As a general rule of thumb, most plants prefer water at room temperature. Some research, however, indicates that warmer – not boiling – water may have some beneficial effects on plant growth, even if those effects are minimal. Around the garden, hot water can also be used to kill weeds and prevent disease. Consider each plant's individual needs before experimenting with hot water, as it can cause serious damage to your garden if used improperly.
The National Student Research Center and John I. Swang, Ph.D., led a group of Louisiana students in an experiment to investigate the effect of hot water on plant growth. Under controlled variables – including water amount, soil type, size of pot, seed type and sunlight exposure – Swang and his students watered three rye grass seeds with three different temperatures of water. Students used water at 36 degrees Celsius, 43 degrees Celsius and 55 degrees Celsius. Plants watered in the latter two groups began growing a day later than the cooler water, but grew slightly taller; plants fed room-temperature water grew up to 10 centimeters; while the plants fed hot water grew up to 11 over the course of the experiment. The NSRC concludes that plants may grow faster when watered with warm water, provided the water is not hot enough to damage root systems.
As noted by the NSRC project, hot water can damage plants, leading to stunted growth or even death. According to gardening expert Ellen Brown of Sustainable Media, hot water has the potential to melt away the coating on a plant's leaves or severely damage the plant's cellular structure. With reduced leaf coating or damaged cellular structure, the plant is unable to retain moisture; as such, it dehydrates and eventually dies. When applied to soil, hot water can kill beneficial microorganisms essential to plant growth and damage shallow root systems.
If used carefully, hot water provides some handy benefits around the garden. Hot water's damaging and dehydrating effects can be used to kill harmful or unsightly weeds. Simply boil a pot of water and pour it over the weed's body. Be sure to target the plant, not the soil, in order to avoid harming the root systems of nearby plants. Indoors, use warm water mixed with a dab of mild soap to clean the leaves of potted plants. This helps fight against disease and insects while maintaining a pleasant appearance. Cover the plant's pot to keep soap from entering the soil.