We have a bronze scuppernog that was 4 years old when we planted it and is now 6 years old, with some bad advice it has become a tangled mess. It has 6 main branches and is on a wire trellis that has been weighed almost to the ground. I was going to prune it back and put it on a steel pipe trellis. Should I prune it all the back to the main branches or just cut off all but two branches and prune them the right way? It was covered in small clusters this past spring but they got no bigger than the head of a pin.Thanks.
Without being there and pointing out specifics, the best advice I can give is that you should carefully examine your vine. Choose 1 good trunk that is relatively straight, or if crooked can be straightened by lashing it to a sturdy stake. From that trunk, choose two sturdy arms extending in opposite directions down the trellis. Remove everything else. It may seem radical to cut off so much, but new vines will be produced this growing season. Even if you make a big mistake, it will only be temporary. It's kind of like what an art professor told me once; to carve a bird, visualize a bird, then remove everything that doesn't look like that bird.
If the undeveloped clusters were flowers, then you don't have adequate pollination. You'll need a pollinator variety. If the undeveloped clusters were tiny grapes, there are other potential causes.