If the amount of hardness minerals in the water is less than 10 grains per gallon, it is doubtful that the sodium content of the water after softening would be harmful to plants.
Most house plants require specific soil conditions for health.
Many thrive best in slightly acid soils. If there is a high hardness concentration in the water before softening, the probability is good that the sodium present after softening could be harmful to plants. For outside use on lawns or gardens, using softened water is wasteful. Also, where the concentration of hardness minerals is high, the sodium content after softening may be high enough to retard plant growth and harm grass. This is especially true in climates where rainfall is scarce.