Plants can be safely given water that has been boiled in a microwave oven. There is no proof that microwaves cause a chemical breakdown of water, despite misinformation on the Internet. Snopes, a popular site that debunks myths and urban legends, demonstrated that microwaved water was harmless by testing nine plants (see resources below for the study). Three plants were given water boiled on a gas stove, three with microwaved water and three with plain tap water. All nine plants thrived equally. Here's how to use microwaved water on plants.
To safely boil water for plants in the microwave, use a container made of glass to prevent plastics from leaking into the water and causing contamination.
Boil water for three minutes to remove any contamination. Use care when you remove the water container from the microwave so you don't scald your hands.
Allow the water to return to room temperature before you use it on the plants. Boiling water can damage a plant, causing shock and even cook the roots.