It is possible to use softened water for gardens.
In the UK, an increase in salt won’t cause a problem for plants because of the amount of natural rainfall we get. During a dry season it isn’t recommended to use softened water in the garden as it can build up sodium levels in soils that can be damaging to some types of plants.
Bypassing outdoor softened water taps:
If your outside tap is fed with softened water, you can still use hard water as and when you need.
Open the bypass valve and shut the inlet and outlet. This allows hard water to flow through your tap. See your instructions for further details.
Remember to reverse this procedure when you have finished watering the garden to make sure your system is processing softened water again.