Japanese Knotweed

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Polygonum cuspidatum or Fallopia japonica

Find it

Japanese Knotweed is an incredibly vigorous plant that forms dense mounds six feet high or more. Once established, it is extraordinarily difficult to remove. Resembling a fleshy bamboo, knotweed’s hollow, jointed stems shoot up in the spring, sometimes several inches a day. Stems are light green at first and darken through the summer, when they become capped with large, floppy seed tassels. Knotweed is often found in wet areas, such as along brooks, and in open areas that have been disturbed by humans.

Why it’s a problem

Native to Japan, knotweed now plagues most European countries and much of North America. It was introduced as an ornamental and for erosion control about 150 years ago, but it is a prime threat to native species and plant diversity. It spreads primarily by rhizomes (underground horizontal stems) that can travel many feet. Rhizomes broken off by ice or water can kick off new colonies far downstream from the parent. The immense strength of its root structure allows it to penetrate asphalt and cracks in concrete and alter landscapes, ultimately excluding other plant life and reducing the value of afflicted properties.

What to do

Eat it! The young shoots can be prepared like rhubarb or asparagus. Older stems can be made into jam or added to stir-fries or baked goods. Its roots are reportedly high in the claimed anti-aging chemical resveratrol, and there are reports of success treating Lyme disease with root tinctures. Don’t plant knotweed in order to eat it—there’s plenty in the wild to harvest.

Remove it. It’s possible to dig up and dispose of (not compost) the complete root structure of small, isolated knotweed plants. But mature dense clumps, common in Granby, are extremely difficult to get under control. As with many invasive plants, multiple approaches over time are usually needed and may require the careful use of herbicides. Non-chemical methods can greatly weaken dense knotweed concentrations. Use a heavy dark tarp or black plastic to completely cover the knotweed patch, weight it down with rocks, and leave it in place for a year or more, allowing the intense summer sun to heat up and cook the roots. A propane torch can also be used to cook the roots. Repeated cutting throughout the season works, but only if all cut pieces are gathered and disposed of.

For more information on invasive plants, events and the NOT WANTED campaign, drop us a note on the Granby Conservation Commission webpage.