By Richard Jauron
Extension Horticulturist
Grapes and raspberries can be grown in the back yard, with a little bit of ingenuity and work. Creating a trellis to support the plants in a home garden is within the means of most gardeners, with a little bit of effort.
Iowa State University Extension and Outreach horticulturists can help answer questions about how to create a backyard trellis. To have additional questions answered, contact the ISU Hortline at 515-294-3108 or hortline@iastate.edu.
How do I construct a raspberry trellis?
There are several different trellis systems. A two-wire permanent trellis is commonly used to support raspberries in the home garden. Its construction requires wooden posts, No.12 or 14 galvanized wire, and two-by-four lumber. The wooden posts should be 3 to 5 inches in diameter and6 to 8 feet long. Posts should be set 2 to 3 feet into the ground and spaced 15 to 20 feet apart.
Near the top of each post, nail or bolt a 24- to 30-inch-long crosspiece. Then run or attach the galvanized wire through the ends of each crosspiece and down the entire length of the row. The two wires should be spaced about 18 to 24 inches apart and positioned 3 to 4 feet above the ground.
A temporary trellis may be constructed of posts and twine. Set the posts approximately 15 feet part. The canes are supported by running twine between the posts. This temporary structure is most suitable for fall-bearing red raspberries grown exclusively for the fall crop.
How do I construct a grape trellis?
Construction of a grape trellis is similar to constructing a farm fence. The trellis must be substantial enough to carry the weight of the vines plus a heavy crop during high winds. Basically, the trellis consists of two or three wires, one above the other, stretched tightly and secured to firmly-set posts.
End posts serve as the anchor points as well as wire supports. End posts are generally 8 feet long, with a diameter of 4 inches, set approximately 2 feet deep in the soil. They may be braced in several ways. A common method is to set an extra post within a few feet of the end post. A heavy piece of wood or another post makes a good brace between the two end posts. Line posts are also 8 feet long, but with a diameter of 3 inches. They are set approximately 2 feet into the ground and spaced about 24 feet apart within the row.
Use galvanized wire for the grape trellis. Galvanized wire is durable and does not cause serious wire chafing of young vines. Wire sizes commonly used include numbers 9, 10, or 11. Wires are secured to end posts in various ways. A common method is to wind the wire around the post once or twice and then twist the end several times around the wire as it is stretched to the next post. Some gardeners use special devices to attach the wires to the end posts because they simplify tightening of the wires. These devices employ cranks that eliminate removing the wires from the end posts when tightening.
Wires are fastened to the line posts with ordinary staples. Space the wires vertically according to the training system to be followed. For example, a four-cane Kniffin system would use two wires. One wire should be 3 feet above the ground and the second wire 6 feet off the ground. The six-cane Kniffin system uses three wires positioned 2, 4 and 6 feet above the ground.
The best time to construct a grape trellis is during the first growing season. Tying new shoots to the trellis wires allows for straight grapevine trunk development in future years.