Some gardeners believe that boiling water is beneficial to plants that are particularly sensitive to water impurities. While there is no proof to this claim, boiling water has been proven to kill unwanted plants when used correctly.
Water that is still boiling or extremely hot from being boiled will kill any plant it touches by scalding it. This makes boiling water an ideal organic weed killer, but not an ideal water source for keeping plants alive.
An urban legend spread across the Internet claims that boiled water, even after it has cooled, is destructive to plants. Snopes, an online archive that debunks or confirms urban legends, found this not to be true. In a study done by Snopes researchers, there was no difference in plant growth or health between plants that were given boiled water and plants that were given regular water.
Some gardeners boil water if they believe in contains impurities or chemicals that could be dangerous to their plants. Boiling water will kill most contaminants, theoretically making the water safer for sensitive plants. The water must cool to room temperature before using, however, or the heat will kill the plants.