There isn’t going to be a defined eletrical resistance you can tune to in general, I think it is going to largely depend on the situation… how much water is in the plants when you do it, how old the plants are, how intertwined roots are… what kind of soil it is etc… However, any method you would use to apply the electrical current to the Japanese Knotweed would also be almost if not the same kind of setup you could measure the resistance of the Japanese Knotweed with correct?

The idea of sensing the electrical resistance of a Japanese Knotweed plant is really just the same very well established idea used all the time in geology called an Electrical Resistance Survey

https://en.m.wikipedia.org/wiki/Electrical_resistance_survey

Four electrode measurement

In a four electrode measurement we apply a direct current voltage across two electrodes (known as the current electrodes). This creates a current and an associated potential field in the subsurface. We can now measure both the current between the current electrodes and the electrical potential across two other electrodes (the potential electrodes). From the current (in Amperes) and potential (in Volts) we can calculate a so called transfer resistance with Ohm’s law (R=V/I). This transfer resistance is a function of the electrical properties of the subsurface and the electrode geometry.

https://subsurfaceinsights.com/electrical-resistivity-method/

Essentially what I am suggesting is to jam to spikes into a japanese knotweed plant, drive a large current through it and measure the resistance as you would with a multimeter in a sense. In geophysics terms you are looking to measure how much resistance the ground has to the electrical current being induced through it, I imagine it is a reasonable hypothesis that a similar method can be applied to a dense network of near surface rootsystems of japanese knotweed and soil at a much smaller scale.

https://kerstenuk.com/blog/Chemical Free Weed Control/using-electricity-to-kill-japanese-knotweed

The Principle -

The system is based on electrophysical technology. The device generates a very high current pulse at the touch of a button, thereby eliminating unwanted plants in a targeted manner. The power module supplies the high voltage required for this, this current flows via the hand lance into the plant and then into the substrate. The circuit is closed again via the current feedback earth spike, which is inserted into the soil via another cable. The energy boils the water inside the plant and thus destroys the plant structure.

The result:

• The plant‘s chlorophyll is immediately damaged, the cells are irreversibly destroyed and die

• The water supply to the cells is interrupted

• The plant dries out

The right amount of energy and optimized ground contact are decisive for the effect: Plants with a high water content and, compared to their leaf mass, with few stems and roots require little energy and are easy to treat. Very dense and woody wild herbs or very large plants require a lot of energy. In these cases, methods combined with mowing, for example, achieve the best results, as the young new shoots carry a particularly large amount of water and therefore direct the flow even better into the plant. Plants with larger rootstocks or rhizomes are severely weakened and sometimes have to be treated several times before they are completely destroyed.

Idk, if you have the funds and time… or can bring them together in your community… why not just do it and wear friendly looking clothes and act calm and authoritative while doing it? Or… just do it at night while sleepwalking.

An example of a company that uses electrical equipment mounted on roadway margin mowing tractor type equipment for large scale infrastructure or agricultural use.

https://zasso.com/control-in-road/

another

The crop.zone approach involves a few critical steps. First, a conductive liquid is applied to the Japanese Knotweed. This liquid enhances the plant’s ability to conduct electricity. Following this, a specialised device delivers an electric charge directly to the plant. This electrical treatment penetrates deep into the plant tissues, reaching the roots and rhizomes that are often missed by conventional methods. By attacking the plant at its source, crop.zone ensures a more comprehensive and lasting solution to Japanese Knotweed infestations.

https://crop.zone/en/blog/tackling-japanese-knotweed/

Also what about microwaves?

https://pmc.ncbi.nlm.nih.gov/articles/PMC10818956/