The idea here is that herbs are herbs, and drugs come from factories. There might also be an issue with noxious weeds, invasives, and so on, but the onus on the grower should be to keep them from spreading, not to not grow such things at all. When seeds are outlawed only outlaws will be able to develop perrenial wheat.
This doesn't legalize importing weed, it eliminates our dependance on imported weed, and as we all know, the end of Prohibition is what got us out of the LAST Great Depression.
This also doesn't legalize heroin, it legalizes opium, maybe. I have a statuatory thing in my platform about dealing with junkies in a civilized fashion.