Like humans, plants are dependent upon water to provide them with the necessary moisture to sustain life and health. Some types of water are more beneficial to plants than others. While plants can survive on tap water, it is not always the best option. Tap water often contains a number of chemicals, such as chlorine, and metals like copper and lead, that can have an adverse effect on plant life. When determining what kind of water to use on your plants, a good general rule is to give them the same water that you drink.
The chemical and metal pollutants that are present in tap water are often detectable by taste. Oftentimes, consumers in areas where the water tastes contaminated will install filters on their faucets or run tap water through pitchers with filters in order to produce cleaner drinking water. If you are filtering your tap water before drinking it, you should also filter it before feeding it to your plants. Allowing the water to sit out overnight will also help release some of the chemicals in tap water. For example, chlorine will evaporate from water that's left out.
The majority of plants will grow better when watered with soft water instead of hard water. Hard water contains large amounts of calcium and magnesium salts, whereas soft water contains low amounts of these chemicals. To find out if your tap water is hard water, either contact your water company or purchase a water testing kit. Filtering and using water softeners does not actually remove the salts from hard tap water. If you have hard water, water your plants with distilled water, which can be found bottled in grocery stores and is always soft water.
As natural and chemical-free as water can be, rainwater is what plants are used to getting when grown out in the wild, and it will do plants less harm than nearly all other types of water. Collect rainwater by placing barrels or other large containers outside in your yard. Melted snow can be used for watering as well.