Wild Cousins That Aren’t Welcome

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You may notice thriving large clumps of greenery with triangle shaped leaves and reddish stems by the roadside. Polygonum cuspidatum, otherwise known as Asiatic knotweed, Mexican bamboo, False bamboo or a number of other names. Those depend on if you’re a fan or not. These plants with striking red stems and almost chartreuse green leaves belong to an herbaceous perennial from the buckwheat family that has made tremendous headway in NC and surrounding states. While it’s not a bamboo, the hollow, jointed stems and dense underground growth habit can make you think they’re kin. There are even a couple of attractive variegated cultivars such as Polygonum cuspidatum ‘spectabile’; Polygonum cuspidatum ‘Variegata’ and Polygonum cuspidatum ‘Freckles’, some of which are said to be clumping, not running. Seeing is believing.

Polygonum cuspidatum

Polygonum cuspidatum (syn. Fallopia japonica, Polygonum japonicum) [Japanese Knotweed, Mexican Bamboo] 

This species version of this plant is impressive. And Invasive. Its trek here started like many others, as an ornamental plant introduced to the US in the late 1800s, just like kudzu. Its creeping rhizome root structure can damage foundations, roads, walls and other structures. A very small piece of rhizome that is moved to another site will give rise to a new plant. While it can form dense clumps on a wide range of site conditions, in areas near the edges of rivers, streams, lakes, and other water bodies it can be a real issue where the logjam of dead stems can cause flooding.

Knotweed stems emerge in late-March to mid-April, depending on soil temperatures, and begin a burst of rapid growth. Roadside knotweed can already be tall enough to cause sight distance problems before the end of April, and the plants are almost full height by the end of May. Flowering usually occurs in July, and the seeds mature in August and September.

If you have it, and don’t want it then there are some options, depending on how much of the plant is on your site. If you’ve just spotted it encroaching on your landscape and it’s a small population, or it’s come into an environmentally sensitive area where herbicides can’t be used then back muscle, a sharp digging tool, fortitude and endurance are required to remove the entire plant including ALL roots and runners. Just like Bamboo and Kudzu removal tactics, being careful not to spread rhizome fragments. Anything left behind will potentially resprout.

Mexican Bamboo

Polygonum cuspidatum, Japanese Knotweed, Mexican Bamboo

Repeated cutting close to the ground several times a year over several years is another method. Cutting stems repeatedly over time will deplete the reserves in the rhizomes. It’s a battle between your energy and the underground plant part’s energy to see who gives up first. But before you think you can just “spray away the problem” with a once-and-you’re-done herbicide treatment, think again. Both mechanical and herbicidal control methods require continued treatment to prevent reestablishment of this tough green fighter. Timing, as they say is everything. Glyphosate applied in spring or early summer my stunt or yellow the plant, but knotweed usually grows out of it. Late summer or early fall chemical treatments are much more effective in preventing regrowth in the following year. The entire stand of the plant must be covered. Otherwise, it re-invades from creeping rootstocks from untreated areas.

Perhaps you can learn to like it or appreciate the physical exercise you can get from battling this gardening challenge. Either way, it’s a learning experience and good example of why “know before you grow” is a really good idea. Pretty is a pretty does.