CRFG Fruit Gardener Vol. 34 No. 2 2002
Canopy Management of High-Density-Subtropical Fruit Trees:
Avocado, Mango and Lychee
Australian subtropical fruit industries—those cultivating avocado, lychee and the more tropical mango—are currently venturing into high-density orchards with little experience and knowledge of canopy management methods required to manipulate trees to limit tree size while maintaining high production.
In this article I hope to provide a basic understanding along with a starting point for growers considering high-density orchards wherein they would control tree size, with an eye toward some application to existing wide-spaced orchards. With that in mind, what follows are tips for canopy management presented in a glossary format, the fundamentals for understanding how to operate in the avocado, lychee and mango canopy-management calendars.
This paper deals collectively with ideas and grower practices currently being used and trialed in Mexico, Florida, Israel, South Africa, Thailand and Australia. The discussion of terms and methods and recommendations for application are therefore based on observations of subtropical canopy management methods currently employed in those venues.
Historically Australian subtropical fruit industries have planted orchards at permanent wide spacings and more recently in the past 10—15 years, planted at double spacing and practiced tree removal as trees compete for space and sunlight. Apart from skirting bottoms of trees, trees have been allowed to grow at will.
The result is very large trees that are difficult to harvest, spray and protect from wind, hail or native birds. Also, tree removal often is delayed and sometimes not done at all, to the detriment of fruit production and profit.
There is currently a worldwide trend, in places such as Florida, Israel and South Africa, to plant subtropical fruit trees on permanent high-density spacings and to manipulate tree growth using canopy management and pruning techniques to control tree-growth patterns and tree shape and ultimately limit tree size, while still maintaining high fruit production of desired size and quality.
Not all canopy-management questions can be answered at this time, and recommendations may not be fully understood until first-hand experience is gained by putting methods into practice. Please refer to the following terms and their definitions for a better understanding of the actual pruning operations recommended.
Terms and Tips for Canopy Management and Pruning
Indiscriminate pruning: Random growth removal by hand or mechanical hedging machines that results primarily in strong regrowth.
Selective pruning An accurate form of hand pruning that usually involves cutting back to a desired lateral or bud(s). Tip pruning, heading back, skirting and flower pruning are all examples of selective pruning.
Determinate shoots: This refers to an emerging flower that does not regrow a vegetative shoot from the end of the flower, as mostly happens with mango and lychee. Sporadic, strong, vertical determinate shoots in avocados that are carrying exposed fruit prone to sunburn should be removed during the early spring prune.
Indeterminate shoots: Avocados usually have indeterminate shoots on strong, upright flowering terminals that produce a vegetative shoot out of the end of the flower. This indeterminate growth is the first vegetative shoot to appear in spring during or immediately after flowering. Very strong indeterminate growth usually needs to be tip-pruned in late spring.
Headback (heading): Selective pruning and removal or partial removal of generally stronger upright growth back to old laterals or basal buds. Excessive regular "heading" can cause strong regrowth and watershoots that will need tip pruning or controlled leader pruning during summer.
Tip prune: Light, selective growing-point removal or pruning of generally strong/medium upright growth back to newly developing laterals on the same branch. Mostly done in older trees that already have developed proper tree shape.
Controlled leader pruning: Selective growing point removal or pruning back of generally strong terminal (top) growth in young trees 1—5 years old, back to 1—3 vertical buds above newly developing laterals on the same branch. This slows the upward growth and develops more lateral growth but still allows vertical buds to produce an upright leader.
Watershoots (suckers): Refers to strong vigorous growth anywhere within the tree framework above the graft, usually coming from dormant buds on mature branches. Do not remove this growth completely, as it has enormous fruiting potential; rather, tip prune or headback to 5—8 strong laterals in mid-to-late summer.
Topping (mechanical hedge rowing): Refers to indiscriminate, non-selective, mostly external branch removal, either by hand or usually by mechanical hedging machinery, to maintain desired tree shape. Hedging results in some strong vigorous regrowth that grows out past the uniform hedged canopy that must be carefully removed by re-hedging or selective hand tip-pruning. Mechanical hedge rowing saves labor, but its repeated overuse can cause multi-branched, dense regrowth that must be selectively hand thinned.
Skirting: Pruning of lower branches or "skirt" of the tree, to aid in weed control, surface mulch applications, mini-sprinkler irrigation distribution and to prevent fruit of the lower branches from touching the ground. The best time to do this is at early flowering during cool weather, when trees are not in a strong vegetative-growth phase. A maximum height of 0.5m is recommended. Repeated skirting by selective hand pruning can eventually result in strong, robust branches able to support fruit without touching the ground.
Stumping: Cutting back trees above graft to bare stumps, usually 1—1 .5m above ground. Comfortable chain saw height is best. Carried out to retrain old crowded feral orchards older than 12 years or to topwork to another variety. Best times for stumping in the subtropics: mango—late September and October; avocado—June, July and August; lychee—May and June. If these times are not possible due to presence of fruit or frosts, then leave a horizontal nurse branch un-pruned to equate about 20 percent of the tree’s volume. Immediately paint any exposed branches and stump with good-quality flat white plastic paint to prevent sunburn.
