Controlling a large stand of invasive knotweed can be a daunting task. We just finished this document: Green Shoots Guidance on Controlling a Large Stand of Knotweed. This
document complements our video: Knotweed Control: 3 Simple Steps for the Non-Professional which is intended for those controlling a small knotweed colony.
In our new piece, we synthesize much of the latest research on and our experience with controlling knotweed. The approach solves several problems confronted in controlling a large infestation: First, how to do you ensure coverage of the entire stand when applying herbicide? Second, how do you accomplish the first objective without introducing excessive amounts of herbicide into the environment? Third, how do deal with the few surviving knotweed crowns that are often difficult to kill.
In short, the approach is to make a broadcast application of herbicide to the knotweed colony in the first year. This is done in such a way as to maximize herbicide effectiveness and minimize harm to neighboring desirable species. Thereafter, we use a targeted approach that combines: spot treatments with herbicides; mechanical control – cutting or digging to remove the surviving knotweed plants; and the introduction of competition from native plants.
The three most common species of invasive knotweed in North America are: Japanese knotweed (Fallopia japonica or Polygonum cuspidatum); Sakhalin knotweed (Fallopia sachalinensis); Bohemian knotweed (Fallopia×bohemica).