Canopy Management of High Density Subtropical Fruit Trees: Avocado, Mango and Lychee - Part 2

What do we want in a fruit orchard?

1. Highest possible profitability
2. Earliest fruit production and $ returns
3. Highest possible yield per hectare
4. Consistent reliable yield every year
5. Long lived orchard 20-30 years
6. High quality fruit of desired size
7 Easy management - pruning, thinning, spraying, harvesting etc.
8. Able to physically protect crop as required    frost, hail, birds etc.
9. Able to grow desired crop on marginal soil types

The above can be achieved if we:
a) Select the correct variety to suit the climate and market
b) Use proper cultural management techniques
c) Select the correct rootstock - not necessarily dwarfing
d) Limit tree size to a maximum height of 4-6m
e) Use correct canopy management techniques

Limiting Tree Size and Maintaining Regular Cropping

Smaller trees will obviously produce less fruit per tree than larger trees. It is therefore necessary to plant smaller trees closer together to achieve the highest yield possible per hectare. There is well documented research that high density, deciduous and citrus orchards with more easily managed small trees will make better use of sunlight and as a result will consistently produce higher yields collectively per unit area than wide spaced orchards in the long term.

The difficulty in the past has been to manipulate and control naturally tall growing subtropical fruit trees by timely pruning and managing the canopy to discourage strong summer growth while still producing enough growth to produce heavy crops. These trees must produce new growth each year to develop and mature the existing crop and at the same time produce extra food reserves in stems and branches for next year's crop.

Recent research in the past 5-10 years has resulted in a better understanding of tree growth cycles or phonological cycles to enable timely tree manipulation to control tree growth and size and to know what to do when. Subtropical fruit growers that do not have an established culture of timely pruning may have difficulty in coming to terms with canopy management. Mangoes and Lychee that have a simple, clear 12 month flowering, fruiting, harvest, growth and dormancy cycle are much easier to manipulate than Avocados. Avocados can have a simple cycle when fruit mature before flowering in warm subtropical areas. However the late maturing Hass variety when grown in cool to cold areas mature fruit so late that often growing cycles overlap and flowers and/or young new season fruit are competing for 2-3 months before the previous season's fruit is harvested. The result is often biennial bearing with an acceptable crop only being harvested every second year. New varieties should be planted in cool areas that not only mature
fruit later but flower later so seasons don't overlap to enable easier canopy management procedures and regular annual cropping.

Controlling Tree Size using Canopy Management Techniques

1. Promoting Heavy Early Bearing

The best way to slow tree growth and contain tree size is to promote heavy early bearing. Once trees are well-established 4-6 months from planting, tree complexity must be rapidly increased to develop as quickly as possible as many horizontal fruiting laterals that have fruiting potential. By carrying out 'controlled leader pruning', the young tree is still able to grow upright while still producing laterals for fruiting. Good levels of well balanced nutrients must be applied to promote, strong growth and heavy fruiting.

2. Promote and Develop Multi Leader Growth Habit

This is important for when trees reach maximum height when we need to implement major limb removal to reduce tree complexity and keep the orchard 'young' to ensure new growth is produced each year and fruit size is maintained. The development of multi leaders happens with little effort as all trees will do this as part of their natural growth habit. The number of leaders that naturally develop is dependant on tree spacing that determines light availability.

3. Maintain Desired Tree Shape

This should start as early as possible after tree establishment and strong growth commences. Most unpruned stand alone trees have a natural growth habit. Some grow upright and some spread out. Almost all subtropical fruit cultivars will grow more or less in a pyramid shape until fruiting occurs. Heavy fruiting has the effect of weighing down branches and shading the branches below. During this period summer growth develops into the open spaces where light is now plentiful to once again to be weighed down next year. Lower branches that become continually shaded by higher branches soon die out due to lack of sunlight. The long term result is that of an inverted pyramid shape wider at the top and narrow at the bottom with fruit being produced higher and higher up the tree as lower branches die out

By selectively pruning back vigorous terminal shoots higher up in the tree at the correct time of year ensures less fruit is produced higher up, lower branches are maintained through good light exposure and produce more fruit down low. A 'narrow pyramid' shape or 'tent shape' should be selected to suit the growth habit of each specific fruit cultivar and pruned at the correct time of year. The correct time of the year to selectively head back or tip prune strong high growth back to tip prune strong high growth back to laterals is 3-4 months prior to cold weather or 'dormancy' to limit regrowth time but still allow mature enough growth to develop and properly flower and set a good crop. Heavy late pruning may result 'm growth that is too immature to set fruit in spite of flowering due to insufficient food reserves. Heavy or too frequent early pruning of the spring growth flush will encourage excessive stronger summer growth that will require more regular selective hand pruning during summer.

