Fuck corn. It is patently one of the worst feedstocks for ethanol. The only reason you hear so much of it is because there is so much money wrapped up in it already and it is a way to use up excess stock. No, switchgrass is the answer. Hearty, more biomass per km² than just about any other crop, has high cellulose content (which is what gets turned into ethanol), and can be cultivated just about anywhere on the continent with little maintenance or involvement. You could probably get away with planting a few fields in pockets around a stronghold which could be checked on a couple of times a week and harvested for an extended period, then you just have to process it as usual. It is even relatively short and dense, so zombies would struggle to hide in it and it would act as a natural barrier to slow the advance of both Zs and any nairdowells that would seek to assault you. Fuel source and defensive emplacement in one.