Large Trees – Heavy Pruning

Jump to these sections in the article:
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.

Staghorning of large trees

 

 

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.

Trees cut back to a height of 1m and stumps painted to protect against sunburn.

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.

 

This tree was staghorned too high - red lines indicate where cuts should have been made.
This tree was staghorned too high – red lines indicate where cuts should have been made.

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.

Staghorned tree with nurse branch.
Staghorned tree with nurse branch.                                  

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.

A staghorned tree requiring regrowth management.
A staghorned tree requiring regrowth management.

 

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.

Orchard beginning to crowd
Orchard beginning to crowd

 

Alternate rows are removed to improve access and light penetration into the orchard.
Alternate rows are removed to improve access and light penetration into the orchard.

Removal of trees on the diagonal in square plantings may involve a change in row direction and this can pose difficulties with respect to irrigation lines.
Removal of trees on the diagonal in square plantings may involve a change in row direction and this can pose difficulties with respect to irrigation lines.

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.

Verticillium wilt

Cause

The fungus Verticillium dahliae.

General comments

Verticillium wilt is a relatively minor problem, but it can cause considerable damage to individual trees. Verticillium affects several other crops including cucurbits, peanuts, potatoes, strawberries and tomatoes. Since the inoculum can survive for many years in the soil, avocados can be affected if planted after these crops, especially if they become stressed. The fungus invades the tree through the roots and grows into the water conducting tissues of the trunk and branches, blocking water flow. The cooler temperatures of winter favour disease development, although the symptoms are not usually seen until water demand rises during warmer weather in spring and summer. Trees where the scion has overgrown the rootstock are more prone to verticillium wilt since this causes stress to the tree.

Identification

A single branch will die suddenly with the leaves turning brown and remaining attached to the tree. The tree will recover unless it is very young, in which case the whole plant may be affected and will usually die. The symptoms often appear when trees are subjected to stress, for example, if they are radically pruned in winter. To confirm verticillium, slice the freshly affected branch with a sharp knife to expose the water-conducting tissue; a discoloured brownish appearance confirms the presence of verticillium. Verticillium wilt can be confused with Phellinus noxius  and waterlogging which both cause leaves to die and remain on the tree for some time.

Verticillium disease usually kills a single branch leaving the rest of the tree unaffected. Affected leaves die suddenly and remain on the tree for some time.

A distinguishing factor associated with verticillium wilt is the brown discolouration of the conducting tissues in a diseased stem (right) compared with a healthy stem (left).

Treatment/prevention

Try to avoid land where other susceptible crops have been grown. Prune back affected branches until clean wood is reached. No other treatment is required as older trees generally recover. Consider removing trees that are repeatedly affected.

Do not:

  • use peanut hay or shells for mulch as this can introduce the pathogen
  • conduct heavy pruning in winter where the disease is known to be present as the stress may trigger disease development
  • collect grafting wood from trees that have displayed symptoms of the disease.

Additional reading