eryn Cusack
