I used to work for a company that sold drug test (Needless to say, I was deep in the bullshit associated that ludicrous industry that only exists for the sole purpose of promoting false information, middle men and money. An actual 5 panel test costs $1.10 wholesale, and some places charge as much as $30 for one)
We also did hair testing. DO NOT CUT YOUR HAIR. It won't matter. The cannibinoids break down in hair after a certain amount of time, just like in urine (Only in hair it's much much slower) and anything longer than an inch and a half doesn't have any cannibinoids left in it anyways, so you would just be ruining your hair. Hair tests CAN go back 180 days, but they only count levels coinsitent with 90 days. Also, if this testing involves any sort of Commercial Drivers License, they can't even do hair testing anyways.

Just letting you know, don't ruin your hair. Even if you shave your head they'll pull it from somewhere else that doesn't grow as fast (Arm pit, back, pubic, chest. You name it)