For a mechanical option, cutting growth should occur four times each summer, say the first week of each month from June through September. One cutting a season is definitely not enough. Think of that huge root, which can be 30 feet long, as the pantry and when you cut the top you only take a little food out of the pantry. The plant will vigorously resprout but it takes stored material out of the pantry each time it has to resprout, so more cutting is GOOD. There is a scientific paper that came out in Natural Areas Management that studied different cutting regimes and 4 times per season was the best.

Another option is to cut the stems next summer around the first week of August when the plant is starting to develp its flowers, them drip herbicide into the cut stem. The herbicide has the be concentrated, so around 20 to 25% glyphosate (the active ingredient) is the recommended dosage. A plastic squeeze bottle with a nozzle that will drip or allow a thin steady stream of the liquid is a good choice. A homeowner doesn't have to be a state-certified pesticide applicator to apply on their own property but would need certification to apply on someone else's property.

A third option is a big smothering operation. I've seen this work on a bank above someone's back yard. They left a pile of sandwiched materials on the knotweek patch for two years and it definitely killed it. One key is to keep adding materials as they breakdown (for example, cardboard or plastic sheeting) and I think using a durable surface like old tin roofing above light-blocking layers like cardboard is the best bet.

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