Basically agree with the info here. Most big box “treated lumber” is treated to ground contact standards. You can order lumber treated for burial and that will last longer. I also put a coat or two of one of the copper based preservatives on the post prior to burial. The most important area to hit is 1’ on either side of the eventual ground level; that is where most of the rot happens.
There are several post wrapping systems out there. The best ones actually glue their plastic part to the wood, it is supposed to make it last the longest. And of course if you use naturally rot resistant wood (chestnut, larch I think, and a few others, not modern redwood) it will likely last as long as the treated stuff, especially if you paint/preserve the ground level area.
There are also things you can do to help it last longer such as not setting wood posts in concrete.
I think a good solution for a long term trellis is to buy a 6 inches PVC pipe and drill a 2 inches hole and stick a 2 inches pvc through for a T trellis and glue it with PVC glue. This will last a life time.
Tony
Lots of good info here OP. I think it comes down to where you live, how much you want to spend, and what look you’re going for. In my climate even treated buried wood rots very quickly. I have ended up using treated metal pipes of different diameters. Where I live The best deal is heavy duty scaffolding type pipes. They come with all kinds of clamps and accessories. Almost as cheap as wood. I drive them directly into the ground with a sledgehammer, or dig a small hole first and fill it with concrete after.
PVC holds up here only if you paint it. Otherwise it becomes brittle after ~10 years or so from the sun. But we are in a sunny location and at a higher elevation than many, but I suspect this holds for most in the SW.
In your case then pour cement in the center then it will stay forever after the PVC breakdown from the UV.
Tony
Just rub your pole with this occasionally to ward off those nasty UV rays… 
1 Like
Might work, you’d need some rebar or a lot of fiber reinforcing though, we get 100+mph winds each year and concrete alone won’t take that too well…
