Hi,
What I've done with my small Perfect Plant gardens, is place a fence all around, then where the gate would normally go, I put a full wall in, decorate with posters, lights and/or Paintings, and then place a people door.
No Deer or Horses can go into garden but Sims can. I don't allow Pets (I delete them) and detest the Zombies and won't play them. So, I don't know how you would keep Zombies away from the plants.
Holly