One thing I know about fences from trial and error: you can’t tell how sturdy a fence post is by looking at it from above ground. Long before you see any evidence of a problem, the post may be rotting under the damp, dark earth. That’s why farmers used to look for heartwood to use for fence posts. This is wood found in the center of the tree. Sapwood near the bark turns into heartwood as a tree grows. During the growth, certain compounds are deposited in the heartwood that make it resistant to disease