No, you cannot water with softened water. That, however, is the water that some people have, so they need to find ways to avoid damaging their houseplants. The damage results from the softening salt that replaces the naturally-occurring calcium and magnesium in the water with sodium. You can’t taste any sodium, but it is there, and it builds up in the container mix, eventually killing the plant if you do nothing.
First, explore other options. Your plumbing system should have an outlet valve that allows you to draw water before it enters the softener, so you can get “hard” water there. Before using it, let that water sit in an open container overnight to allow the chlorine from a municipal water system to evaporate and for the water to come to room temperature. Another option is to use a rain barrel to collect and use rainwater. In some areas rainwater is very acidic. If yours is, add a teaspoon of ground limestone to each 6-inch diameter container of potting medium to counteract the acidity.
If none of these options are viable, use the softened water and take the following steps. Always use containers with drainage holes and never let the containers sit in a pool of the softened water. Use a houseplant fertilizer that contains trace minerals, and use it “weakly, weekly.” For instance, if the label says one teaspoon of fertilizer per quart of water monthly, then use 1/4 teaspoon weekly. Divide the recommended dosage so that you use an appropriately reduced amount every time you water. Then, every time you water make sure the amount of water is enough to go entirely through and run completely out the bottom so that it is carrying away any built-up sodium. As time goes by, the soft water sodium causes the potting mix to harden as the salts accumulate. Watch for this symptom and repot as often as it happens. Usually once a year is enough, but you must wash away all of the potting mix from the roots using tepid water and start over with new mix. That routine will do as much as possible to reduce the damage to the houseplants.
If any of your houseplants are in huge containers, just do what you can. I have a two-wheeled dolly to lift and wheel them out to the patio to sit in the summer rain sometimes to leach out their soil. In the winter my ficus likes to be leached by taking a shower in my walk-in shower stall. That’s one way to leach away the yucky stuff from a monster pot. Put your thinking cap on, and you will find a way.
As for annuals in outdoor containers, soft water may not have time to do much damage in only one summer growing season, as opposed to houseplants that are expected to grow and thrive for many years. Fertilize the same “weakly, weekly” and water thoroughly every time you water to leach out the salts from the previous watering. Then use new container mix each season and any build-up of salts should be eliminated in the process.
Answer by Martha Ferguson.