Hot water is not always bad for plants, but whether it is depends on how hot the water is and how frequently the hot water is used on the plants. Very hot water, as well as very cold water, can shock a plant. In most instances, lukewarm or room temperature water is best.
Continually watering plants with hot water will "cook" a plant's cell walls, effectively killing them and killing the plant. Plants with dead cells will lose their turgidity, and they will wilt all over and die.
Brief periods of watering with hot water will effectively kill many plant pests. Most plants can withstand a few minutes of a hot water "bath." Immersing a potted plant in water with a temperature of about 120 degrees Fahrenheit for about eight minutes can kill insects such as mealybugs, mites and scale without injuring the plant.
While frequently watering with hot water is bad for plants, so is watering with ice-cold water. This is especially true for tropical foliage plants, which are used to the warm, wet conditions of rain forests. For that reason, it's best to water your plants with water that has been allowed to come to room temperature.