Constructing a trellis
The anchor should not be an afterthought.
When planting an orchard, don’t forget to leave room for the anchor, which is the most critical component of a trellis. It should be as far away from the end post as the height of the post, and the end post should lean slightly towards the anchor.
Anchors are the most important element in the construction of a trellis, says Kent Waliser, manager of Sagemoor Farms at Pasco, Washington. “It all starts and ends here. The problems with trellises falling down often go back to the anchor.”
Bill Johnson, with Wilson Irrigation and Vineyard Supply in Wenatchee, Washington, said an augur, or screw anchor is the type most commonly used for trellis systems, though for hard ground with rocks or caliche, a rock anchor made of heavy rib-back steel can be pounded into the ground.
Too often, the anchor is an afterthought, and growers don’t leave enough room for it, Johnson said. “It really is a bad situation because people will put the anchor as close as five feet to the end post and it becomes a fulcrum—it doesn’t anchor anything. It’s just pulling down on the top of the pole.”
The anchor should be as far away from the end pole as the height of the pole. Waliser leans end posts 15 to 18 degrees from vertical towards the anchor. The anchor shaft should point towards the point of attachment on the end post.
Screw anchors should be installed in undisturbed soil. If you dig a hole, put the anchor in, and cover it up with soil, chances are it will not hold, Waliser warned. Screw anchors come with shafts of various lengths. A three-foot-long shaft is suitable for short rows; a four-foot length is standard, and a five-foot shaft is best for longer rows or where there’s a lot of stress on the trellis. The diameter of the shaft can range from 5/8 inch to ¾ inch. Waliser uses an anchor with a 6-inch plate, but said an 8-inch plate might be necessary for longer rows, sandy soils, or a Tatura trellis.
The wire running from the anchor to the top of the end post should be as strong as the trellis wires combined. So if, for example, a three-wire trellis has 12.5-gauge wires with a breaking strength of 1,380 pounds each, then the anchor wire must be stronger than 4,000 pounds. It’s a common mistake to have a single wire from the anchor to the end post that’s equivalent to only one of the trellis wires, Waliser said. Either the wire must be stronger, or all the trellis wires can be taken past the end post and fastened to the anchor, but that might be more work. Consider using 10-gauge high-tensile wire for the anchor, not soft wire, Waliser suggested.
The wire around the anchor should be covered with PVC pipe to make it more visible to workers in the orchard and reduce the risk of damage by equipment.
Waliser said he is not a fan of the H-brace, because it’s difficult to get right and the failure rate is high. If it’s not done correctly, it will slowly, but surely, work its way out of the ground until the trellis falls.
Bill Johnson said the standard end pole is made of treated lodgepole pine and is 4 to 5 inches in diameter at the narrow end. An alternative for organic orchards, where treated wood cannot be used, are poles made of rib-back steel, which have holes every two inches where the wires can be fastened. They come in weights of 3 or 4 pounds per foot and are sturdy enough to support long runs and heavy cropping. Another nonwood option is used well-drilling pipe, which comes in various lengths. It does not have holes, so the grower will need to weld attachment points.
Waliser likes to use fully pressure-treated CCA (chromated copper arsenate) wooden line poles. He recommends driving 12-foot poles at least 30 inches in the ground for most eastern Washington sites. “If you want more height, get longer posts,” he suggests.
Johnson said there’s been a move towards 14-foot poles for growing taller trees with more bearing surface. Steel poles, suitable for organic production, come with notches where the wires can easily be placed, which reduces the cost of installation. Metal is also suitable for V and A trellis systems.
Pole spacing depends on the rootstock, row length, tree height, and soil condition, but Johnson recommends that line poles be spaced no more than 40 feet apart. Though closer spacings make the trellis more expensive at first, it is cheaper to do it right than to have to fix a collapsed trellis, he said.
Typically, line wires are 12.5-gauge Class 3 galvanized steel with a minimum tensile strength of 180,000 pounds per square inch, and a breaking strength of 1,380 pounds. Use a spinning jenny wire dereeler and take care when working with wire, Waliser advised. Workers need gloves and eye protection.
Johnson said there’s a trend to order large stands of wire weighing about 2,000 pounds, rather than the 100-pound rolls. A stand contains an average of 7,400 feet of wire, and reduces the need for mid-row splicing.
The lines should be tightened to a tension of 200 to 250 pounds p.s.i. so that they can hold a crop without sagging. If the trellis is installed in hot summer weather, bear in mind that the tension of the wire will be significantly greater when it contracts in the winter.
“If you have really long runs, that wire can shorten up to a couple of feet,” Waliser said. A wheel tightener can be used to tighten the wires if they become loose after being installed.
Because tree rows are usually oriented north to south, and the wind comes from the west, Waliser staples the wires to the upwind (west) side of the posts so that the staple is merely holding the wire, and the tree is pushing the wire into the post rather than away from the post.
He uses two-inch barbed and galvanized staples inserted so they straddle the grain of the wood. If the row goes over a hill, more staples may be required in the posts at the top as the wires are trying to pull down. Three- or four-inch spike nails can be used for added reinforcement in any line or end post. When the row goes through a valley, the wires are trying to pull up, and another anchor might be needed at the bottom of the dip.
Waliser advises against making holes through the posts to thread the wires through. Wooden posts have a protective treatment on the surface, and he believes that the fewer entries through that protective layer, the longer the post will last.
The trellis should be designed to last the life of the orchard, or even longer if the orchard is grafted over to a new variety. “You don’t really want to scrimp, at least in the construction part of it,” he emphasized.