How cant I control knotweed? My neighbour has it and it is coming up on my side of the fence. I have tried cutting it to the ground and covering it with a tarp, but it shoots up elsewhere.
Japanese Knotweed, Polygonum cuspidatum, or Fallopia japonica is an aggressive semi woody perennial introduced to Canada in the 1800’s as an ornamental plant.
Japanese Knotweed spreads via huge underground roots (rhizomes), that can grow 2 metres deep and 15 metres horizontally away from the above ground clump of stems. These factors have to be taken into consideration when attempting to control it. In Ontario, the OIPC (Ontario Invasive Plant Council) recommend digging up plants as soon as they sprout in the spring, being careful to dig up the entire root (rhizome). Young plants can be controlled by removal, but established clumps require an ongoing effort. Start by cutting the plant down to ground level in the spring and covering the stumps in a thick, pliable, light blocking tarp. This must remain in place until the knotweed is controlled. Since this can take several years, it is not a very attractive option for the home garden. To improve the aesthetic of your garden, you may choose to cover the tarp with mulch. If you have a slight slope you can add a layer of soil, about 6 inches thick, and plant with ground covers suited to your light conditions, especially species that spread by runners (stolons) or have shallow root systems. If your tarp is on a medium slope, you may need to retain the soil with heavy stones/pavers/logs, being careful not to tear through the fabric. If your site has a steep slope, don’t add anything on top of your tarp. If you wish to grow larger plants, you can build/purchase a planter(s) and place on top of the tarp. Fill planters with soil and plant as usual. For more details see: https://www.ontarioinvasiveplants.ca/files/OIPC_BMP_Presentation_Japanese_KnotweedFinal2014.pdf
Another method to deter the weed, known as soil solarization, is cutting shoots to the ground in spring, covering in thick black plastic and allowing the heat accumulation under the plastic, over the hot summer months, to kill off a reasonable percentage of the rhizomes. This works best in areas that get direct sunlight and the plastic needs to remain in place for 6 to 8 weeks without anything on top of it. You may need to remove the plastic, check for shoots and cut the stalks to the ground, then replace the plastic, 2-3 times over the summer for best results. This technique may need to be repeated for several summers until the weed is under control. For more information: https://ontariowildflowers.com/mondaygarden/article.php?id=110
Since your neighbour has Japanese Knotweed, a root barrier is also recommended to control the spread of the deep knotweed rhizomes. This would require digging a deep trench, to 2 metres, along the fence line and the installation of a vertical root barrier. You can hire a professional to do this if it is beyond your scope.
Once you have Japanese Knotweed under control the OIPC recommends planting out the area, or covering it in a thick layer of mulch, to reduce the chance of other invasive species moving in. Good luck with controlling this beast!