How should I build a trellis for grapes grown in a high tunnel?
Answer: The specifics of how you build your grape trellis will depend on the system you select, but there are many common factors across all trellis systems. It will behoove you to set your trellis posts before planting the grape vines in the high tunnel, so as to avoid damaging vines in the process of setting posts. In addition, it may be easier to set trellis posts before the high tunnel construction is finished so that the high tunnel bows do not obstruct the equipment you are using to set posts. However, you will want to have the ground anchors for the high tunnel laid out and set before beginning on the trellis to know exactly where the footprint of the high tunnel will be.
The end posts for your trellis should be nine to 10 feet tall and 4.5 to six inches in diameter, set three feet into the ground at a slight angle leaning away from the row. Treated pine or locust posts are commonly used, and pressure-treated posts will last an additional 10 to 15 years. Organic growers are not allowed to use pressure-treated wood, however, and should refer to the ATTRA publication Pressure-Treated Wood: Organic and Natural Alternatives for an explanation of the rule and a summary of options. Products such as WoodGuard™ are available for organic growers and will have a lifespan similar to pressure-treated lumber, though the cost will likely be higher.
A screw anchor or H-brace should be used to brace the end post. The line posts used within the row should be eight to nine feet tall, at least three inches in diameter, and set three feet deep. Although field spacing for line posts is typically 18 to 24 feet, in the high tunnel it is recommended to space line posts 15 feet apart to support the vigorous growth of the vines and heavy fruit set. High-tensile-strength galvanized steel wire should be used for the trellis wire, 12-gauge or heavier. A wire tightener should be inserted in each training wire to allow for tightening each winter after pruning. It is recommended to use metal cross-arms for a Geneva Double Curtain trellis system and metal support arms for a Modified Double High Cordon system.
Don’t miss the new ATTRA publication High Tunnel Grapes: Pruning, Trellising, and Training to learn much more on this topic. This publication is the second in a series on high tunnel grape production, based on research led by Dr. Elena Garcia at the University of Arkansas. It covers the topics of selecting and constructing a trellis system, pruning grapevines, and training them to the trellis system.
Note: Mention of specific brand-name products is for educational purposes only and does not constitute endorsement by ATTRA, NCAT, or USDA.