Also, weather conditions can factor into what you use for posts. The UC photos shown are of very heavily pruned plants. If those get fuller, weight is going to be an issue, unless theyre keeping them thin full time, since theyre research plants, not crop producing. But in California, you also have to factor in wind. Heavy Santa Ana winds would bend those posts if the plants became more widespread and fuller, so that's something to consider. My 'post' of choice is still pressure treated lumber, coated with tar or sealant for the underground sections, and then wrapped in burlap. The 4x4 allows you to nail cross posts or wire to the sides, is thick enough to drill out holes through the wood without compromising the strength, and since its wood, you can more easily attach angle posts if necessary. Its not 'THE' way to go, but I find that it works best for my needs. A lot of this is more based on where you are, what space you have and the conditions youre planting in.