When growing grape vines on wires, you must ensure the strength and stability of the posts and wire supports long before the vines are even planted. Since grape vines do not produce their own methods of support and must depend on other structures for vertical height, you have to provide a sound trellis for them. Because the fully grown plants can be very heavy, the supporting posts and wires must be installed properly before the vines begin using them. If the installation is improperly done, it is extremely difficult to replace the posts and wire after the grape vines have begun growing.
Dig a hole with a posthole digger at least 2 feet deep and at a 30-degree angle away from the ends of each row of grape vines.
Set a wooden fence post into each hole.
Shovel the dirt around the posts and pack it firmly. These are the anchor posts for each row. Therefore, they should be as stable as possible.
Drive a metal fence post 2 feet into the ground every 48 inches between the anchor posts on each row. These are the line posts. It is easiest to do this when the ground is slightly moist but not muddy. Use metal fence posts that have holes or notches already formed in them for stringing wire. Pack them in well by stomping on the earth around their bases. Each line post should be the same height as the anchor posts, forming a perfectly straight row. Position them so that their holes or notches line up with the row to make it easy to run the wire.
Wrap the end of a length of high-tensile strength, galvanised wire around the top of one anchor post and clamp it tightly with a wire clamp. String the wire through the holes or notches on the line posts. The wire must be one piece and not sag. Pull the wire taught, wrap it around the top of the opposite anchor post and clamp it using a second wire clamp.
String another length of wire in the same manner 2 feet below the top one. Again, it should be one piece and not sag. If the type of grape vine you are growing requires it, string another wire 2 feet below this one.
Use pressure-treated wood for the anchor posts. It is very important that the wood not rot. If you are growing grapes organically and cannot use chemically treated wood, use a hardwood native to your area that is not prone to decay. There are many styles of metal fence posts. Some have holes and some have notches for stringing wire through. A home improvement centre or farming supply store will have a selection you can choose from. They will also have a variety of appropriate wire clamps available. Use only galvanised wire so that it does not rust and fail in the weather. If you are planting a large vineyard, it is easier to plant the grape vines prior to installing the posts and wire. If your anchor posts show signs of weakening, secure them with guy wires anchored into the ground.