1. During medieval times, hair washing was about as important (or not) as bathing.
The wealthy — because their finances allowed them to afford the collection of clean water, servants, and the time to indulge more often in such luxuries as bathing and hair washing — washed their hair more frequently than peasant classes.
2. In spite of flowing finances, many royal kings, queens, and those in the noble classes, washed their hair only a few times per year. They preferred instead to wear crowns, hats, and headdresses.
In place of washing, powder was used to soak up scalp oils & attempt to suffocate head lice.