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Japanese Knotweed

Japanese Knotweed

Crossing Cambridge, and a few neighbors along with it.

This beautiful and prolific plant is one of the most aggressive invasive species in the world. Just as purple loosestrife now blankets marshes across Massachusetts, out-competing native species, Japanese knotweed is beginning to blanket Cambridge. Just take a walk around the block and you’ll find it growing between houses, in alleyways, popping up in manicured gardens, in public parks, and in patches along the Charles River. And Japanese knotweed uses its good looks to great advantage. If you don’t know it’s invasive, you probably love it.

Good people sit on both sides of the fence. Some think it’s lovely and see its vigor as simply survival of the fittest, while others see it as a threat to our local ecology, at times pitting neighbor against neighbor.

Ironically, the plant was introduced by Frederick Law Olmstead, designer of the “Emerald Necklace,” the series of parks connecting Boston and Brookline. In Olmstead’s day, exotic plants were freely used as ornamental, and the larger consequences were unknown.

What’s the big deal?
Like many imported plants, knotweed came here without its insect predators—the bugs that keep its population in check. Freed from evolutionary checks and balances, it easily monopolizes green space and elbows out plants that have spent hundreds of years evolving here, building the interdependence and diversity that is so necessary for ecological stability.

Japanese knotweed is fast growing and mainly spreads by the roots, which can reach 23 feet horizontally and almost 10 feet deep. Digging up knotweed inevitably leaves root fragments behind, and pieces just one inch long can produce a new stand, making it almost impossible to displace. While the fragmented urban environment can sometimes prevent the roots from spreading too far, its seeds remain viable for years, allowing knotweed to hop across roads and concrete barriers, setting up shop just about anywhere.

What can be done?
The MA Department of Transportation and Arlington’s Friends of Great Meadows tried two very different strategies to eliminate Japanese knotweed and restore native species. In Arlington they relied on scores of volunteers willing to spend time cutting and digging repeatedly throughout the growing season for several years. The MA DOT, lacking those human resources, pursued a plan of cutting and applying herbicide to remove roadside knotweed.

In Cambridge, the Public Works Department lacks both the human and financial resources to tackle invasive plants, and the city hasn’t used chemicals on plants since the 1980’s. This leaves it to residents to identify and tackle any knotweed on their own property, using whatever method appeals to them.

And what if your neighbor loves Japanese knotweed, or doesn’t have the time or energy to deal with it? The truth is, this hardy species will outlive all of us, so the best advice is to just do your part and love thy neighbor.