I'd prefer the single wire at 6ft or GDC. The yield should be somewhat higher with GDC because the canopy will be larger and intercept more light.

Having a wire at 3ft and 6ft never worked well IME. The canes on lower wire get shaded out by the higher wire and add little to production. Just don't see the reason for having fruit buried below a higher canopy.

I've grown all of those cane pruned with good results. Haven't tried spur pruning.

I use rebar, but @ 40ft might get too expensive. Don't have any problem with production on the bottom row.

This photo was taken 2mo ago. The grapes have lost all their leaves now.

Thanks Fruitnut, and glad to hear about the cane pruning since easier. Are the 'somewhat higher yields' worth the trouble of GDC? Do GDC get tangled? I don't mind building it, but I would prefer easy vine maintainance unless the yields are significantly higher with GDC (1.5-2x?), or the vigor of the vines are easier to manage with GDC. If I do the single wire (which I am leaning towards) it will be one at 6 feet, and I will have 8 foot per vine. Suggestions if that is enough room per vine?

Thanks campv- I have a big spool of 9 gauge already. Why is rebar better than a heavy wire? Not having to re-tension?

midlin:

My GDC is at 7.5ft height. I like this because I can thin and harvest from below. Besides it looks nice. But it won't out yield one wire by 50% unless the wires are more than 36 inches apart.

My vines don't get tangled any more in GDC than with one wire. I love pruning both.

Eight ft per vine is enough.

Az is different than a more humid climate as to having fruit buried in the canopy. And the vine pictured above looks barely trained to me.

Here's a picture from under my GDC this summer. These are first crop, second leaf, potted plants so not much fruit.

{{gwi:120620}}

Mid-lin:

I grow Canadice and Concord on a 4-arm kniffen system. I have the lower wire at 3' and the top wire at 6'. While this system works for me, I too have found that the canes on the top wire produce much more fruit than those on the bottom wire. I leave about 10-15 buds per cane on both top
and bottom wires when pruning so I can only assume the difference in yield is due to sunlight differences.

I grew up with grapes pruned to a 4-arm kniffen system so
I am not changing now, but cordon pruning has always intrigued me.

Beautiful pictures as usual, FN. But how do you arrange for pollination inside your green houses? It is a question that occurred to me when I saw no wasp damage, then I remembered that you grow indoors. Obviously, insects must have free access in spring and late spring.