Untreated wood posts rot first at the ground line, and everything else hangs on the posts. Get them right and the rest is upkeep.
Why posts rot where they do
A University of Kentucky extension sheet explains that untreated or only dip-treated posts rot first at the ground line, where water and air combine to let fungi grow. Properly treated and installed pine posts can last 20 to 35 years; untreated pine posts might last 3 to 7.
Buy posts rated for ground contact
The Western Wood Preservers Institute says fence posts should be pressure-treated for ground contact. Its use categories list sawn fence posts under UC4A, the ground contact category. UC4B is meant for ground contact in severe environments and climates with a high potential for decay. Ground-contact posts carry a tag showing enough preservative for that use, so check the tag at the store.
If you cut a treated post, the cut exposes untreated wood inside. Re-treat a cut end with a wood preservative.
Set posts so water runs off
YellaWood's advice for posts set in concrete is to bring the footing above ground and taper the top so water drains away from the post. A vinyl maker's instructions put gravel in the bottom of the hole with concrete around the post, and set the depth by the local frost line. Check your local code for the frost line depth where you live.
Restain on a schedule
General Services Administration guidance on wood (written for historic buildings, not fences) says a single coat of semi-transparent penetrating oil stain on smooth wood should last 2 to 4 years, and a two-coat system up to 8 years depending on exposure. Fading color is the sign it's time. Stain wears faster near the coast, in cold climates (especially at high altitude) and on most south-facing sides.
Restaining costs money too: Fixr puts staining a fence at $1,000 to $3,000 on average.
Sources: University of Kentucky Extension, FORFS 03-05: Installing Wood Posts for Greatest Longevity (PDF); Western Wood Preservers Institute: Residential uses; Western Wood Preservers Institute: Preservative Use Categories; YellaWood: A Great Fence (planning a fence installation); Catalyst Fence Solutions: Privacy Vinyl Fence Kit installation instructions (PDF); US General Services Administration: Applying a semi-transparent or opaque stain to wood; Fixr fence repair costs (August 2025).