First, I hope the posts you set in concrete are not wood and are only direct set in concrete. If they are, they will rot in a little as a couple of years, but certainly long before the kiwi plants reach their old age. If so, the best time to redo them is now. You can use gravel or a steel sleeve to protect the posts.

I'd put a post in the middle. You say the end posts are 20' apart but the plants are each 7' apart, so the numbers don't add up. A middle post each 10' should be enough, Since you have three plants that probably means a post between each of the plants. The posts won't be handling much direct weight, they'll be helping keep the trellis line tensioned properly.

If you use pressure treated 6x6 or 8x8 end posts and 4x4 middle posts, this setup will handle a brutal amount of weight. User 1/4" aircraft cable for your trellis line. You'll also need a crimper and crimps to make loops at the ends of the cable, and turnbuckles. The turnbuckles allow you to increase tension as needed and also make installing the aircraft cable much easier.

On the end posts, wrap the cable around the posts several times, using an eyebolt or large staple to hold the last loop in place. You need a turnbuckle on each line at each end post. The trellis line is a single wire with loops on each end that hook into the turnbuckles.

For the middle post(s), set them in the ground slightly offset with a large eyebolt facing forward so that the aircraft cable travels straight through the eyebolt from end post to endpost without being pulled to the side.

You'll want to to anchor the end posts. Set a short 4x4 post in the ground at a 45 degree angle back about 6'. Use a loop of cable and turnbuckle on the angled post set next to the ground, then drill a hole through the end post about 8' up (where the top support line is) and run a loop of cable through it. Attach the two loops with a 3rd turnbuckle.

All of that said, there are other ways to make a trellis work. This is just my favorite method.

Wow, thanks for all the information.

A couple of questions. First, they used pressure treated 6x6's for the end posts. They are set in concrete. Do these need to be replaced, even though they're treated, or will they resist rotting. Also, in terms of anchoring the end posts, unfortunately the kiwi plants are right up agains the house, so there's no way to anchor the posts. Any suggestions? Think they can make it work w/o anchors?

Thanks!

Pressure treated posts will still rot in water, which is basically what happens when they are set direct in concrete. If this is a long term project -- and kiwis generally are -- I would replace the posts while you still have a chance of doing it without disturbing the roots too much. Personally, I think using steel post caps is the easier and more effective approach -- unfortunately it's also the more expensive. You can Google until you are totally confused about proper ways to set posts, but AFAIK, only gravel and end caps are long term solutions. While you are at it you will learn about reverse tapered holes for the concrete and other probably over-engineered criteria. You may also want to check in the Fruit & Orchards forum for more ideas.

It may work without end posts. The problem is that vines get really heavy. Aggressive pruning could keep this in check. Next to the house, I think you'd need to do this anyway 'lest the kiwis climb on the roof. But if you are resetting the posts, I would just sink them extra deep and maybe consider upgrading that post to 8x8. With adding 2 posts in between, you'd probably be fine.

Another option is to consider converting to an arbor. If you can spread the weight out over more posts, it's less of an issue. If you like arbors.