Gardening and Horticulture - Depth of posts with metal post anchors

I'm building an arbor with a 5 foot section of trellis on either side. Because we're on rocky ground, I'm not sure how deep I can dig the holes for the 6 4X4 posts that I need to put in. My plan is to dig a hole about 18 inches deep, use an 8 inch sonotube, fill it with concrete, and then put a metal post anchor into the wet concrete. Once all the concrete has cured, I'm going to install the 4X4 posts, which should be no taller than 6 feet high.

My question is whether or not the concrete in the hole will be deep enough or strong enough to support the posts. We live in an area that doesn't get much frost. Would it be easier to just dig a hole and fill it with concrete, rather than using a sonotube? Are the metal post anchors sturdy enough to support a trellis (6X6 inch square lattice) that's about 5 ft X 6 ft?

put them as deep as possible, not more then 30" or so though. anything will work, the sonotube, or just concrete. perhaps save some trouble and put the post in, get it straight, then fill around it with concrete. depending on wind conditions it will hold through anything short of a hurricane. :gmorning:

If the structure is stable otherwise, the posts on metal mounts will be satisfactory. Pouring the footers then installing the metal mounts will keep the wood from rotting away underground. Sonotubes tend to be a bit on the expensive side. I would shoot for digging a straightsided hole and pouring into the undisturbed earth. If frost heave is of no concern, then 18 inches should be enough.

Hope this helps.