How deep should a post be in the ground, compared to how tall it's going to be above?

Question

I have some grape plants coming in next year. My plan is to make a sort of trellis-fence to partly break up the view of the road from my yard, and of course help the grape plants vine out.

What's the ratio for post that's buried in the ground vs post that's showing above ground?

I live in Zone 6, so we have some pretty mild winters (rarely gets below 0, never below -10 I think). I was thinking of using just 4x4 posts with about 8 feet above the ground. I was thinking maybe I would put 3 feet in the ground and have 8 feet above. Would that be enough?

The ground here is made of about half dirt and half clay. So I would imagine it would hold the posts pretty well.

Answer 1

When you are going that high, I'd use at least a 6x6. I'm in zone six, with similar soil, and the frost line has never gone below 2', so I usually go 2 1/2-3' down. There isn't really a ratio, but grape vines once mature can pull down some serious supports.

Make sure you really brace the outside posts, or they will pull in. I'd also recommend attaching planks along the top of the support to keep the posts from pulling together, and to protect the ends of the posts.