Let people grow pot on farms—safely and legally—and they'll grow pot on farms. Ban growing pot on farms and people will grow it unsafely and illegally:

Six days after the Rim fire broke out in the middle of the Northern California forest, Twain Harte Fire and Rescue Chief Todd McNeal told a community meeting the blaze was definitely human-caused. In his Aug. 23 talk, a video of which has been posted on YouTube, McNeal said that the fire started in a section of the Stanislaus National Forest inaccessible by foot or vehicle and that it was “highly suspected” that an illegal marijuana growing operation that sparked the blaze.

Fighting the Rim fire has already cost California and the federal government more than $60 million dollars already and the fire is only 75% contained. So it's going to cost millions more. The cost of battling this fire should be added to the cost of fighting the idiotic, un-winnable war on pot.