I don't qualify as herb-washer because I never used saponin-containing herbs, but here is my experience with herbs:
I dilute shampoo in nettle tea. 2 tea bags brewed for a couple of hours in 16oz of water. After the tea cooled off I pour it in a squeeze bottle, and mix it with shampoo. I do it once a week if I remember to plan ahead and if I'm not clarifying.
I noticed the scalp stays cleaner and 'calmer' (not dry, itchy neither overly oily in some parts) if I do that and I can stretch washes by one day (that makes a difference, I don't like to stay with wet hair in a cold apartment!).
In the summer, when I sweat a lot, I boil rosemary and thyme and use the cold tea to pour on my scalp in between washes, to get rid of the sweat. It's refreshing when the weather is humid and hot.
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