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Japanese knotweed, also known as Fallopia japonica or Polygonum cuspidatum, is an invasive plant species that is incredibly difficult to remove; it is often found on forest edges, along roadways, and along waterways, as well as in yards and gardens. Japanese knotweed removal is generally a long and extensive process requiring a number of different options, including cutting the plants back, spraying and/or injecting them with weed killer, digging up the root systems, and then repeating the process the next spring, when new shoots of the plant invariably appear. Most regions encourage Japanese knotweed removal, however, because the plant spreads rapidly through its underground root systems, and quickly smothers any other plants in the vicinity.
For smaller areas of Japanese knotweed, experts occasionally recommend covering the area with a tarp to smother the growth of the plants. This can be a good first step, but it can take some time, as the plants are quite resilient. Choosing to do this will mean regularly walking out to the area to step down on the tarp to kill any shoots that are coming up, and it is also only the first step in the process, as the plants will still need to be dug up and removed anyway. Spraying the plants with weed killer is often a better option.
The weed killer most often recommended for Japanese knotweed removal is glyphosate, which is available in many name-brand variations. It should be sprayed on the foliage of the plant while it is flowering, which typically occurs in the late summer or early fall. Some people, however, choose to simply stunt the growth of Japanese knotweed by spraying the weed killer at regular intervals throughout the year, though this will not lead to true Japanese knotweed removal. For that to take place, it will be necessary to cut the plants back; some people will then directly inject glysophate into the stumps of the plant, or some choose to just begin digging up the roots.
The roots can go quite deeply, a few feet into the ground and out to each side, so it is necessary to be very thorough. It is extremely likely, if not a virtual guarantee, that this process will need to be repeated in the spring, because it is very difficult to remove all the roots. It is unfortunate that Japanese knotweed was deliberately introduced into North American ecosystems at all, especially so because Japanese knotweed removal is such a lengthy and difficult process. With persistence, however, it will be possible to greatly reduce the numbers of this invasive plant. Be sure to also consider the available methods for plant disposal, and learn about any laws regulating it.