I also thought it was a metaphor for the stubbornness and thick-skinned personality necessary to be a farmer. It also seemed to be a metaphor for the farm itself and a lot of the hard work required to keep it up and running. There is always more to be done and tasks that should be done in the future, but never enough time or money to tackle them.
I thought it was also important the part where it acted as a natural barrier for the livestock. Other parts with fences needed to be repaired or the cows would sometimes escape if they found a weak spot. But where the Osage-orange hedge was, they never bothered or escaped there, even though they could have by pushing through the branches and thorns.
This symbolized to me that even though there was fence keeping them imprisoned, they still treated this obstacle as real as the prison bars of a fence. Sure it would have snagged and been tough to get through the hedge, but it was still possible to leave it. The same goes for the life style and the farm. Nothing was physically holding the family there, imprisoned. But they chose to stay because it would have been menacing to leave those invisible hedges keeping them rooted there.