4. How and When to Prune

Suggested recommendations are listed in detail in crop canopy management calendars. Avocados are light pruned during the latter stages of the spring flush but mostly in midsummer 3-4 weeks after the main summer flush commences. Provided trees have been pruned back to the desired shape from year 2, little fruit will be removed as it will be back inside the outer tree canopy. Pruning back summer flush allows light into the tree and encourages the development of fruiting wood within the inner framework.

Mangoes and Lychee are basically outside tip pruned back to the desired tree shape, and size immediately after harvest to bring the entire tree, into a uniform regrowth cycle in late summer.

5. Major Limb Removal (window pruning)

This is carried out in early flowering to partially rejuvenate the tree by selectively removing a major vertical upright limb to make the tree less complex from year 5-6 onwards. A maximum of 15-20% of total tree only should be removed otherwise excessive vegetative growth and fruit drop may result during the following late spring/early summer growth flush. Any excessive vegetative growth must be selectively pruned as necessary. However removing the correct amount of major limb during winter should result in low vigour growth coming back evenly over the entire tree after fruit set with the spring growth flush, and some watershoot development regrowing with the summer flush. Repeated major limb removal means some watershoots  must be allowed to regrow to eventually replace major limbs during summer.

6. Restricting Root Development in Fertile Soils as an Aid to Canopy Management

Controlling tree growth in highly fertile soils in hot subtropical climates can be difficult. In Mexico, Florida and South Africa, various methods are employed to restrict root development, that in turn aid in canopy management and strong vegetative growth control. In hot dry areas, controlling irrigation and water distribution effectively restricts root system development and coupled with fertigation, gives the desired result.

Deep ripping, an essential part of land preparation for any fruit tree planting, in particular Avocados, can be a very effective means of restricting root development. By only ripping with a single ripper down to 0.5m or by double tynes 30cm apart down to only 30cm in very fertile volcanic soils works well. Conversely in shallow soils or poorly aerated infertile soils, deep ripping preplant on a 0.5 metre cross grid to a depth of lm+ can mean the difference between successful rapid establishment of young trees or slow, erratic growth and poor long term productivity.

7. Natural Restrictive Root Development Effect of Shallow Soils

In Florida, solid 'coral' deposits are machine trenched along the row and across the row. Trenches are approx. lm wide and lm deep. Trees are planted where trenches cross. The result is trees can only grow to a certain size as they are virtually growing in 'a pot' with highly restricted root area and regular rejuvenation pruning must be carried out to produce new growth and corresponding fruit production.

This same principle applies with most soils that have 'a bottom' or an impervious clay, sandstone or rock 'C' horizon 1-2-3m below ground level and explains why so many unpruned subtropical fruit orchards become non-productive after a given period of time depending on available soil depth.

Canopy management practices are easier to apply in shallow soils that aid in the control of tree size when trees reach a maximum size relevant to soil depth.

Summary of recommendations

1. Increase complexity of small trees for heavy early fruiting
2. Promote and develop multi leader growth habit for future major limb removal after year 5-6
3. Maintaining desired tree shape from year 2
4. How and when to prune - crop canopy management calendars
5. Major limb removal
6. Restricting soil preparation and root development as an aid to canopy management

Conclusions

Growers planting permanent high density orchards must look closely at the overall concept and implement the total canopy management package to achieve success. Varietal selection, planting density, row direction, correct soil preparation, well balanced nutrition both pre and post planting, and application of correct pruning techniques at the correct time all must fit together to achieve success.

Long term canopy management systems operating in high density orchards in Israel and Florida and more recent research being carried out in South Africa indicates that success can be achieved. Systems and methods employed in manipulating high density orchards must be easily understood and easy to do for industry to adopt them as a standard practice. It is hoped this paper has addressed some of these issues to give growers the confidence to venture into canopy management of high density subtropical fruit orchards Avocado, Lychee and Mango.

References

Stassen, P.l.C. and Davie, S.J. (1996). Principles of a Tree Management Programme. Institute for Tropical and Subtropical Crops Information Bulletin No. 285.
Goren, M. and Gazit, S. (1992). Small-Statured Litchi Orchards; A New Approach to the Growing of Litchi. ACTA Hort.349:69-72.
Middleton,S.G. (1996). Theoretical/Practical Design of Orchards - Proceedings of the 4'h National Lychee Seminar, Australia, pp 24-29
Wolstenholine, B.N. and Whiley, A-W. (1996). Strategies for Maximising Avocado Productivity - A Review. World Avocado Conference III Israel.
Snijder, B. and Stassen, P.J.C. Manipulation of Hass Avocado Trees - Pruning - A Progress Report. South African Avocado Growers Association Yearbook, pp 73-76.

Authored by: 
Peter Young, Birdwood Nursery
Sourced from: 
Rare Fruit Council Brisbane Branch Newsletter February-March 2001
Date sourced: 
24/01/2013