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Staghorning
Tree Thinning / Removal
As avocado orchards age and grow, space within the orchard may become congested. A solution to this particular scenario may involve quite heavy canopy management. The two techniques outlined below include staghorning and tree thinning / removal.
Staghorning
Procedure:
Involves cutting trees back to a stump above the graft union and allowing them to re-grow. Carried out to rejuvenate older crowded orchards. Can also be used to top-work trees over to another variety.
Costs Involved:
Costs range from $22 – 55/tree ($4400 – 11,000/ha based on 200 trees/ha) this includes cutting down trees, chipping/mulching of limbs and painting of stumps.
Trees are cut down to a stump using either chainsaws or mechanical pruning saws. Mechanical saws can be used to reduce tree height and width (saws can handle branches up to 10-15cm diameter). Larger limbs are then cut down using chainsaws. A forestry flail mulcher mounted on an excavator arm can also be used to bring trees down.
Smaller branches can be mulched using slashing equipment. Larger branches can be mulched (tractor operated forestry mulching equipment can handle branches up to 25cm diameter), chipped (commercial operators available @ $235/hr) or removed from the orchard.
Considerations:
Timing
Trees are staghorned after harvest. In subtropical climates, trees are staghorned between June-August and in cool temperate climates between October-December.
Height of the Stump
Cut trees back to a stump of about 1m high but ensure that the cut is made above the graft union. Trees staghorned higher can become too large before they come back into production.
Sunburn Protection
Exposed branches and stumps should be immediately painted with white plastic paint to prevent sunburn. Diluting the paint with water so it can be sprayed onto the exposed branches will reduce the time taken to apply protection. Adding a copper fungicide treatment (e.g. copper oxychloride) may assist in controlling disease at the pruned surface. Other preparations including bentonite clay and calcium carbonate sprays can also provide sunburn protection.
Time Out of Production
Trees can be out of production for 2-4 years after staghorning. To maintain cash flow, sections of the orchard can be staghorned at intervals so at least part of the orchard is always in production.
Replant Versus Staghorning
Since trees can be out of production for 2-4 years after staghorning, some growers prefer to replace older trees with new plantings utilising new rootstocks. Fallowing is recommended (eg. a sorghum cover crop) for Phytophthora control. In some situations, tree removal can pose a disease risk. Ensure that all stumps,sticks and roots down to a size of 1cm diameter are removed from the orchard and allow time for smaller sticks and roots to rot away prior to planting particularly in those orchards adjacent to rainforest and wet sclerophyll forests to prevent infection by the wood-rotting fungus Phellinus noxius. Replanting in these infested sites is not advised as the fungus can survive in root debris for several years.
Staghorning Alternate Rows or Alternate Trees Within a Row
Not recommended. For the stump to regrow effectively, adequate sunlight is required. Remaining trees tend to fill in the space provided and reduce the amount of light reaching the staghorned tree or row.
Tree Health at Time of Stumping
It is recommended that trees should be injected with phosphonate 4-6 weeks prior to staghorning to assist in Phytophthora control.
Nurse Branches
If trees are unhealthy, leave a nurse branch to support root growth. The nurse branch can also produce fruit thereby maintaining some production in a staghorned block. This method is also used when top-working to other varieties.
Regrowth Management
It is important to manage the regrowth from staghorned trees. In this example the number of branches should have been reduced to ensure light penetration into the centre of the tree.
Selectively prune or mechanically prune regrowth to manage tree size as described in Section: Young Trees – Maintenance pruning. Strong vigorous shoots or water shoots can be removed or cut back to lateral branches.
Plant Growth Regulators
At the date of publication both SUNNY® (active ingredient uniconazole-P) and AuSTAR® (active ingredient paclobutrazol) were registered for use in avocado, however check the status of the registrations before planning an application. Also note that AuSTAR® cannot be used when there is mature fruit on the tree.
Trials in subtropical environments indicate that foliar application of SUNNY® (Active constituent: 50g/L uniconazole-P) and AuSTAR® (Active constituent: 250g/L paclobutrazol) at a rate of 5-10L/1000L to the summer and autumn growth flush can reduce shoot growth, increase flowering and yield in staghorned trees. New growth was treated when shoots are 50-100mm in length.
Do not apply plant growth regulators to trees with low vigour, under stress or showing symptoms of Phytophthora root rot.
Tree Thinning / Removal
Procedure:
Alternate rows of trees within a row are removed as orchards begin to crowd.
Costs involved
Costs range from $22 – 55/tree ($4400 – 11,000/ha based on 200 trees/ha) this includes cutting down trees, chipping/mulching of limbs and removal of stumps.
Trees are cut down using either chainsaws or mechanical pruners. Mechanical saws can be used to reduce tree height and width (saws can handle branches up to 10-15cm diameter). Larger limbs are then cut down using chainsaws. A forestry flail mulcher mounted on an excavator arm can also be used to bring trees down.
Smaller branches can be mulched using slashing equipment. Larger branches can be mulched (tractor operated forestry mulching equipment can handle branches up to 25cm diameter), chipped (commercial operators available @ $235/hr) or removed from the orchard.
Considerations
Remaining Trees
The remaining trees are pruned using selective limb removal or mechanical pruning techniques to prevent crowding.
Disease Risk
In some situations, tree removal can pose a disease risk. Ensure stumps and as many roots as possible are removed particularly in those orchards adjacent to rainforest and wet sclerophyll forests to prevent infection by the wood-rotting fungus Phellinus noxius.