Knotweed in flower
You’ve almost certainly seen it in dense thickets by the side of the road or pushing up through cracks in the concrete—a thick, red stem possessed by a demonic strength. Japanese knotweed is a plant that defines the term invasive. Grub it out when it first appears on your property and you may keep it at bay. Allow it to establish itself over a season or two, and you will be living with Polygonum cuspidatum forever.
First a description: Knotweed is a perennial shrub that can grow to over ten feet tall. Its leaves are heart-shaped and about six inches long. Its stems are hollow and snap easily. At this time of year knotweed has achieved its full height and is in flower, covered in surprisingly delicate white blossoms that grow in sprays at the end of stems. Shortly the blossoms will give way to the tiny winged fruit that carry the seeds. In the winter the foliage dies back and the skeletal stems rattle menacingly in the wind.
Like so many plants that have outstayed their welcome, knotweed was introduced via Europe to the United States for ornamental purposes in the nineteenth century. Because its roots create a mass of earth-grabbing vegetation, it was also valued for its ability to prevent erosion.
As tasty as rhubarb?
Before long, the story turned sour. Unopposed by the natural enemies that kept it in check in Asia, knotweed was soon displacing native species and choking whole ecosystems. Able to thrive in damp and drought, in salty soil and rock-strewn spare lots, in extreme heat and cold, knotweed was a brilliant opportunist.
Part of knotweed’s success lies underground. Like its fellow invasive and partner-in-crime black swallow-wort, knotweed spreads by means of rhizomes that can generate a new plant from a mere scrap of root matter. If you find it in your garden, catch it young and dig it all out. Driving a stake through its heart may be tempting, but bagging knotweed in plastic is the recommended method of disposal.
I found some this spring pushing up between the foundations of my house and the concrete driveway. Unable to dig and unwilling to use a conventional herbicide, I fed it with a strong vinegar solution for a couple of days. To my surprise, the knotweed withered and died. I’d been expecting more of a fight.
A knotweed wood in Watertown
Only recently have I discovered that knotweed is good to eat—like rhubarb people say. Harvest it when under two feet tall, peel the stems, and eat it raw or cook with it. The New England Wild Flower Society, which has an understandable loathing for the plant, publishes a page of knotweed recipes, including Go Anywhere Knotweed Squares!
And for those who want the complete Knotweed Experience, I’d recommend a wander through a plantation of the stuff by the Charles River in Watertown. Park your car in that little lot by the Arsenal Street Bridge on the Allston side of the river. Cross the bridge on foot and plunge into the shrubbery at the first opportunity. You’ll find yourself in an empire of vegetable evil, a mind-boggling jungle of knotweed that arches over you, excluding all other species and most of the light.
If you didn’t already, you’ll understand what invasive means.
For more info:
a "Least Wanted" poster from the National Parks Service
a video about knotweed from the USDA
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