Hydrogen peroxide is a great way to kill root rot. Used accordingly and sparingly, it can protect a plant from a variety of diseases. Some people like to put a small amount in with the soil for this very reason.
However, hydrogen peroxide, like with human skin, can be harmful to plants if used without care. It must be used with a significant amount of water - around 3% of hydrogen peroxide to 97% water - to provide a gentle amount of chemicals that won’t harm the plant.
Hydrogen peroxide is great for preventing the spread of root rot when repotting diseased plants. After removing the rotten roots, you should clean the existing or new pot with disinfectant thoroughly before repotting the plant to stop the spread of rot. You can use higher concentrations of hydrogen peroxide in this instance.
Plant owners like to spray the leaves of their plants with a spray bottle of water, and a small amount of hydrogen peroxide can be used in this. Not only will this provide hydration to the leaves without soaking the soil and increasing the chances of root rot, but it will also work to combat other issues that plants may experience such as pests like spider mites and slugs.
However, using hydrogen peroxide around plants comes with potential impacts on the ecosystem. High concentrations of hydrogen peroxide can be damaging to the plant as it can corrode the leaves and stem. Also, whilst hydrogen peroxide is useful for protecting the plant from pests and bugs, most insects like snails and earthworms are useful to a garden ecosystem.
As long as you use the smallest amount of hydrogen peroxide to water, it can be a wonderful cure for killing and preventing root rot in plants.