My father told me that if you cut hair or garden grass during waning moon, they will grow slower. Is it true?
Of course not. Even if grass or hair would be affected by the moon phase (which I doubt), the speed with which they grow would be affected by what the phase is, not what the phase was when you cut it. Hair is dead, and the root of the hair do not know when the top was cut.
There is no references for this answer, because I can't find anyone that has researched this. :-)