While mineral water may seem like a good alternative to plain, old-fashioned tap water for your plants, think again. The very minerals and salts that are found in mineral water and are good for people, are not so good for plants. Mineral water likely won't kill your plants, but it may inhibit growth or dry out soil.
Best known under the commercial labels of Perrier and San Pellegrino, mineral water contains salts and minerals not usually found in tap water. The minerals and salts may be naturally occurring at the site where the water is collected or may be added during bottling. Sodium, calcium, potassium or sulfates are among the salts and minerals that may be found in mineral water.
It's unlikely that mineral water will kill your plants, but it may cause them to grow more slowly or be less healthy than a plant watered with tap water. Because mineral water contains additives that plants do not need, these minerals are left in the plant's soil and, ultimately, may change its composition. For example, if unused minerals and salts are left in the soil, the soil may dry out more quickly, so the plant may require more frequent watering.
While mineral water is branded and sold, be aware that you may be giving your plants additional mineral and salts by watering with tap water. Some regions have so-called hard water, which is chock-full of extra chemicals, including magnesium and calcium. Since salt is used to treat hard water, softened water also contains extras, particularly sodium. If you live in an area with hard water, consider watering your indoor or potted plants with water that has been treated with a charcoal filter, such as those found in Brita containers.
In most cases, the best water for plants is rainwater. Consider collecting rainwater in plastic jugs or buckets and using this reclaimed water for your plants.