Thinning Avocados for Increased Production
Adopting a new thinning technique to improve avocado production
Avocado trees should be regularly shaped to maintain their maximum height of between 4 and 6 metres, allow light to penetrate the canopy, allow access for fruit pickers on Hydraladders and to regularly renew bearing wood; allowing the original tree spacing to be maintained without thinning and promote optimum fruit yield per hectare.
New Zealand has only been commercially growing avocados for about 30 years and the life of a tree is thought to be around twice that time. New Zealand is on the climatic limit of avocado production because the plant is a sub-tropical species. Trees are also subject to a root disease called phytophora and are very shallow rooted and sensitive to water-logging. Once successfully established, trees grow vigorously and fruit on new wood, so if unpruned, they have to keep getting bigger to keep fruiting. The initial silviculture recommendations were to plant on 7 x 7m spacings (or about 200 trees per hectare), let the tree canopies grow mainly untrimmed and begin thinning trees from about year 8 onwards, as the canopies start touching and thereby restricting each other’s light. At the time of first thinning (taking out every second tree) the stayers would be perhaps 5m high. They keep on growing and expanding until a second thinning was recommended (perhaps year 15 onwards) by which time remaining trees could be 8-10m high. At this time the fruit is getting out of reach of the pickers on the highest Hydraladders and the air-blast sprayers cannot reach the highest leaves and fruit. After the second thinning there may be around 50 trees per hectare.
While a mature tree of 10m-plus high and at least 25m canopy diameter is very productive, it also contains a huge branch structure of almost empty space and covers a large area of shaded ground and leaf litter where the shallow roots reside. They are also subject to structural damage and prone to drop fruit in high winds. Fruit can also be blemished by leaf rollers and greenhouse thrips because it has been inadequately sprayed or because it is exposed to wind-rub. Avocado trees can grow taller than their orchard shelter belts, which exposes them to storm damage. In terms of fruit production per hectare, a large number of smaller or medium-sized trees with maximum sunlight and crop protection is better than a small number of very large trees and open spaces. When mature trees have been razed on an avocado orchard it is very hard to establish seedlings in the same ground.
From around four years of age multi-leader avocado trees can be shaped by selective cutting of branches, removing cross-over branches, opening up “windows” in the canopy for Hydraladder pickers and for spray penetration and bringing sunlight into the centre of the tree. Avocado trees commonly have five to seven leaders arranged in a rough vase shape and perhaps one taller central leader, often branched. The new shaping approach prioritises removal of the central leader, which is prone to getting too long and bearing inaccessible fruit; and one of the side leaders, usually the one towards the north and the direction of day-long sunshine. That enables sun to penetrate to the “inside” of the tree canopy and the shape of the tree initially resembles an armchair facing north. In the first year of pruning 30-50% of the canopy is removed and low vigour “flush” cuts should be made. Where necessary in heavily pruned trees some high vigour “stumped” cuts can be made to promote regrowth. Avocado limbs exposed to light quickly send out regrowth shoots which are very numerous, and can be selectively pruned. In every subsequent year, other side leaders or main branches will be taken out or shortened so that over a number of years the tree structure is renewed while being controlled.
Immediately after limb removal, the potential fruit yield of that tree may be reduced for a season, but regrowth flowers and sets fruit in the second year. Shaped trees are not being allowed to fruit to their maximum yields, but collectively a larger number of 80% yielding trees produce more avocado tonnage that a smaller number of 100% trees. The whole orchard is kept at a more manageable size for smaller Hydraladders and good spray coverage. Smaller Hydraladders, for instance the 3.6m entry model, are cheaper, safer and more easily manoeuvrable, which means faster fruit picking. The ground space of the whole orchard is productive and the difficulties of establishing new seedlings in an existing orchard are minimised.
In the spring of 2012, the Whangarei avocado growing region experienced cool day and night temperatures which hindered bee pollination and greatly reduced fruit set. Because the fruit yields for the 2013-14 picking season will be very low (perhaps 10% of the year before), growers can do tree shaping with the knowledge they are not cutting off large volumes of immature fruit. This an ideal time to catch up on overdue trimming or to cut back very large trees. There is evidence to suggest that annual tree shaping will help avoid “biennial bearing”, when the trees have a large crop one year followed by a small crop.