Can you water the garden with softened water?
You can water the garden with softened water but it would use an enormous amount of salt.
After a lot of watering and no natural rain you would build up a level of sodium in the plants and soil, which may be detrimental to some types of plants. In England that should not be a problem because of the amount of rain we get.
If your outside tap is fed with softened water you can bypass the softener whilst watering the garden. See the instructions with the softener. Don't forget to turn the softener back on again afterwards.