Q: What is an apron fence, and why is it helpful?
A: An apron fence is a fence that helps to keep out digging predators. But it isn't normally a special type of fence that you buy; it's more of a special way to install your fence. You can install a dug apron fence or a bent apron fence.
For a dug apron fence, you might bury a foot or so of your fence in the ground... so if you buy a six foot fence, one foot of it would be buried, and the fence would be five feet high. When a predator like a rodent, dog, or fox tries to dig in, it finds a barrier. Eventually, of course, a very determined predator could get in, but rodents don't tend to reason that "it must end somewhere!" and keep going. Instead, they just look for an easier meal elsewhere.
A bent apron fence doesn't require digging the fence down, so it's a little easier to install, particularly if you have very rocky or hard soil. With this type of fence, you would bend the bottom foot of your fence out along the ground. That works because digging predators typically try to dig right along the fence, and encounter a barrier right away; they just can't dig down. And again, they don't tend to reason that, "I'll just have to dig further back to get in." When they dog, they dig at the fence line. If they can't get in at the fence line, they go elsewhere.
The danger with a bent apron is that a clever, small predator like a raccoon could simply lift the apron and crawl under, so a bent apron fence tends to work better in areas with dense growth of plant life. In those areas, the grass grows up through the apron, not only hiding it (if you can't see the edge, there is nothing to prise up), but also anchoring it down. Where vegetation is sparse, a bent apron is less likely to baffle clever predators.
Folks in the desert southwest with barren, rocky soils, we're sorry to tell you that you have a hard choice: dig a trench through the rocks, or risk a bent apron. But those in greener areas have their own concerns: aprons will eventually rust and deteriorate in wet areas, whether buried or laying on the ground. Then you have to replace the whole fence, right? Well, one employee has come up with what she hopes is a solution. She installed a regular fence without an apron... and then attached a bent apron to it, connecting it securely with wire. When the apron eventually rusts, she's planning to just replace that separate apron, without having to disassemble the entire fence.
And as always, when choosing a fence for your run, remember that chicken wire is for keeping chickens in, not for keeping predators out! For predator safe fencing, use something like quarter or half inch welded wire hardware cloth. Predators can chew or tear right through chicken wire.