Restoring cyclone-damaged trees

Cyclones can cause severe injury or damage to fruit or nut trees.

Immediate care for damaged trees depends on:

  • tree age
  • extent and type of damage
  • severity of root damage
  • soil type and drainage.

Tree age

A tree's age will largely determine its ability to recover. A young, vigorous tree will be more likely to survive than an older one, though sometimes an older tree's deep roots will help it withstand the force of the damage.

Tree damage could include:

  • tree rollouts, where the whole tree has been uprooted
  • partially uprooted trees, where 10 to 50% of the roots on one side of the tree are exposed
  • trees on a severe lean or flat on the ground, where roots may be damaged but remain underground
  • trees that have been knocked to the ground by falling branches from windbreaks or shade trees
  • severe twisting and breakage at ground level
  • breakage of the main trunk from ground level and above
  • breakage of primary and secondary limbs
  • sunburn on exposed branches, stems and trunks.

Fallen trees

The sooner you can attend to fallen fruit and nut trees after a cyclone, the better their chances of surviving. You can either try to reset the tree (stand it up again) or prune it and encourage it to reshoot.

In general, it is best to reset only smaller trees, since large trees will be weakened and may fall again. Try to reset trees while the soil is still wet.

For most trees that reshoot readily, consider:

  • stumping the tree about 30cm above the graft (if grafted) or 1m above the ground if not grafted
  • staking the stump to prevent further movement
  • pruning the exposed roots and covering them with river sand or the surrounding soil and mulch heavily
  • painting the upper side and end of the stump with a 1:4:20 mix of water-based, white acrylic paint, micro fine lime and water (i.e. 1L paint, 4kg lime and 20L water) to prevent sunburn.

It is very difficult to replant trees that have been completely uprooted. Unless they are heavily pruned, replanted and staked within 24 hours following the cyclone, they will probably die.

Partially uprooted trees (where 10–50% of the roots on one side of the tree are exposed) or trees on a severe lean, or flat on the ground, where roots may be damaged but remain underground, should be straightened while the soil is wet. After straightening, the trees should be braced. Keep braces in place for at least 2 years.

Make sure all jagged and irregular root breaks are cut, smoothed and painted with a 1:1:1 mix of acrylic paint, water, and fungicide (e.g. copper hydroxide) mix. Cover any exposed roots with soil and mulch. Keep the tree well-watered.

Prune damaged trees just enough to balance root losses. Cut out broken, diseased and malformed branches to give the tree a desirable shape. Apply a 1:4:20 mix of water-based, white acrylic paint, micro fine lime, and water to exposed branches, stems and trunks to prevent sunburn. Small amounts of fertiliser may help stimulate growth.

Young trees (less than 2–3 years old) may be reset after you prune exposed roots and remove soil on the uprooted side before lifting. Use this to cover any exposed roots.

Reset the trees while the soil is still wet. Apply a 1:4:20 mix of water-based, white acrylic paint, micro fine lime, and water to prevent sunburn.

Stop lifting the tree if you hear cracking due to roots snapping on the downwind side. Stake the lifted or partially lifted tree to minimise further movement from wind. You should remove up to half the canopy to compensate for root loss.

Older trees (more than 5 years old) are less likely to be easily reset without causing further severe root damage on the downwind side, therefore follow the guidelines for fallen trees.

Some success has been achieved resetting older mangosteen trees, but this is the exception.

A tree might still die after being reset if there is severe root damage or fungal attack that you can't see.

If you remove dead trees, it's best to remove the stumps and roots as well because they can harbour fungal diseases which can infect healthy trees. Leave about 1.2m of stump standing as removal will be cheaper and easier if you can pull, rather than dig, stumps out.

Injured or damaged trees

This type of injury is likely to be fatal. However, if some of the main trunk remains intact there is a chance the trees will reshoot if you make a clean cut below the damaged area. If this is below the graft it will result in the loss of the variety, taking the tree back to the seedling stock. This isn't an issue in seedling trees such as rollinia or soursop, or in marcotted trees such as lychee or longan. But in trees with specific varieties, such as avocado, durian, jackfruit or rambutan, the rootstock that is left is unlikely to be worth saving.

Paint the upper side and the end of the stump with a 1:4:20 mix of water-based, white acrylic paint, micro fine lime and water (i.e. 1L paint, 4kg lime and 20L water), and some fungicide, to help prevent sunburn and wood rot.

This damage will affect the shape as well as the general health of the tree.

Consider:

  • pruning the tree properly to avoid additional damage
  • making a clean cut behind the damaged and splintered wood (ensure a downward-facing cut so that that moisture does not pool on the cut)
  • painting the stump, pruning cuts and exposed branches with with 1:4:20 mix of water-based, white acrylic paint, micro fine lime and water, and some fungicide, to help prevent sunburn and wood rot.

