Do settlers need access to the ground to farm?
Solution 1:
Your settlers do not need to have physical access to the the tasks they've been assigned to. Scavengers and farmers will still collect resources even if they cannot reach them.
However, I have noticed that settlers will not be at the last locations they were when I fast travel away and subsequently return to settlements. This means you may find settlers tending to crops on the ground when returning to your settlement.