This is a final research report from Hort Innovation’s historical archives. Please note that as these reports may date back as far as the 1990s, the content and recommendations within them may be superseded by more recent research.

What was it all about?

This study was undertaken due to concerns within the macadamia industry that yielded in high density plantings (<8x5m) would decline with age and it was unclear what was the appropriate management strategy to follow.

The first process in tackling this problem was to determine whether there was a decline due to inter tree competition. To do this intensive investigations of yield response were carried out in orchards at a range of stages of canopy development. Production from 21 sites of cultivar 344 from a range of latitudes were monitored for up to 4 years. Analyses showed that, of several indices of orchard crowding, percentage ground cover (the percentage of the orchard floor covered by canopy) was the best measure for predicting yield.

The project showed that yield decline due to orchard crowding did not occur as early in the life of the orchard as previously expected. The results indicated that high yields tended to occur in orchards with high percentage ground cover, even up to complete canopy closure. The orchards at this end of the range which did not show any evidence of decline during the project comprised 16 year old trees planted at 7 x 4 m. Previously, Trochoulias (1994) and James (1994) had indicated that yield declined due to overcrowding at year 11 in 5 x 3 m and 7x 4 m plantings respectively.

Yield per ha at the most crowded site showed a significant decline during the project. However because decline occurred at only one site it could not be considered conclusive evidence of yield decline due to crowding. We could not rule out the possibility that some other factor not related to orchard crowding was causing yield decline at that particular site.

Further monitoring of sites was required to identify if and when yield decline occurs.

Unfortunately, serious damage was done to the project at the end of the fourth harvest when trees were removed from three key sites. These sites were our most crowded sites after the one that had already shown yield decline. This had left a considerable gap in the range of crowding in the orchards we were monitoring and had probably delayed the opportunity to identify yield decline by several seasons.

The second component of this project was to examine different canopy management techniques to maintain production. Treatments examined included: a light annual hedging to maintain a two meter inter row space; removing every second tree from every second row; and leaving trees untouched. Hedging had no significant effect on yield. Tree removal decreased yield in the first year in which the trees were removed and this significant relative reduction was maintained in the subsequent seasons.

The compiled evidence from grower tree removal trials generally showed reduced yield. In the short term, removal of trees appeared to be a poor option for managing orchard crowding.

Of the alternatives assessed the best method for characterising canopy structure remainred measuring the dimensions of trees and inter-row space. Neither leaf nutrient levels nor indirect estimates of leaf area index were able to separate differences in canopy structure as precisely as empirical measures. In testing indirect measurement of leaf area index it was found that mature macadamias had a higher leaf area index than any other commercial horticultural crop.

Detailed study of leaf characters confirmed that leaf age, leaf light environment and leaf nutrient content were affected by shading. These differences were not consistent across sites but were found in all cultivars investigated. Leaf nutrient content of the trial trees at most sites was high compared to industry recommendations. Yields were also high at the monitored sites and there were no commercially important detrimental effects on nut quality. The use of leaf nutrient content to manage tree nutrition needed revision in view of these results.