Unfortunately just cutting and destroying the stems of the plant does not mean the entire plant will die. The roots will live on and the next growing season sprout leaving you back at square one.
Sprayed herbicides applied during the late summer on dry, windless days are highly effective. Weather conditions need to be suitable otherwise the herbicide will be washed or blown off the plant. The herbicide needs to stay on the surface of the plant for a sufficient amount of time to penetrate into the plant. Coverage is critical to success, you want to cover as much of the exposed surfaces of the plant to facilitate a quicker death. Visible results of herbicides can range from a week to months, and follow up applications may be required, particularly on dense stands where access is poor.
Herbicide can also be applied via a weed wiper or stem injected. Both these treatments are a highly targeted approach using a concentrated dose of herbicide, whereas a sprayer application is diluted with water
Systemic herbicides – (glyphosate, etc) once absorbed will translocate through the plant either from the leaves down to the roots or absorbed from the soil through the roots to all parts of the plant. Killing the leaves, shoots and roots.
Residual Herbicides – (Picloram, etc) are effective at restricting growth but problematic for no-target vegetation and sensitive receptors. They can also restrict later movement of soils.
Excavation is a quicker approach than herbicide applications. This option is more expensive than herbicide and likely to be used on large scale clearances needing a quick solution. Once excavated the choice is to either deal with the knotweed on-site or dispose at an off-site licensed disposal facility. If burning the excavated material, the controlled burn should be completed on the same land where the knotweed was removed. The resulting ash will need to be spread back over the area where the knotweed originated from.
Use of insect predators, physical barrier systems which damage the plant as it grows and on-site processing systems using intensive treatment of soils are currently being explored but have yet to be officially adopted into codes of practice.
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