by Mark De Kleine, De Kleine Machine Co; and Karen Lewis, WSU Extension Specialist
Engineering Analysis for High Density Trellis Structures is a WTFRC funded project being led by Mark De Kleine of De Kleine Machine Co. in Prosser, WA. Co-PI’s include Karen Lewis, WSU Extension, Paul Booker, Century Steep, Othello and Chuck Pezeshki, WSU ME Department. An industry advisory committee has been involved to identify trellis current industry practices, failures, success and engineering needs. Objectives of the project include: I. Engineer a trellis structure based on fruit load in high density plantings; II. Analyze the trellis structure for fixed and variable loads and; III. Develop a design tool and planning guide for stakeholders. We have completed a field diagnostic/survey process and the following introduces some preliminary findings.
Trellis failures in Washington have been expensive and fortunately, so-far, injury free. The expense of lost capital varies mostly between two scenarios. Scenario 1 – the trees and trellis need to pushed out, new trees purchased and planted, and new trellis constructed. Scenario 2 – trellis can be salvaged and trees either cut back or top-worked. In both cases opportunity costs are lost.
Diagnostics of failures
Thirteen trellis failures were diagnosed by farm personnel as reported in a trellis background project survey (https://www.surveymonkey.com/r/DeKeineTrellisWTFRC) and/or during site visits by the project team.
Reported trellis failures
- 90% of failures occurred in systems that were less than 10 years old and of that 90%, 78% failed within the first 7 years.
- Both angled and vertical systems had failures at just about a 50-50 rate.
- In-row post spacing ranged from 40-55 feet, and more than half were spaced greater than 40 feet between posts.
- Cropload ranged from 40 bins/acre to 130 bins/acre.
- Half the failures occurred in Sept-Oct.
- In some failures, several feet of crop was growing above the top wire.
- High wind events were reported in 40% of the surveys.
- Roughly 40% were anchor or anchor-wire related failures including loose anchors, and rusted anchor wires.
- 60% were in-row post related (poles snapped or up ended).
- One failure occurred after a 20 hr watering interval in sandy to sandy-loam soil.
Participating field managers were able to describe weaknesses in their systems after the failures occurred and in most cases they could identify an event (wind, water, material, or layout) and a failure point in the trellis system. Inspection and mindfulness of critical points could prevent future failures, mitigating loss of capital and opportunity costs.
The next phase of the project continues with the engineering analysis of trellis systems and common materials used.. There are many questions that will be addressed through the duration of the project. What we strongly encourage at this point is an integrity inspection for your trellis systems, regardless of age and cropload. Identify a trellis czar! Comprehensive trellis inspections need to occur each season and outside of the formal inspections. Educate everyone in your crew what to look for and how to report concerns. Be sure to include the irrigators – they see more miles of trellis, more frequently than anyone. Make sure they know why it is important to identify, tag and report system weaknesses or issues. Make them the assistant czar!
What to look for during the inspection
- In-row wires: wires should not have kinks and breaks need to be fixed.
- In-row posts: check for poles that have become loose or are leaning beyond the intended angle. Look for bowing wood posts or, if applicable, bending at wire notches on metal posts.
- Anchor wires: Where is the eyelet? Below ground, exposed to the soil environment? Check wires for rust and breaks.
- Anchors: Check that they have not become loose.
The issue of cropload above the top wire will be addressed as most growers figure they need to grow at least 18 inches of crop above the top wire to meet their production goal. We saw crop 36-48 inches above the top wire in one particular failure. In some cases, this above the top wire canopy included 18-24 inches of structural wood. Croploads grown higher (distance from ground) in the trellis will increase the center-of-gravity of the system. It is advantageous, and good practice, to keep the center of gravity within the trellis system.
Give us a call if you have a trellis experience that you think will add to our collective trellis and super structure knowledge.
Mark De Kleine