Hydrogen Peroxide is an unstable chemical compound - H2O2. Technically, that's water with an extra oxygen molecule. When moisture is added (when you add it to your watering can, laundry or put it on a cut), the HP explodes apart, releasing the extra oxygen molecule. As the HP explodes, it destroys anything biological. This is good if you are using it to clean your laundry but bad if you put it on a plant or a cut.
Once open, the HP in the container has a very short life expectancy so chances are, you watered your plant with expensive water. The bad news is if the chemical reaction is still taking place (its still fizzing) when you put it on a plant, it will destroy healthy plant tissue. The good news is the reaction happens very fast so you are most likely using water with an extra oxygen molecule.
I use a couple drops of HP in water when I am attempting to root cuttings but just a couple drops. It adds an extra oxygen molecule to the water for a while, aiding in rooting.
