Apples and pears will be heavy.
Use gravel to keep wood dry, cement then fill with gravel. Concrete is dirt cheap, easy to mix. If you were by me I would give you some. I had a chimney done and the crew left 600 pounds of unused concrete. Enough for 3 lifetimes...
You could use anchors for additional support.
I have both 4x4's in cement, and steel conduit risers for my trellis systems. I don't use cement with the conduit, but they are 3-4 feet in the ground. Well so are my 4x4's! I would buy 10 foot posts.
Also I would mound your trees in the raised beds as garden soil has a lot of organic matter which will disappear with time. If you leave level your trees will sink down. Not good!

Obviously you can't get 7 feet of exposure out of a 6-foot post.
The 8-foot length would only allow burying one foot deep.

If I was doing this project, I would start out with 10-foot posts, bury 30-36" deep, use packed gravel or concrete. The tops can then be trimmed if needed. Then in 20 years I can go pound on the posts and they won't wobble.

You could use 6-foot posts buried deeply and extend them upward with more wood.

This post was edited by larry_gene on Mon, Oct 27, 14 at 22:40

I am thinking of doing something similar to your setup, and I don't want to use anchors on the end of each row. I'm not sure how this will work, but I might use 10' metal posts for chain link fence secured horizontally at the top of each 4x6 to prevent the wooden posts from collapsing in toward each other.

Benny

Posts in the above image will be unstable for forces perpendicular to the row. You need at least a tripod arrangement using anchors for such a shallowly buried large post.

Using heavier-duty methods for garden structures requires more work up front, but then you don't have to mess with or modify it over the years.

I have found that the methods commonly used, concrete, burying 3 feet etc. are correct. If only one foot, it will fail. I have pole beans pull down a trellis, they weighed nothing compared to apples and pears. So yeah, do it right. The branches may bear some of the weight, but being tied, the tree depends on the support, I would think? If the branches bend, the weight is going on the trellis. Plus if only 1 foot down, above the frost line, the concrete will heave upward.
The 3 feet is also to make sure you are below the frost line.

Drew,
I believe that in zone 8b the upheaval from frost would not be a problem. I could be wrong but I don't think so. And Sym only had 8 foot posts max.

The inside braces will counteract any inward & downward stresses same as outside anchors but would stay within the 20 foot parameter.

Larry, first these are only 2 trees and 20 feet and Sym is using 4x6 posts, quite hefty for the load.

Second, these are espaliers so they will be very open and allow the wind easy passage so the perpendicular forces would not be all that powerful.
Third, the trees themselves will be staked adding additional strength.

And lastly, keep in mind that it is only 20 feet andd 2 trees. If the rows were longer and in much colder zone I would agree with both you and Drew , but as this project was described by Sym... I must respectfully disagree.

Mike

Here are some pears that I found on the internet. I had a similar support for my pear espalier when I lived in Italy.

Sym...

BTW if you have 8 footers you can sink them 2 feet into the ground. There is nothing wrong with having the top trellis wire at the top of the post at the 6 foot level.

Mike

I am thinking of doing something similar to your setup, and I don't want to use anchors on the end of each row. I'm not sure how this will work, but I might use 10' metal posts for chain link fence secured horizontally at the top of each 4x6 to prevent the wooden posts from collapsing in toward each other.

Benny,

I have used 3/4" metal electrical conduit for this purpose (I painted mine to sort of match the posts, but you could just let it weather). I'm using 10 ft posts sunk 2.5 ft in the ground, no concrete. Things rot here. Too many fences lean over in 5-10 years because the posts have rotted at the base where they meet the concrete and then the entire mess has to be excavated to get out the concrete mass. I'd just as soon replace the post without all the hassle when the time comes. Properly packed in, the posts seem pretty solid to me.

Steve as I mentioned earlier in the thread, i too use 3/4 inch and it rocks! Two grades of steel here. I bought the cheaper grade, not as thick, but still super strong.
Also it seems to work for long distances too. My trellis for raspberries is 24 feet long. Although the raspberries add little weight.

Thank you all for your responses! I did find out that the 4"x6" posts i have are 8ft long. I am worried about the wood rotting over time, since i technically leave in a wetland zone, so i'm thinking bury the posts 1 1/2-2ft down, and since they are in a 1 ft raised bed, that will still give me 5 feet or so to work with for the different tiers of branches, and i might only use gravel and really pack it in, just in case i have to replace the post sooner than later due to the moisture levels. does this sound okay?

Gravel around the poles will not only be OK but it will drain all of the water away from the wood poles. Concrete tends to let water pool and accelerates the rotting. This is info that I got from wooden fence builders in California. 1 1/2' is good for the depth. You can also paint on treatment for the wood which will help you with the number of years that you'll from your support.

I would disagree with that. Concrete doesn't absorb any water. The post will be bone dry where the concrete is. You want to use both or cement the whole thing. I have seen may rotted posts, and none were ever rotted where the concrete was.

Perhaps not, but that has not been my experience here at the coast with hydrophobic dune sand for "soil". Fence posts rot off at the level of the concrete and often when the concrete is finally excavated the wood inside it is soft and flaky. Concrete can shrink away from the posts as it hardens and even a small gap is an entry point for water. Of course, since it seems determined never to rain again here this may eventually become a non-issue...

Here about over 25 years ago I bought the arsenic treated wood. Not one piece of it exists that was laying ON TOP of the dirt. The posts I pulled from an espalier and sank in concrete for deer hanging are still there in spite of the post holes being too small and the concrete cracking at the corners of the posts.
Modern copper treated wood is probably not going to last as long.
Mound the concrete so the water runs off, don't put dirt over top of it to have grass up to the posts.

You're looking at 30 years or more. That's a long time for dirt to invade gravel. And it's a high water table?
Not that replacing the posts will be a big deal. By the time the posts rot the trees will stand on their own and the support is just there to help the tree with weight. It would be no problem to take the cables out of a post, pull it and leave the tree blow in the breeze while new posts set for 24 hours.

I'll probably be doing it with a grape trellis which is going to be a lot worse