If branches are split at the crotches, but are worth saving, pull them back into place and secure them.

Tree health

Following a cyclone, the high amount of root damage and tree stress may result in fruit and nut trees being susceptible to soil pathogens such as Phytophthora and Pythium species, which are a normal component of the soil environment. There are registered products (e.g. phosphoric acid and Ridomil) for control of those disease pathogens in some crops (see the Australian Pesticides and Veterinary Medicines Authority).

Diseases in the tops of the tree may be best controlled by regular applications of copper fungicide over the 1–2 months following a cyclone.

Cover badly damaged limbs, or breaks that cannot be cut cleanly, with a mix of copper hydroxide, water, and acrylic paint (approximately 1:1:1 – thicken or dilute as required) to help prevent disease. A more dilute version of the mix without the fungicide will also be suitable for sunburn prevention.

Tree trunks and major limbs that have been suddenly exposed to full light will be prone to sunburn. To protect them spray them with a 1:4:20 mix of water-based white acrylic paint, micro fine lime and water (i.e. 1L paint, 4kg lime and 20L water).

Cloudy and wet weather is helpful to trees that need time to recover.

Trees that have suffered major foliage loss still require water. Irrigation regimes should be more frequent, although for shorter durations, to ensure that the root zone remains moist, but not waterlogged. Overwatering may cause root rot.

Fertiliser inputs to damaged trees are recommended in small doses. Generally, the fertiliser amount should be reduced in proportion to the leaf or canopy loss. As new leaves develop, use a foliar application or frequent small amount of your usual fertiliser application.

Restoring trees after cyclone damage

There are many things you can do to manage cyclone-affected tree crops. This advice is for specific tree types:

  • restoring damaged trees
  • remedial measures to minimise future loss
  • managing fruit.

Fruit on the tree may have been affected by wind damage. Fruit may be bruised, and some may have cuts and chunks out of them. A copper spray at this stage will help protect the fruit from both bacterial and fungal breakdown that will be aggravated by this damage. It will also protect damaged leaves and branches. Choose a product that leaves minimal residue, such as red copper. Alternatively, you can apply an azoxystrobin fungicide (within the label's resistance management guidelines), against anthracnose after the cyclone, as a curative and preventive fungicide.

Bruising and fruit damage will be clearly visible after a few days, and pickers should avoid harvesting damaged fruit. Treat fruit with normal post-harvest treatments, making sure that equipment is cleaned carefully if any fruit with bacterial soft rot goes through the packing line.

Some trees that have blown over can be salvaged if root damage is not too extensive. Prune and prop the trees to help them recover. When propping, lift the trees while the soil is still wet, being careful not to damage the remaining roots. Stop lifting the tree if you hear cracking due to the root snapping on the downwind side. If in doubt, leave the tree as it fell and prune to a new shape over several years. Prune off the canopy proportionally to the amount of root loss or damage. Treat limbs for sunburn with a 1:4:20 mix of water-based white acrylic paint, micro fine lime and water (i.e. 1L paint, 4kg lime and 20L water). Treat the ground with a fungicide (e.g. phosphoric acid or Ridomil) for phytophthora.

Consider sunburn protection for fruit remaining on trees, as fruit will be more exposed when leaves have been stripped from the tree. This is particularly the case with Hass. A spray-on crop protectant is recommended. You will need to gauge the volume for each tree, as this will depend on the amount of canopy lost. Apply enough protectant to get good coverage, but not to the point of run-off.

Normally a phosphorous acid injection is recommended for March, and this injection is more important than ever following heavy rain during a cyclone. Injections are favoured over foliar sprays because the leaf area needed to absorb the phosphorous acid has been reduced by the cyclone, to the point where there is not enough leaf area for sufficient absorption.

Macadamias are particularly brittle and prone to cyclone damage. Consider strategies for clearing tree rows, salvaging trees where possible and recover fallen nuts from areas where you can gain machinery access.

You should prune trees to take out any damaged and broken or twisted limbs, and apply a 1:4:20 mix of water-based white acrylic paint, micro fine lime and water to protect exposed limbs against sunburn. Pruning trees to a central main leader can provide more resilience to future high-wind events.

Many trees break or twist off at ground level, and you will need to replant these. Where trees have not broken, prop and prune to reduce the canopy to help trees recover. Open out the centre of the tree by removing selected internal branches.

The first step to recovery is to stand trees back up as soon as possible and while the soil is still wet. When this is done, broken branches are pruned and debris is cleared, spray all trees with a preventative fungicide such as mancozeb or copper.

If the canopy has been significantly reduced, apply 1:4:20 mix of water-based white acrylic paint, micro fine lime and water to protect exposed limbs against sunburn.

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