The Cost of Japanese Knotweed

This is the Japanese Knotweed outside of my apartment. Impressive, I know, but not for the right reasons. While it has its own sort of charm, the reality is that this plant is highly vexatious. Japanese Knotweed, also known as Devil's Bamboo or Demon Weed, is a woody-stemmed herbaceous perennial rhizomatous plant native to eastern Asia. It was introduced to North America as a horticulture plant in the late 19th century and has been a menace ever since. In early spring, I noticed these bamboo impersonators poking up, only inches high, and each week they slowly encroached toward the house. According to the NH Department of Agriculture, the stems can grow up to eight centimeters per day, so no wonder I noticed its rapid growth! Their relentless nature makes it notoriously difficult to remove and creates many structural problems for roadways and even homes. The National Park Service claims it to be one of the most invasive plants in the world, growing 10 ft. in just six weeks and up to 15 ft. or more in one growing season. Their roots can grow 10 feet deep and up to 70 feet horizontally from the originating plant. Its complex root system allows it to creep through cracks in concrete and weak areas in walls, causing structural damage to infrastructure, foundations and even septic systems — eek! But the threat of knotweed is not only structural. Their dense thickets of stems and persistent growth allow it to quickly take over and smother other plant life — as noticeable in my "yard". Their colonies block sunlight, while their rhizomes release biochemicals that suppress the growth and germination of other plants. This then lends to a domino effect leading other native species like birds and insects to vacate the area as well. Also back in spring, my landlord informed us that he would be removing it; it's now nearly mid-June and I don't have high hopes for that happening. I will not be doing this myself, but I was interested to see what it would entail to remove the monstrous vegetation. It turns out there are two options, and both are fairly involved. Smothering: The first and most natural way to remove the plants is to smother them. In accordance with the NH Department of Agriculture, these are the steps one must take to successfully smother this vegetation villain: Allow the knotweed to grow in the spring without attempting to control it. Cut the knotweed at the base around the first week in June to help weaken the root system. Pile the cut stems on an impervious surface such as a tarp, plastic or pavement to let them dry out. Once the stems have browned, they are safe to compost. Apply a layer of mulch, grass clippings or other cushiony material over the cut stems to prevent them from puncturing the plastic. Cover the entire area with a large, heavy-duty, dark-colored plastic, tarp or weed fabric. (7 millimeters thick is recommended.) Cover the ground surface at least 5-10 feet beyond the knotweed in all directions. If multiple pieces of plastic are used, be sure to overlap the seams by about two feet. Weight the top of the plastic and seal the edges with rocks, sticks, soil, sand, mulch, etc. Do not puncture the plastic! Any holes must be patched to prevent the plant's survival. After five years, the covering material can be removed, and the area replanted. Foliar Herbicide Treatment: A faster, but inorganic way to manage a knotweed colony is with herbicide. While this method may be faster, herbicides are known to be toxic to humans and the environment. Follow smothering steps 1 through 3. After cutting, allow the knotweed to resprout and do not do any management until after flowering — usually in early to mid-September. Just after flowering, apply a foliar spray herbicide using a 5% solution of a glyphosate-based product. Apply to thoroughly wet all foliage, but not to the point of runoff. Always read and follow the product label. Foraging Japanese Knotweed: This is less of a management practice and more of an interesting way to take advantage of this species. Apparently, knotweed is also called "donkey rhubarb" and has a close resemblance to asparagus with a somewhat sour taste like rhubarb. You'll have to wait until next year, but when the shoots are between four to six inches in the spring, harvest them at the base as you would asparagus. They can be eaten raw or cooked, used in the same way you would rhubarb, or eaten as you would asparagus! As Japanese Knotweed continues to pose a biological threat across the world, research is underway with the USDA to evaluate certain Japanese insects that may contribute to its control; but, as we've seen with knotweed and countless other species, that may open its own can of worms.