Nearly all plants require a water source to help them take in nutrients and create sugar via photosynthesis, which is why you water the plants in your garden or around your home on a regular basis. Using milk as a liquid food for your plants in lieu of water may seem like a good idea because of the added nutrients in milk, but it can actually be detrimental.
Viscosity is essentially a measure of the "thickness" of a liquid. The thinner a liquid, the more easily it can be transported. Milk is about 150 percent more viscous than water, meaning it will not be able to flow through the plant's xylem to distribute nutrients as easily. The extra energy that the plant will need to move the milk through its parts will exceed the resulting nutrients it acquires.
Milk is composed of much more than water -- filled with protein, lipids, carbohydrates and lactose. Over time, bacteria will break down this lactose, causing the milk to spoil. This can lead to mold and other germs that can kill your plant. The curdled milk can also block pores that the plant may use for photosynthesizing or even block up roots. However, pure water is free of such spoiling and, while it may pick up bacteria, it will never curdle or spoil.
Milk does not evaporate as well as pure water because it is a much heavier substance that is chock full of other nutrients and fats. In plants, water evaporates through the leaves much in the same way sweat evaporates through our skin, keeping the plants cool. If plants were watered with milk, evaporation would be much more difficult and would lead to overheating.
In small quantities, milk can provide benefits to plants. For example, spraying a plant's leaves with extremely diluted milk can increase the pH of the leaves' surfaces, making them less susceptible to bacterial and fungal infections. The proper solution should be about one part milk to 10 parts water to counteract milk's natural viscosity. Using a diluted milk spray can also stave off mildew, creating a sort of protective barrier. Despite the benefits, however, milk should only be used on plants in a diluted form and never as a substitute for pure water.