I am going to take a guess here that the untreated water would be the best solution. However, If you have a way of collecting it how about rain water? Last year I used a lo9t of bucket to collect snow and as it melted I used that to water a lot of my plants I also collected rain water in large storage tubs. I live in the city and have city water but when I can provide extra rain or snow water I use it since the city water is full of chemicals.

Penny

Softener salt is not good for plants. At first they won't mind, but eventually the salts build up in the soil and wreak havoc, starting with sensitive plants first.

At our house we have well water. The incoming pipe is split as it goes into the house. One fork goes for the outside hose system, the other goes to the water softener and everything in the house. I didn't have to request it, it was built that way, probably because people don't want to pay to soften water that is only going outside. Check your pipes to see if you might have a similar situation without knowing it.

First try to reduce the watering. Deeply prepared beds and mulching can help a great deal, as can growing plants that don't necessarily need a lot of water.

If you don't have to water, then the problem disappears.

Also, you could try alternating with the softened water and the untreated water. Definately do not use any chemical fertilizers as they contain salts as well. Stick with organics. But I think the ideal situation would be to put in one or two rain barrels and use lots of mulch on your flower beds.