First, I realize that nobody has waited for this thread but that is only because you are entirely unprepared for the zombiecalypse and will be wiped out anyway.
So, here is the reasoning:
First wheat, rye and corn need to be prepared to be able to be consumed. Given the limited resources available after the complete breakdown of society thats bad.
Then, one hectar of potatoes yields around 15-20 tons of potatoes which is probably half fuckload in the imperial system.
The killer argument though is that wheat and corn grows pretty high, in a corn field a whole flock of zombies could sneak up on you and tear you apart and you would not even see them coming.
Not so with wheat, but crawlers would still be a problem.
Potatoes on the other hand dont grow to far out of the ground, nothing could sneak up on you, so they are perfect for a zombie safe harvest.
In Land of the Dead, Dennis Hopper kills someone to flee with FIVE MILLION DOLLARS !!11!!
To go where?
To buy what?