Whenever you install a fence, you have to install the posts securely in the ground, and you do this by digging post holes. However, in certain circumstances, the post holes aren't enough, and you need to dig a trench and set the fence (or other material) in the trench in addition to deeper post holes. The main driver in the decision is whether you need to block or support something.

Animal Control

Whether you've got a dog that's a determined digger or are battling wild critters who want your vegetable garden for themselves, a trench is the best way to exclude or restrain animals. Many pets try to get out from yards by either jumping over or digging under the fence, and a trench that contains the bottom portion of the fence or another blocking material helps stop their escape. At the same time, that trenching stops critters from invading your yard by tunneling under the fence.

Fence Support

Occasionally, you'll need to bury the bottom part of the fence in a trench in order to support the fence. In most cases, the posts will be enough to support the weight of the fence. But if you're going to attach anything to the fence that could place strain on the posts, for example, you'll want extra support. For example, if you're attaching supports for an espaliered fruit tree to the fence, you'll want extra support -- you don't want the tree to pull the fence down in a few years when the tree is bigger and if it becomes unstable due to soil erosion around the roots.

Noise Blocking

Noise is sneaky, and sound waves can travel far and wide if they aren't stopped. If you build a fence as a way to screen out noise, you don't want to have the bottom of the fence completely open as that will just allow the noise to pass through. New fences can be trenched to help block that noise, while existing fences can have additional material added at the bottom, extending the base of the fence into a trench.

While you can rent post-hole diggers yourself, trenching is best left to a fencing company that offers trenching services. These workers know the laws regarding trench safety, and they know local regulations regarding utility avoidance and permitting. Have a professional specializing in fence trenching services take a look at your plans so you can work out a deal.