Ask the master gardeners
Q: Why is “softened” water not good for plants?
A: Water softeners remove minerals, mostly calcium and magnesium, that cause water to be deemed “hard.” When water is hard, a myriad of difficulties can arise for a homeowner such as the occurrence of excess lime deposits and the frustration of soap not dissolving properly or at all. Therefore a water softener can be beneficial to everyday life for the homeowner. Most water softeners are briny, that is they contain salt such as sodium or potassium chloride.
Plants depend on a fragile mechanism called osmosis to survive. What happens in osmosis is that water passes by diffusion from a weak solution (high water concentration) to a strong solution (low water concentration) until an equality is reached between the two. In the case of a plant, the water from the soil (weak) into the plant roots (strong) provides the plant with the moisture it needs to survive.
When water that has been softened is discharged into the garden, the brine from the softener can alter the osmotic pressure that plants rely upon to regulate their water needs. This imbalance should be avoided.
— Judie Phillips, New Rochelle , master gardener, Cornell Cooperative Extension of Westchester