Most plants grow better in neither hot nor cold water, but flourish in lukewarm water. However, plants differ depending on the species of the plant and the climate in which it grows. For instance, if water is too cold, African violets acquire water spots.
North Carolina State University advises watering of plants at room temperature, especially if the plants are tropical. Tap water may be used, but should sit out overnight until it comes to room temperature, especially if it contains fluoride.
According to How Stuff Works, cooking water can be used to water plants, as long as the water cools down to room temperature, there is not an excessive amount of salt in it and oil has not been added to it. Also, this method should not be used for indoor plants because of lingering smells from the cooking water. On the upside, it is a good way to conserve water.
In order to improve plant growth, plants should be watered regularly according to the plant and soil type. Some plants naturally require more water if they grow quickly or are exposed to a lot of heat and sunlight. Other plants require less water, such as cacti and succulents.