In many cases, water from a mechanical softener does have a harmful affect on plant growth, but there is not a great deal of research to indicate which plants are injured. Plants have widely varying tolerances to softened water.
Hard water contains large amounts of calcium and magnesium, which reduce the effectiveness of soap.
When these minerals combine with soap they create a ring that is often visible on tubs and washbowls. However, while the original calcium and magnesium in the water are plant nutrients, sodium is only needed in very small amounts and, over long periods of time, can become toxic to plants. Sodium also has a tendency to damage the soil quality around the root system. As sodium enters the soil, it breaks down the structure in the soil and reduce the drainage potential. A good, crumbly soil mix containing too much sodium can become a sticky mass around the roots of your plants.