How to water herbs when you have extremely hard water?

We had awful hard water until recently. What I would do is add a teaspoon of baking soda to a gallon of water.

We filter our water with a Pur filter attached to our kitchen faucet. It costs about $25 every 6 months or so, but filters out all the chemicals and all. Hard water is not bad for herbs, but the faucet additives can harm them. If you cannot or prefer not to use a filter, my Grandma used to set a gallon of water over night to let all the chemicals settle to the bottom. She used the top 2/3's of the jug is all. She dumped the rest out. For as long as I knew her she did this and she always had the prettiest garden and flowers in town. She also had the best herbs inside her house.

You need to make sure that when you water you don't get the water on the leaves. Perhaps a tube watering system would save your herbs.

http://theselfsufficientliving.com/self-watering-pots-containers/

https://www.familyhandyman.com/landscaping/planters/build-your-own-self-watering-planter/view-all/

Water only the soil, keep it off the leaves. Perhaps and RO water filter system that gives you purified, not distilled water would help. It takes out all the chemicals out of the water. This is what they use in the factories that makes the purified drinking water. We use a system in our basement for adding water to our salt water fish tanks and store the water in a big plastic garbage can.

In my office, the plant company guy fills a bucket of tap water, then lets it sit for several days, the man claims that the "hard" parts will settle near the bottom and then he scoops water from the top and it is better for the plants.

Fill buckets and allow the water to sit for a few days

Fill buckets and allow the water to sit for a few days

http://wellwaterguide.net/iron-well-water.aspx

Maybe this will help.