Jun 16, 2016

Study shows it’s possible to strengthen tree support system

Apple growers today want to produce large apples on small trees, with the trees packed as tightly as possible. Such a system can improve efficiency and yields, but has inherent structural problems. Heavy fruit loads, weak tree bases (due to grafted rootstocks) and narrow, densely packed layouts make the trees more susceptible to damage from wind, snow and ice. Growers need a durable tree support system, or trellis, to protect their long-term investment.

Peter Havard and Josiah McNutt, researchers with Dalhousie University in Nova Scotia, Canada, are studying ways to strengthen orchard trellis systems. They submitted a progress report to the International Fruit Tree Association earlier this year, summarizing their findings so far.

According to the report, the researchers studied trellis systems at three orchards in Nova Scotia’s Annapolis Valley. They recorded data on factors like post movement, standing angle and the location of each pole. They also collected soil samples and made general observations about trellis conditions.

In general, the orchards studied tended to lean in the east-west direction, which is also the direction of prevailing winds. The standard deviation showed that east-west posts had the greatest variability. Attempts to improve trellises should look at ways to stabilize post movement in the east-west direction – or lateral to the trellis lines – because it appears to be the weakest direction of the structure, according to the report.

The report detailed conditions in the three orchards studied:

Orchard 1 had several areas where posts and trees were noticeably leaning. Many trees were significantly bowed, and individual poles, as well as entire rows, were slanted. Despite the significant lean in the trellis system, the poles seemed quite solid and did not sway much when force was applied. There was no significant gap or gully in the soil around the pole. The researchers speculated that the reason for the severe slant in the trellis system was due to high moisture content in the fine sandy loam soil, due to rain or snow melt. The saturated soil appeared to have low strength, allowing the poles to shift, then re-solidifying as the soil drained, leaving the pole firmly in place. This theory explained why the poles were solid despite being slanted, and also why there was no visible gap in the soil around the pole, according to the report.

The researchers tested a potential solution. Water was added to the soil surrounding the pole until it was saturated, the pole was straightened (relatively) by hand and the soil repacked. It seemed to be an effective short-term solution, but is likely to be undone by future rain, snow and wind events, according to the report.

Orchard 2 had more mature trees than those surveyed at the other orchards. Because of the maturity of the trees (size of the trunks), the combination of tree and trellis probably gave exceptional support. The posts in Orchard 2 had the least movement with the least variation. Along the row they were similar to the other orchards, but lateral to the row they were the most vertical. Literature states that a well-supported tree will put less energy into building woody material, according to the report.

The portion of Orchard 3 surveyed was the newest of the trellises in the study, with few visible problems. The majority of the trellises in this orchard differed from the other two in the number of steel wires used in each row. Having up to six steel wires in a single row was common. That many wires allowed excellent direct support, with no signs of any tree movement along the wires. The trellis system was on a coarse soil, compared to the fine sandy loam supporting the system at Orchard 1. In some areas, soil in Orchard 3 had recently been tilled and mounded along the row to stimulate root development, which would likely improve stability too, and drain water away from the trees and posts, according to the report.

Early results

The trellis study included a laboratory component. Considerable time was spent running post-stabilizing solutions through a computer program called SolidWorks. The SolidWorks simulations used general property values found from literature, but actual field values would be more accurate, according to the report.

Lab work included testing a small- scale model of a post to compare soil-strengthening and post-enhancing methods. The model was inserted into a box filled with soil that was collected from the orchards. A Universal Testing Machine “applied a load on the post and recorded the overturn resistance.” Initial results clearly indicated that soil strength can be dramatically increased with different treatments, according to the report.

Early testing has shown it is possible to achieve statistically significant improvements to post stability. Other options being investigated include steel rods inserted vertically in the soil to enhance soil strength, soil cement around posts and bracing wings attached to posts. The next step is to apply the most promising options to the posts in the study and monitor their stability over the next year, according to the report.

Initial data suggested that the angle and movement of the posts had no effect on fruit yield, according to the report.

— Matt Milkovich, managing editor