For outside use on lawns or gardens, using softened water is wasteful. 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. Areas that receive regular rain are not as sensitive to sodium accumulation because the rain "rinses" the earth.
Ask the dealer who is installing your water softener about a bypass for lawn and garden faucets, and a faucet for unsoftened water.