How does electricity kill weeds?
When you pass an electric current through a plant, electrical resistance within the plant turns the electrical energy into heat. This causes water inside the plant to boil, which bursts the plant cells both above and below ground, resulting in their death.
Electric weeder action
Electrical power is applied to the plant by the treatment electrode. The current flows through the leaves and shoots, then the roots. The earth return electrode allows the electrical circuit to be completed. The diagram shows this circuit created by a roller electric weeding machine.
Electric weeding machines operate between 3kV and 10kV. Current flow depends on the resistance of the plant but is typically around 50mA, increasing to 500mA as the structure of the plant breaks down, lowering the resistance of the electric circuit.
Systemic weed killing
The electricity acts systemically, killing the roots as well as the shoots and leaves. The plants will typically die-back after a few days with long-lasting results since there are no live parts of the root from which to regrow.
In some circumstances, where the plant has a particularly large below-ground to above-ground biomass ratio, then multiple treatments may be required, as you would with chemical weedkillers. Typical examples of this would be invasive weeds such as Japanese Knotweed or Horsetail, which both have systems of deep rhizomes.
Does it really work?
The image shows some burdock plants that were dug up two weeks after treatment.
The plant on the left had not been treated.
The plant in the centre had a larger tap root than other plants in the same patch, and it shows the top portion of the tap root has been damaged and has begun to decay. Research has shown that killing the top 20% of a tap root is sufficient to prevent any regrowth.
The plant on the right has a smaller root and the same treatment has effected the entire depth of the tap root and caused significant decay within two weeks.
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