Japanese Knotweed

Japanese Knotweed

Lessons from Harvard and MIT

In a previous post, I described the “beauty and the beast” nature of Japanese knotweed, the city’s limited resources to manage it, and a couple of references to state and local efforts to control it. That post generated a lot of attention, so I thought it would be interesting to find out what our esteemed institutional neighbors with beautiful grounds and huge endowments do, if anything, about it.

It turns out both Harvard and MIT grounds crews have marching orders on knotweed. Walk around either campus and you’d never guess they have a problem with it. Paul Smith of Harvard’s Landscape Services said you’re not likely to see Japanese knotweed in Harvard’s manicured areas because they constantly pull up new shoots as they see them. “It's the border areas--the backs of buildings or narrow strips between fences and buildings—that’s where knotweed really gets traction.”

The same is true in Riverside, and probably all across Cambridge. The neglected narrow strips between tightly packed triple-deckers; rental properties with offsite landlords; grassy strips behind parking lots; these are the major hotspots for knotweed.

Between fences
Japanese knotweed thriving between fences

Through conversations with crews at Harvard and MIT, I came up with some best practices that can be used to halt its spread across Cambridge. And surprisingly, also came up with a new best practice to offer them as well.

Step 1. Pull shoots
Pulling up the first signs of growth is the best way to prevent new stands of knotweed. Whenever Harvard and MIT groundskeepers see plants in this stage, they yank it up. Norman Magnuson, Grounds Services Manager at MIT recommends digging up the entire root at this point, before it takes hold and you’re stuck with a larger battle. He also cautions against shaking the dirt off, since the roots break easily and you’ll most likely wind up right back where you started.

Young knotweed
Early stage Japanese knotweed

Step 2. Hit with herbicide, then cut
For larger plants, Harvard sprays knotweed with BurnOut, a citric acid-based organic herbicide. This knocks the plant back within a day or two, and is supposed to be effective on roots as well. Once the plant is dead, it’s cut down to the ground and removed. For larger, well-established stands, this process is repeated several times during the growing season until successful.

Since the stalk of mature knotweed is hollow like bamboo, one Riverside resident said she cuts the plant down to the ground and pours RoundUp into the stalk to directly access the roots. While effective, RoundUp is not on Harvard’s list of approved chemicals for organic landscaping, but they suggested trying the same with BurnOut.

MIT also avoids using RoundUp. Norman Magnuson suggested trying household vinegar on roots as an alternative, or horticultural vinegar, which is more acidic.

Mature stand
Mature stand in full bloom

Step 3. Don’t compost it!
This is where things get really interesting. Composting knotweed is a really bad idea, since the seeds and root fragments often survive the composting process, leading gardeners to unwittingly spread compost laced with knotweed seeds all over their gardens. But both Harvard and MIT said they don’t put yard waste in the trash because state law prohibits it. Both send their yard waste away – Japanese knotweed included – with private yard waste haulers, who turn it into compost.

I checked with Randi Mail, Recycling Director for Cambridge, and she confirmed that the Massachusetts Waste Ban prohibits putting more than 20% of any banned material, including yard waste, in the trash. When I explained the unintended consequence of knotweed winding up in commercial compost, she contacted the Mass DEP and requested a waiver from the city's waste facility, so residents can now put knotweed out with the trash where it belongs.

Better yet, she’s sharing the outcome with her contacts at Harvard and MIT so this could be replicated there as well.

So if we can resist the charms of this aggressive invasive plant and put Japanese knotweed out with the trash instead of into turning it into compost, we just might have a shot at controlling it.

NOTE: For those who want to learn more, the Massachusetts Association of Conservation Commissions and Clark University are sponsoring a conference on managing invasive species, including Japanese knotweed, on November 16 from 8:00 am – 3:45 pm in Worcester.


Interesting story, great photos!
Our local forage expert, David Craft, says knotweed leaves, gathered in spring, can be used as greens -- another way to combat these tough plants.

I've never heard of anyone eating the leaves, but in early spring when new stalks are just coming up, they taste just like rhubarb! Years ago, I did an edible wild plants walk with Russ Cohen of Arlington, and we made strawberry knotweed pie. What better way to deal with an invasive plant than eating it! Here is a great summary of its nutritional value and how to use it: http://www.wildmanstevebrill.com/Plants.Folder/Knotweed.html

Maybe we should organize a Cambridge-wide cook-off next spring! (though sadly, Cambridge soil, particularly where knotweed grows, is not necessarily the best for edibles...)

I love this article. Very helpful. I heard about an invasive weed in Cambridge, and we do have one in my neighborhood, Inman/East, but now I see that it is not the Japanese knotweed. Ours is a small leaf, bright green, and very shiny. It grows in all the places you mention it this story, but also vines around other plants. It is very invasive and hard to pull out of the ground. Do you know what it is? kristina

It's hard to say without seeing a photo. I'd suggest emailing the city Arborist within the Public Works Department, and send a photo if you can. If you can find some on city property, I'm sure he'd come and take a look. His name is David Lefcourt, and his email is dlefcourt@cambridgema.gov. Good luck!