Stump regrowth pruning: All vegetative shoots are allowed to regrow from the remaining stump. When any strong vigorous shoots reach a height of 1—l.5m from stump, head back to laterals. Do not allow any strong terminal growth to develop above 2—2.5m until a heavy crop is set 12 months after stumping. Then carry out normal controlled leader pruning and tip pruning during the second summer growth period.
Access pruning: Should not be necessary in an orchard with a well-managed canopy. If access alleys into trees to aid in harvesting or servicing irrigation are required, do this during the skirting operation at commencement of flowering when not in a vegetative growth phase.
Flower pruning: Early flowers are selectively cut off or pinched out in cold areas, to encourage re-flowering and later fruit set when weather is warmer. Timing of this operation is critical and dependent on variety and climate.
Snapping/pinching: A form of selective pruning—using only the hands—on young flowers or soft new growth, often used on mangos to encourage lateral branching and form complexity on 1—2 year old trees.
Primary crotch and branches: Main or first crotch of tree above ground level, with main branches coming from or near this crotch.
Secondary crotch and branches: Smaller branches coming from crotches above primary crotch and primary branches.
Cincturing (girdling or ringbarking): Complete bark removal down to, but not into, hard wood. Used in early maturing lychee cultivars to prevent a second late-summer or autumn growth flush. The cincture is normally done in late summer/early autumn when the first growth flush after post-harvest pruning has hardened, and if a second growth flush is imminent. Width of cincture is between 2—4 mm and must heal over by ideal flowering time. Cincturing avocado and mango is not a standard practice anywhere to date.
New laterals: Lower-developing lateral growth or side branches that are part of but below a strong and actively growing upright watershoot or main shoot or leader.
Old laterals: Mature, hardened, lateral growth or side branches at the base of a previously pruned leader or any non-vigorous lateral growth within the tree framework.
Bud(s): In the context of canopy management in this article, this refers to a bud, usually on a vertical branch, that has the potential for vegetative summer growth.
Terminal bud/growth (dominant apical growing point): The dominant end bud at the very end of a vertical branch that has the potential to produce the strongest growth. The strong vertical growth of this bud often suppresses secondary regrowth of laterals and reduces tree complexity and fruiting potential.
Major limb renewal (window pruning): This selective major limb removal operation occurs in mature bearing orchards from year 5—6 onwards in trees trained with 4—5 multi-leaders from planting. A maximum of only 15—20% of total tree volume should be removed at any one time. Select the most dominant single upright limb or alternatively remove two smaller higher upright secondary limbs evenly spaced within tree canopy. This should be done at early flowering during cool weather when the tree is in a non-vegetative phase.
Tying up: Refers to tying up heavily fruiting branches mostly in mangos using single continuous pole-mounted horizontal wire along row above trees or by wiring smaller branches back onto strong upright limbs.
Propping: Timber supports to hold up heavily fruiting lower branches to prevent fruit from touching the ground. Less pre~erred than tying up, as it restricts use of boom sprayers for weed control.
Dormancy: Period during which a subtropical evergreen tree stops growing and either develops flower buds or remains idle during winter, until a trigger or stimulus occurs causing bud break and flower development and/or emergence.
Rejuvenation pruning: Refers usually to mechanical hedging, topping or cutting back to a desired tree shape and size in 10—12 year old feral orchards that have never been pruned, or in mature high-density orchards that have failed to set a crop and are in an off year.
It is essential to carry out selective, controlled leader pruning and tip pruning on strong vegetative regrowth throughout spring/summer growing period. The best time for rejuvenation pruning is as for stumping. If a crop must be harvested or if cyclones remove fruit in midsummer and pruning must be done during the summer growing season, consider pruning only half of each tree along the row at one time with the other half done when regrowth has hardened or do the following year at the correct time.
Tree shape: Tree shape refers to the natural growth habit unique to every specific cultivar or variety. It is advised to work with the natural growth habit of the tree rather than against it. Generally the following two basic shapes can be identified in fruit varieties and a recommended guide is as follows.
NARROW PYRAMID TENT
Reed Green Gold
Tommy Atkins Nam Dok Mai
Valencia Pride Glenn
Kwai May Pink (B3) Tai So
Chacapat (Emperor) Brewster (Floridian)
Fay Zee Siu Kwai May Red (B 10)
Permanent High-Density Tree Spacing
Low-vigor cultivar 6x3 7x3
Medium-vigor cultivar 7x3 8x3
High-vigor cultivar 7x4 8x4
Row Direction:North-south orientation along row preferred for high density orchards, ideally up and down the slope if mounding to aid in water runoff, or across steep slopes to give a minimum 4—5% to max 8—10% slope along rows.
Peter Young operates Birdwood Nursery Ltd., 71-83 Blackall Range Rd., Nambour Qld. 4560. Ph 07 5442 1611; Ph 07 5442 1053. Readers can e-mail him at firstname.lastname@example.org