Usually reserved for your medicine cabinet, hydrogen peroxide has many household uses, but one of the commonly overlooked uses is in the garden.  Hydrogen peroxide occurs naturally in rain.  It forms when water vapor reacts with ozone in the atmosphere.  Ozone (O3) is an unstable molecule that wants to give up an atom of oxygen to form normal O2.  It most easily reacts with water (H2O) to form H2O2, hydrogen peroxide.  H2O + O3 = H2O2 & O2

Hydrogen peroxide itself is also an unstable molecule, wanting to rid itself of the extra oxygen atom.  It does this by oxidizing materials.  This is beneficial because it can oxidize simple organisms like bacteria and viruses killing them in the process.  It can also oxidize inorganic pollutants in the environment neutralizing them.  However, in low concentrations it is harmless to larger organisms, like plants and animals, hence its pharmaceutical use as a disinfectant.

In the garden hydrogen peroxide has many benefits.  At the most basic level it helps with oxygenation around plant root systems.  It is commonly used by hydroponic growers to help increase oxygen levels around the roots, but the main benefit is as a disinfectant.

Hydrogen peroxide when mixed with your water can help kill harmful bacteria, viruses, and fungi that can grow on plants and in the soil.  It can also be used when pruning trees to help clean fresh cuts.  Here at Bob's we use concentrated hydrogen peroxide to control mold, mildew, and algal growth in the greenhouse and also sterilize the facility where we root cuttings.

For use in the garden only small concentrations are needed.  The standard concentration that you can find in most retail stores is around 3%.  Mix ½ cup per 1 gallon of water for use when watering plants.  Higher concentrations (10-20%) can be used for mold, mildew, and algal growth, but not on or around plants.  Also, concentrations above 3% are considered a hazardous chemical because of hydrogen peroxide's oxidizing power.  It will corrode many materials including skin, so we don't recommend using industrial concentrations at home.

For the best results use a dose of hydrogen peroxide every couple of weeks when watering.  This both helps develop a healthy root system and also reduce harmful pathogens, fungi, etc. on the plants and in the soil.