Is the climate of Yenan suitable for growing opipum seed? Maybe it is, I don't know.
The opium itself is useless until you sell it. Whom did the CC sell he opium to? It had to be large scale sale to be profitable in order to sustain the population of Yenan. The transactions must be in secret. Dai Li's men around Yenan must be aware of the transactions. Chiang Kai-shek was a fool not to seize upon the opportunity for propaganda purpose.