If you're looking to build a trellis for your grapes, you might be wondering which direction to go and what materials you might need to accomplish the task. I'll go over why we chose to trellis our muscadine vines using what is known as the Geneva Double Curtain or GDC, our trellis dimensions, and the materials we used. I'll also go over how to train your new muscadine vine to the trellis and how to avoid drooping vines when they are laden with fruit.
Why GDC
The GDC trellis is used for vigorous grape cultivars, and if you've ever seen wild muscadine growing in the south, then you know just how vigorous this vine can be. GDC trellising can also save space while providing a higher yield per plant. It is a little bulkier than growing your vines on an existing fence but vines will be a lot easier to prune and maintain on a proper trellis. When compared with the "T" trellis, the material requirements don't differ much but you'll have twice as many offshoots from the main stock that can produce grapes, therefore increasing the yield. So, you basically get more bang for your buck, or in this case more grapes for your wine (or juice or pie or... well the list could go on, but you get my point).
The Set Up
Your goal with the GDC trellis is to create a birds-eye view of an "H" with each vine. To do this you'll need to create a "T" with your supports then thread wire through each side of your capital "T". Don't worry, it sounds more complicated than it is. I'll provide you with some drawings and dimensions along with some pictures so you can install your trellis with ease.
Materials
This is a materials list for a 50ft trellis but you can adjust the plans to suit your needs
4 - 4x4x6 pressure treated posts
2 - 2x4x10 pressure treated boards
6 - 6 inch bolts and nuts
2 - 6 inch eye bolts and nuts
16 - washers
150 ft of 17 gauge electric fence wire (feel free to use a thicker gauge but so far this has worked just fine for us)
post hole diggers or shovel
blunt nose pliers
monkey wrench
Drill
Layout
Before you start to get happy with those post hole diggers you'll want to determine the best location for your trellis and the length. Grapes need full sun to thrive, so be sure your site gets plenty of sunshine. Also, be sure to leave enough room for your vines to grow without taking over any other nearby plants or buildings. We built our two trellises side by side with 5 feet between them to walk. It wasn't enough. In the summer the two sides come together to form a single canopy and we have to either crawl under the vines to harvest the grapes between the trellises or prune prior to harvesting. It's kind of a pain in the knees. I think 10 feet would have been better between trellises and 5 feet sufficient between the vine and a solid object like a wall or fence.
We space our plants 10ft apart and planted 5 different varieties on each trellis. Each trellis has 3 "T" supports and anchor posts on each end (we let the vines grow along the wire leading to the anchor posts for an additional 10 ft of trellis space). Some varieties could easily take up a 30ft stretch by themselves and others barely reach the 10ft mark. I personally feel as though the 10 ft spacing is adequate but 15-20ft would also work well.
The Build
First things first, decide where you want your trellis. We chose a spot that receives full sun. We have our trellis situated lengthwise going from East to West to eke out as much sunshine in the day as possible.
Measure the length of the trellis, then mark where you will be placing your supports and braces. The total length of our trellis is 60 ft including the supports.
We placed our supports 25 ft apart. So, we marked spots at 0ft (this will be the first brace), 5 ft (for the first "T" support), 30 ft (for the center "T" support), 55ft (for the final "T" support), and 60 ft (as the end brace).
Cut one of the pressure treated 4x4x6 posts in half to create the two end braces. These end braces will only be about 1 ft out of the ground. Drill a hole 2-3 inches down from the top of each post. Make sure it goes straight through. Thread an eye bolt through each of the brace posts and tighten down.
Cut the 2x4x10 boards into three 5 ft sections to create the cross boards that form the "T" shape of the supports. Drill a hole on each end of the board about 2-3 inches in and centered by width (for threading fence wire through). Then pre-drill two holes in the center of the board to bolt to the posts.
It helps to pre-drill the posts and pre-assemble the "T" support prior to putting them in the ground (this is something we didn't have the foresight to do and getting an extension cord long enough to reach our trellis site was tricky). Make sure your drilled holes line up and are large enough to place the bolt through. The other three 4x4x6 ft posts you have left will be used to assemble the "T" supports. Simply line up the holes and assemble one board on one end of each of your three posts, creating your "T"s.
Once all of your posts are assembled into the three "T" supports and two end braces, it's time to get those post hole diggers out. Dig each of the spots you marked 2 ft deep and place the the three "T"s in the three center holes and the two brace posts( with the eye bolt facing inward toward the "T" supports) in each of the end holes. Backfill your holes with sand and water.
I don't really recommend adding concrete to the supports in case they need to be replaced in the future. Having to pull up concrete with a broken or rotted post more difficult and frustrating than just pulling up a post in soil.
The supports do not have to be perfectly level in order for the trellis to function properly, as long as they are all approximately the same height as one another (with the exception of the two end brace posts, which are only a foot out of the ground each).
Now that the supports are up and sturdy, it's time to thread the fencing wire through it all. Starting at the brace post, thread the wire through the eye bolt and tie it off. Here's a great video that shows you how to tie a knot in fencing wire. In this video they are tying the knot to a fence post. For your trellis, you'll just thread the wire through the eye bolt instead of around a fence then follow the same instructions for tying the knot.
Once the wire is tied off, roll out enough wire to thread through one side of the trellis all the way to the other brace post plus a few extra feet. cut the wire and thread it through one side.
DO NOT tie your wire to the opposite eye bolt yet! FIRST you'll need to install the heavy duty in-line strainer. This device will allow you to add tension to the trellis wire so that it doesn't sag with time and the weight of the vines and ample fruit. We usually have to tighten the tension once every year or two or after a big storm or hurricane. This is a great video to follow to help you install an in-line strainer.
After attaching the strainer, finish the first side by attaching the other end of the wire to the other eye bolt. Repeat these steps for the other side of your trellis. Tighten the strainer to add tension to the trellis wire on both sides.
Now all that's left for you to do is decide which vines you want to grow on your trellis. Once you have that figured out, check out how to train them to your new trellis HERE.
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