The softened water contains a high amount of sodium because of the salt used in the softener. This water does not have enough sodium to taste salty, but the sodium levels are high enough to affect plants and affect grasses in higher concentrations.
Softened water does not kill plants or grasses right away. However, too much sodium over time can negatively affect the health of plants, eventually leading to death if the plants are only given soft water.
Some plants are more tolerant of salt than others. These include oak trees, maple trees and ferns. Grasses are the most tolerant of salt and can handle much larger concentrations of salt, up to 2300 mg for each kilogram of leaves. Other plants are usually damaged at lower concentrations, such as 100 mg per kilogram.