Plants require water for proper growth. They use it to absorb nutrients from the surrounding soil and distribute it throughout their structures. The temperature of the water is indeed a factor for most plants and many will grow faster and larger in warmer water over colder water. However, water that is too warm could be detrimental to the plant's health.
Some experiments have shown that plants tend to grow better in warmer water. One experiment conducted by John I. Swang, Ph.D., showed that various plants grew faster and taller in water warmed to 55 degrees Celsius. The plants in the colder water (36 degrees Celsius) grew significantly slower and were far shorter.
Warm water may be the ideal temperature for plants because it is not cold enough or hot enough to damage it. Water that is near freezing will slow the metabolic processes of the plant while water that is too hot may physically damage it, particularly its root system. Cold water can also damage the plant's roots.
Temperature effects other factors besides the rate of growth. Warmer water will make aquatic plants more buoyant since the plant's increased photosynthetic activity creates more gases. Warmer temperatures will also increase the length of certain plants' growing seasons. Many aquatic plants in Florida have longer growing seasons than the same plants in colder parts of the United States because of the warm temperatures.
It is recommended that plants are watered at a warm to room temperature, around 68 to 70 degrees Fahrenheit. Let cold water sit out overnight so that it reaches room temperature before giving it to a plant. The warm/room temperature water is also appropriate for most aquatic plants kept in home aquariums. There are situations where you will not want to promote plant growth, like algae in ponds, in which case aeration will keep cold water circulating throughout and thus stunt algal growth.