Building a new coop and am wondering if the hens will do damage to garden produce if allowed to free range in it.
Ken
Chickens and gardens don't mix well in the growing season. Chickens like to take dust baths in bare dirt, they can smother/crush anything growing. The love to scratch in mulch, looking for bugs. The mulch gets scattered everywhere and plants are often scratched up. They seem to love spouts, just one peck and the plant is gone at that stage. They will eat certain of the produce, often not eating it totally but just pecking it enough to ruin it. depends on what the produce is.
Some people turn their chickens into the garden after the growing season. The poop helps fertilize it for the next growing season, they may help clean it up, and they can eat certain pests that try to overwinter. How much benefit this is depends on your chicken density, how many chickens in how big an area.
So yes, I think you will need a fence. Chickens can fly a lot better than many people realize if they want to fly, even the larger breeds. There is a difference in keeping them in versus keeping them out. In both cases you want to make them not want to. Since you are keeping them out the further the coop is from your garden the better.
Chickens like to perch. If the top of your fence looks like a good place to perch they may fly up there just for fun. Who knows which side they may hop down on? Putting a solid rail on top of your fence or having post tops that look substantial is generally a bad idea as far as keeping them out. Most wire mesh fencing has enough stiffness that if you leave 6" or more sticking up above your posts or fence top if does not give them a good place to land. I use a 48" high electric netting that has nothing for them to perch on. That keeps them in, which is harder than keeping them out. There are some other tricks to keeping them in which you don't have to worry about.
You will need some type of mesh fencing, either wire, plastic, or cloth. The holes in the mesh need to be small enough they cannot squeeze through. Another point is that fencing often doesn't prevent holes under it, especially on uneven ground. Chickens can squeeze under a fence just as easily as a rabbit. If you use a wire mesh fence and put an apron around it on the outside you can maybe keep chickens and rabbits out. Watch your gates though, those are often weak points.
Many years ago I had three hens that learned they could get out of a relatively small 5' high run. One was trying to get away from an amorous rooster and went vertical to get away. She taught a couple of her buddies and they started flying out every day. That's a difference in keeping them in versus out. The point of this is that they can learn, so I'd get the fence up before they learned there was good stuff in the garden.
I don't know how big an area they have to roam in or other things unique to your situation, but I'd think a 5' high fence with nothing for them to perch on would work great. 4' high would probably work.
Good luck!