Cut privet on "a dark night in September"

My folks didn't put much stock in superstitions, as far as I could tell, and "planting by the signs" seemed to fall under that category.  However, Mom did say that her father said that to keep privet from coming back, cut it on a dark night in September.  Of course a "dark night" would be one that is near the new moon, when the night is literally dark for lack of moonlight.  Sometimes folk wisdom is just superstition, but sometimes it might be based on the relation of plant growth to the time of year (and possibly root growth cycles with gravitational pulls of the earth and moon).  Anyway, the new moon was night before last, so if you have privet to cut down, get with it!