Not all pressure-treated wood is equally decay-resistant. Different parts of the tree absorb treatment differently. Learn how to spot the best quality 4x4 posts by checking the growth rings.
Centered growth rings indicate the post is made from peeler core and won't accept pressure treatment well (right), while off-center rings mean the post is not the log's center (left).
Pressure-treated 8-ft. 4x4s are typically made from “peeler cores,” which are the byproduct of plywood production. When a log is turned on a lathe to produce plywood veneer, the center that remains, called the peeler core, is used as a post. These cores often contain the pith, or center of the log, which doesn't accept pressure treatment as well. Peeler-core posts are more likely to warp and twist than posts cut farther away from a tree's center.
When shopping for 4x4 posts, look at the ends. If growth rings start near the center of the post and expand outward, it's almost certainly made from a peeler core. If the rings are off center, it's not the log's center. Buy 10-ft. posts instead of peeler-core 8-footers and trim them to fit.