Where will they get the cannabis from? they don't have enough lands, import from the US/Canada i bet? ;)
Modern opium war
I believe the most pragmatic approach is to decriminalise but regulate soft drugs like cannabis. This breaks them away completely from the criminal ecosystem. Next keep the trade in hard drugs illegal, but decriminalise use and possession of small amounts that are actually typical of users. This helps address health and welfare issues, plus if users know they will not be prosecuted, it drives a wedge between them and dealers.
We’re never going to end the drug trade. It as futile as prohibition. What we can do is implement practical policies to mitigate its worst effects.
"they don't have enough lands"? Are you sure about that? Given that less than 10% of Germans consume cannabis on a regular basis, I don't think you'd need all that much land to supply them all with generous amounts of weed. Also, since weed is going to be quite a profitable crop, I'm pretty sure some farmers will be eager to cease their corn/wheat/whatever production for it.