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Orchard Trellis Design – A Balance of Forces

Industry Best Practice

A Washington State study into designing high density orchard trellises has some useful messages and handy online tool for Australian growers.

A new online tool which can reduce the risk of trellis failure under high load by customising orchard systems to meet the demands of local conditions is the key output of a recent Washington State study.

But while the online Trellx model – downloadable from https://www.trellx.com/resources – allows the grower to enter orchard specific data on soil type, trellis type, wind speeds and projected crop load to guide construction, monitoring remains critical to preventing trellis failure.

Orchard trellis systems are a major capital investment. With more intensive plantings delivering higher yields. an increasing need for hail netting and growing adoption of overhead cooling the orchard trellis must be more capable than ever. A correctly engineered trellis can be the difference between a bumper crop and the costly loss of of fruit and infrastructure.

The Washington State apple industry study was initiated to put some firmer numbers behind orchard trellis systems in order to identify their current industry practices, failures, successes, and engineering needs.

It revealed some interesting messages which can help with trellis design principles here in Australia. A study of trellis failures reviewed as part of the project found that:

90 percent of failures occurred on plantings which were less than 10 years old:

  • Failures were found to occur equally in angled trellis and vertical.
  • Generally the failures occurred near harvest and crop loads ranged greatly from 50 to 120 tonnes per hectare.
  • 60pc were reported as in-row post related (poles snapped or up ended)
  • More than 60 percent of the failures had post spacing greater than 12 metres
  • Roughly 40pc were reported as anchor/anchor wire related (loose anchor, rusted anchor wires).
  • A number of failures also occurred where fruit was unsupported and growing well above the actual trellis height.

Researchers found that in most instances it was quite clear what went wrong in each failure. Whether a post is metal or pine, any material which begins to fatigue has effectively lost its structural integrity so once a crack occurs the trellis structure is compromised and will never recover. If a trellis is already fully loaded, then adding an extra piece on top of the structure will likely cause a problem.

Project leader Dr Mark De Kleine who presented the results at the International Fruit Tree Association Conference in Wenatchee, Washington State, earlier this year explained that “there is no wrong trellis if the trellis is engineered correctly.

“A trellis is a balance of understanding both the above ground forces and the below ground forces.   The above ground forces are mainly wind, canopy type, expected fruit load, wire span and structure.  The below ground is about soil and foundation which is what to do if the soils is not adequate to hold a structure.”

The study concluded trellis systems are a collection of individual components that must collectively work together to resist external loading. The weakest link often gets exploited and creates a potential for costly capital failure. A model to predict loads on trellises was developed as a tool to aid in the analysis and planning of current and future trellis systems.

The principle of the model is that every component of a trellis will have a point beyond which extra load will cause failure. The model aims to ensure that designs ensure every part of the trellis system is conservatively within that point.

Analysis of the failures studied led to the engineering analysis which underpinned the model

  • Embedment depth can be predicted for adequate foundation
  • Above ground forces need to be balanced through a trellis member to below ground forces
  • A fixed base, or fully embedded trellis member, reduces the soil variability to a predictable value
  • Wind loads are proportional to canopy area and porosity factor
  • Wind load is the critical design variable for trellis members that are embedded correctly
  • Post spacing can be determined using the Trellis Engineering Model

The main project output is the Trellx trellis engineering model available online..  This  tool can be applied worldwide and the website provides a spreadsheet which can be used to input parameters for new trellises such as trellis height, canopy type, wind speed, soil type and provides a guide on the design of the trellis based on these.  It gives a good comparison of relative material strengths of materials such as the commonly used metal versus pine posts

Wind speed is one of the trellis engineering model variables.  Wind also depends on the height of the trellis and whether it’s hitting either a more solid wall canopy/net or a more porous fruiting wall or an even more porous formal trained canopy where there are a lot of spaces for air to pass through.

In some circumstances it is suggested that it may be worthwhile varying the structure based on wind load, so with high winds it is recommended to increase the strength of the posts of the end rows where the direction of the wind is high.

The study makes no specific recommendations on the best anchor types, as long as the anchor chosen holds up the trellis.   Rather the recommendation is to have every trellis post as an anchor, because when the end posts are the only anchor it will not sufficiently protect the row from swinging side to side.  Cross wiring on the top adds strength side to side.

Where soil structure is the limiting factor the advice is that it is worth considering a more post dependent-system which is where more of the structure is put together and held above the ground rather than a soil dependent-system.

While the Trellx engineering model provides the much needed-numbers lacked in orchard trellis design, the main principle of preventing trellis failure remains, which is to monitor the trellis to identify any emerging issues early.

Look for:

  • signs of fatigue,
  • kinked or broken wires,
  • loose or bowing posts
  • Rust or breaks in Anchor wires
  • Crop above the trellis

A trellis will be at more risk of failure if they are built of poor quality material, neglected, allowed to lean, overstressed and unconstrained

General principles of good trellis design are

  • one third of the post must be in the ground,
  • use quality material of the correct size length and diameter.
  • With height ensure the crop stays within the trellis and not go above.
  • Ensure a good foundation.

Metal trellis with added structure to the top to assist with the application of the Drape Net. Prevents the Drape Net from resting on the actual tree canopy and the tree vertical growth is no longer limited. Picture: Orange, NSW, March 2017.

V-Trellis using concrete posts imported in bulk from Europe provides extra material strength compared with pine at a similar investment. Picture: Adelaide, March 2017.

An example of a post based-system where the metal pipe is pre-welded and assembled in the field. Picture: Washington State, February 2017.

Partial trellis failure caused by combination of fatigued material and heavy crop.

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Future Orchards orchard design and density Pre-plant preparation

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