Water for plants can come from many sources. Unsoftened house water is almost always suitable for plants. The sodium that is added to soften water, though, can cause problems.
Tap Water
Water that is fit to drink is almost always fit for plants. The outstanding exception to this is water that has been treated with a water softener. The softener replaces the calcium and magnesium in the water with sodium. The sodium makes the water soft, but it isn’t healthy for houseplants.
Well Water
Water from wells frequently contains dissolved minerals. Although some of these minerals, such as calcium and iron, cause household problems, they do no harm to plants and can be used on houseplants or outdoor plants without difficulty.
Plants are sensitive to a few minerals, however, and may experience problems if one of them is present in your well water. Perhaps the most common of these is boron. Although it is a necessary plant nutrient in minute quantities, in only slightly high concentrations, it is toxic to plants.
Surface Water
Water from ponds, rivers, and streams is almost always good for plants. A few rivers and creeks that drain agricultural land contain high levels of dissolved minerals from the fields. Even this water can be used, but a little excess water should be applied to leach the minerals from the soil and prevent them accumulating.
Other Sources of Water
Distilled water and deionized water are pure water, with no dissolved minerals. They make excellent irrigation water, but are expensive.
Some people collect rain water for use on the garden. Rain water is naturally distilled, and is as pure as the air it comes from. It can be collected from roof runoff and stored in barrels or cisterns.
During drought, you can use “gray water” to irrigate your garden. Gray water is used water from bathtubs and the rinse water (not the wash water) from dishwashers and washing machines. Although not fit for household use, this water can be recycled in the garden as irrigation water.