1-Make a limit on places to grow the drugs.
2-Make it in the active city to forbidden people that can get in helicopter and jump on the roof of any building and start growing and that gonna be hard to catch.
3-Cops able to harvest those plants and deliver it to police department.
4-Make it more challenge that cop will be on radar as he have drugs.
5-People can stole his car or make it undriveable and steal the drugs from the car.