It seems like Japanese knotweed — a noxious invasive that creates thickets so dense no other plant life can survive — is everywhere this year. Actually, all knotweed is everywhere.
Nancy Olmstead, invasive plant biologist with the Maine Department of Agriculture, Conservation and Forestry, told me in a telephone interview recently that Maine actually has three types of knotweed.
Japanese knotweed (Fallopia japonica) is the most widespread and can be found throughout the state. It grows about 10 feet tall, producing billowy white flowers at the top of the plant. Those flowers turn into seeds. Most —but not all — of those seeds are sterile. That doesn’t mean you can ignore them.
“If only a fraction of it is fertile, but each plant carries hundreds and hundreds of seeds, it’s going to be a problem,” Olmstead said.
To make things worse, Maine also has giant knotweed (Fallopia sachalinensis), which grows up to 17 feet tall and has large leaves. The seeds of giant knotweed are always fertile. When it crossbreeds with Japanese knotweed, it produces the third type of knotweed, namely Bohemian knotweed (Fallopia x bohemia), which also can be fertile.
So, yes, knotweed can be spread by seed. But that is not the how it spreads most often or most easily.
Even if the plant was pulled or cut, if any part of its stem containing a node or joint, or a rhizome, gets to bare soil, it can create a new plant.
Much of the knotweed in Maine gets moved around when contractors or others dig out soil in one location and, either not noticing the plant or unaware of how harmful it can be, move it to a job site miles away. That new location ends up with knotweed.
Knotweed prefers moist soil, although as an invasive, it can thrive pretty much anywhere. It grows especially easily along riverbanks and whenever there is high water, floats downstream to areas where it didn’t previously grow. Hurricane Irene, in 2011, spread knotweed widely in New England, including to some areas where progress had been made toward eradicating it, Olmstead said. Not only did high waters loosen stream-side plants and move them downstream, but also construction crews repairing washed-out roads and bridges unknowingly let fertile parts of knotweed plants hitch a ride with them. Case in point, knotweed now grows along the Saco River upstream to Crawford Notch, southwest of Mount Washington in New Hampshire, probably because of repairs after Irene, Olmstead said.
The other way knotweed spreads, more slowly but more relentlessly, is underground. If a neighbor has knotweed near your property line, the rhizomes (which are part of the root system) will spread underground onto your property, and there is almost nothing you can do to stop it.
There are no fast and easy ways to get rid of knotweed.
I mentioned to Olmstead that Eric Sideman of the Maine Organic Farmers and Gardeners Association told me if you have an area that can be mowed, and you mow the plot weekly, eventually the knotweed will disappear.
True, she said — with a lot of exceptions. You would not want to mow near wetlands or protected areas, because bits of the knotweed plants could get blown into those areas and root. Also, you would have to clean the mower thoroughly after mowing the knotweed before mowing any knotweed-free parts of your property. And you definitely wouldn’t want to take that mower to do work on someone else’s property.
Another method for eradicating knotweed is to smother it. Olmstead described the process and also sent a link to a New Hampshire Department of Agriculture publication that provides additional details. It sounds laborious and it ain’t fast.
First, cut the knotweed, putting the material on an impervious surface so it will dry out, turn brown and no longer be a threat. Cover the cut knotweed stumps with three to four inches of mulch, sand, mud or other material. Cover that with thick, dark plastic (tarps or landscape fabric) going three or more feet beyond the edge of the knotweed patch and, if more than one piece of plastic is used, overlapping pieces by two feet. Weigh the plastic down with rocks, logs or other materials. To make it look better, you can add a heavy covering of mulch, as well.
Then wait five (according to the publication) or preferably seven (according to Olmstead) years to see if the knotweed is actually dead. Yeah, good luck with that.
Chemical treatment, with glysophate or other material, also can work – but some communities have banned it. Professionals have to be licensed to apply the chemicals. If homeowners apply it on their own property, Olmstead cautioned that they must read the instructions (long and in tiny type) and follow them to the letter. And guess what? Glysophate doesn’t always eradicate the plants.
The fact that knotweed is hard to kill and spread easily is part of the reason it is an evil plant. It is not native, so does not support local wildlife, and it forms very dense thickets, which prevent native plants – that do support native wildlife, birds and insects – from growing.
So, if knotweed is so awful, how did it get here? As with many other problem plants, someone thought the prolific flowers and supple stems were pretty. As far back as in 1825, it was brought from East Asia to Kew Gardens, a famous botanical garden in England. From there, sometime before 1875, it made its way across the Atlantic to the United States, and boy did it thrive here.
There is no biocontrol for knotweed, though biologists around the country are trying to get the federal government to approve tests on Aphalara itadori, a sap-sucking insect called a psyllid that eats knotweed. Testing on that will take years, Olmstead said.
So, there is hope, but I think I won’t live to see knotweed eradicated.
Tom Atwell has been writing the Maine Gardener column since 2004. He is a freelance writer gardening in Cape Elizabeth and can be contacted at 767-2297 or at:
tomatwell@me.com
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