What was it all about?

Increasing yields from young trees and reducing the time to first commercial harvest is a key target for growers establishing new macadamia orchards. Planting more trees per hectare in high‐density plantings can increase early yields, but the additional cost of the extra trees can be a disincentive. Also, if trees planted at high density don’t reach full production until after they’ve become crowded, then there’s limited benefit in having more trees per hectare. So precocity, encouraging higher yields from young trees, is important.

This project, which ran from 2015 to 2019, investigated options to reduce the time to first commercial harvest for young macadamia trees. In addition to looking at reducing the purchase cost of trees and the cost of establishing trees, the project evaluated a range of management techniques to improve tree structure, and promote flowering and fruiting. The findings are likely to be of relevance to other tree crops, too. The work was funded through the Hort Innovation Macadamia Fund using contributions from Plant & Food Research Australia and the Australian Government.

Using plant growth regulators

Young macadamia trees exhibit very strong apical dominance, with almost complete suppression of axillary shoot growth. This can make it difficult to produce trees with a strong fruiting framework and result in considerable work for growers to prune and train young trees.

The researchers trialled plant growth regulators to overcome apical dominance in the nursery. One regulator was found to stimulate already‐dormant epicormic buds into becoming active and producing new shoot growth on bare sections of wood on larger trees. This might be useful when older trees need rejuvenating.

Another growth regulator product was found to reduce shoot extension growth on young trees, avoiding one or two rounds of pruning and training in the first two years of growth without compromising yield. In fact, there were indications that treatments which created a smaller, denser canopy increased flowering.

Mini-grafted trees

Working with a commercial grower near Maryborough in Queensland, the project also evaluated the growth and productivity of micro‐grafted, mini‐grafted, traditional‐grafted and own‐rooted cutting‐grown trees. This work found that the less expensive mini‐grafted trees were just as early to come into production as traditional grafted trees.

Trunk girdling

Trunk girdling three‐ and four‐year-old macadamia trees was also evaluated, with four‐year‐old trees doubling the crop to over 2kg/tree, nut in shell. This advanced the timing of the first commercial harvest from these trees by one year, with potential to harvest by machine. More research is needed to confirm the findings in other cultivars.

Full details of all project components can be found in the final research report, available for download at the top of this page.