Effects of girdling on fruit abscission, yield and shoot growth in macadamia

•Girdling caused slight mitigation of the negative effects of pruning on fruit abscission and yield.•Girdling reduced shoot growth and tree height increment.•Girdling did not affect cumulative yield over four years.•Further monitoring is required to fully assess girdling as a tree size control strategy.

In Australian macadamia (Macadamia integrifolia Maiden and Betche, M. tetraphylla Johnson and hybrids) orchards, trees are generally side-hedged in early spring to maintain the inter-row for efficient orchard management. Hedging at this time increases fruit abscission and decreases yield due to competition for carbohydrates between fruit and post-pruning shoot growth, and to the loss of photosynthetic area. Girdling increases fruit set in many crops and the possibility that girdling might mitigate the effect of pruning on fruit abscission and yield was investigated in this study. Girdling was also investigated for its potential to control shoot growth and tree size. In the first experiment, in early spring around anthesis, trees were pruned (P), pruned and girdled (PG), or were left as unpruned and ungirdled control trees. Fruit per raceme over time, percent of racemes with fruit at 20 weeks post anthesis, fruit number per tree, nut weight, yield, and the shoot length and number of the post-pruning flush were measured. In a second experiment, trees were pruned (P) or pruned and girdled (PG) in autumn, and the shoot length and number of the post-pruning flush were measured. In a third experiment, trees were girdled at anthesis in 3 years over a 4-year period and shoot growth, tree height, yield, fruit number and nut weight were compared with ungirdled control trees. Girdling mitigated the effect of pruning on fruit abscission and yield but the mitigation was slight with PG trees producing 28% less yield than unpruned control trees. Girdling reduced shoot length and number but more so on unpruned branches (26% and 28%, respectively) than on pruned branches (10% and 14%, respectively). The tree height increment in girdled trees was around half that of ungirdled trees. In the 4-year study, girdling increased yield by around 10% in two out of four seasons, and decreased it in one season. Cumulative yields were similar for girdled and ungirdled trees. This study has shown that girdling was not useful in mitigating the effect of pruning on yield. However girdling was effective in controlling shoot growth and tree size, with no reduction in cumulative yield, and may be useful as a tree size control strategy. Further monitoring of long-term effects of repeat girdling is required to confirm this.