I am looking to build a trellis to be shared between grapes, blackberries, figs, and other espaliered fruits and vegetables along an 80 foot retaining wall.
The retaining wall currently under construction, and I am reluctant to set the posts in the gravel backfill as we go. It just seems a bit tedious.
Is it possible to set free standing posts into something like a deck block, or a notched stone? Or somehow incorporate it into the retaining wall?
If the metal T-posts are sufficient on their own to support lots of fruit, then that would be desirable as I wouldnt need to dig a hole for the installation.
What is the maximum length of wire that can span between two structural posts? If setting wood posts in the ground is the only way to have a strong trellis, then I want to do as few as possible obviously.
I would really like to set the posts after the retaining wall is finished.
Without knowing what kind of load the wires will be under is difficult. I suppose it depends on the wire gauge and tension applied. If you can pull them tight like a high tensile fence, you will have less sag. I guess it depends on how many plants. You could always add posts as needed if you see sag...but that could be tough. Why not just put in 8 posts, one every 10 feet....or you could go one every 8 feet and set 10. I tend to over do this type of thing.
You can set wood posts without digging a hole, sharpen one end to a point and use a loader tractor or whatever you have to press it in. Works great, compacts the soil as it is driven. I like the look of wood over metal. Setting wood posts in gravel is almost better than concrete, if you tamp in the gravel as you go.
You could cast concrete forms that can hold the posts, but the foliage may overload the weight of the concrete in a heavy wind. I think they would have to be crazy heavy. It may be less work to set posts.
posted 6 years ago
I think I will try my hand at digging a hole in the gravel backfill once the wall is finished, or just dig it into the dirt, or try the sharpened post method. All I have is a sledge, so let's hope that works.
For a small pure vertical load, you can get away without concrete--just driven in the ground. For a substantial vertical load, you should dig a hole and put the post in a concrete footer and then you can backfill with dirt. You can use a rock or urbanite just to prevent further sinking but actually setting the post is best. Any side load increases the need for concrete. For a cantilevered trellis like you pictured in the other thread, you will need a substantial amount of concrete in each hole.
I can't tell the total length, but usually it is best to build super-strong end posts and let the tension of the wire do most of the work, with the middle posts only holding a little vertical load.
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Some people are selling concrete post supports now. It is a pretty octagonal block of concrete with squares in the top for either 4 X 4 or 6 X 6 posts. they cost $6, about the same as a bag of concrete. I plan to use mine on top of an existing concrete pad, where I will not have frost heave.
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