Are you sure they are doing hair test for drugs? It cost more than urine test most company don't go that route, you know cut cost increase profit.
To be safe, cut your hair again at 1in and style it so it' not even all the way around, this way your a man of fashionable hair cut and not because you're trying to hide something. Depends on how fast your hair grow in a month, 1-2in left should be good the older length has THC encoded.