did you look this up? because i don't think you're going to be happy about this.
Hair testing
Hair follicle tests assess drug use for up to 90 days. After use, weed reaches the hair follicles via small blood vessels. Trace amounts may remain in the hair.
Since hair grows approximately 0.5 inches per month, a 1.5-inch hair segment taken close to the scalp can provide a window of weed use for the past three months.