How Correct Care Can Extend The Safe, Useful Life Of A Tree

Quercus ilex subsp. ballota, Barbary oak - 4
Many Australian botanic gardens are reviewing their oak collections due to the impact of climate change. Barbary oak, Quercus ilex subsp. ballota, Albury Botanic Gardens, NSW.

“Hi Jerry, I live in Gordon, Sydney and a friend recommended I contact you as I have big concerns about my beautiful oak tree and need to know what to do; the pest controller noticed termites in the fence and tree, but was unsure about helping the tree. Any advice will be greatly appreciated, Kathy”.

Reply: Hi Kathy,

During my time in Sydney, I had plenty of time to observe a wide range of oak (Quercus) species growing from the New England region of NSW down through the Central Coast and Highlands through Canberra to Melbourne. Oaks have been commonly planted in parks, streets and as memorial avenues. Several botanic gardens, including those of Sydney, Albury and Melbourne, have oak collections.

Pin Oak Avenue, Quercus palustris, Tenterfield - 1
Memorial avenue of pin oak, Quercus palustris, Tenterfield, NSW.


Oaks grow faster in a warm temperate climate, which includes Sydney, than they do in cool temperate climates which have long winters and regular frosts or snow. In our warming climate, Australian botanic gardens are re-assessing the future of their oak tree collections. Many of their trees are senescent and superior replacement species may be planted.

In SE Australia, apart from faster growth, the majority of oak species tend to live shorter lives and they are more subject to a range of root decay fungi (like Armillaria luteo-bubalina, Phytophthora spp., Phellinus noxious), and pest damage, including termite attack.

Thank you for sending clear images, taken from different angles, this really helps, because an armchair diagnosis can help only so far. It cannot replace a physical examination.

Oak tree
Image 1: English oak, Quercus robur, Gordon, NSW.

Image 1 shows a tree with an old, large, unhealthy wound where it has lost one of its two major limbs. There are also several really bad flush cuts which haven’t fully healed, a consequence of poor care in the past.

A flush cut is where a limb has been removed by a saw leaving no base or stub, it is removed entirely, leaving a smooth cut flush with the trunk. This leaves the largest possible wound for the tree to heal.

Forty years ago it was well established by professional research that flush cuts encourage a range of problems in trees. Twenty years ago on television I explained how to correctly prune off a dead tree branch at Taronga Zoo by pruning to a branch collar.

Sterculia quinqueloba/ branch collar cut - 3
Image 2: Sterculia quinqueloba, showing where a branch collar cut is made. It has sealed and healed perfectly. Mt Coot-tha Botanic Garden, Brisbane.


Image 2 shows in profile where a branch collar cut has been made and healed over. Almost every tree branch will widen at the base where it grows out from the main trunk or a another limb. This enlarged zone is the branch collar. Making a cut at this point creates a smaller wound than a flush cut and it stimulates secondary growth – callusing. The callus is produced by the living cambium layer underneath the bark. This grows over the wound, sealing it from the external environment. To completely enclose a wound, callusing may take several years depending on the age, vigour and species of tree, but while this is occurring, the cambium may also exude kino.

Oak tree 4
Image 3: Gums, resins and kinos are exuded to protect a tree wound during healing.

Image 3: Look at your image of bark exuding its protective gum. Gums, kinos and resins protect healthy trees, including protection from the egg laying of bark boring beetles. This exudate is a sign that there is vitality in the tree, it traps and kills bark boring insects (including termites) investigating opportunities for food for their larvae. You’re probably familiar with amber, ancient tree resin, commonly exuded by conifers, and it often includes long dead insects trapped within.

Exposed heartwood also changes its structure and inner chemistry to better resist decay fungi and bacteria. This is a step by step process, known as compartmentalisation.

I was prompted to record this pruning tip for gardeners because I saw how much damage was still being done to trees in the name of ‘arboriculture’ in Australia.

Painting wounds, also long discredited, has been proven to encourage disease, decay and dieback in trees. The paint seals disease into wounds, helping it establish. If the paint contains artificial growth hormone, this disrupts naturally produced hormones in the tree.

Oak tree 2
Image 4: The large stump left on this oak tree has not healed, it is decaying and dying back.

Image 4: Your current problems start from the fact that when one of the major limbs was removed is wasn’t correctly removed to the collar, instead a large stump has been left behind.

This stump hasn’t healed over, it has died back. Look at your image showing how the living tissue – the cambium layer under the bark – has died leaving a gaping wound open to problems.

The tree has also been badly damaged by a series of unhealed flush cuts made on the bole (Image 1), they have become secondary sites for infection.

Image 5: All the exposed wounds and dead wood are an open invitation to bark boring beetles.

These beetles generally avoid healthy trees because the process of compartmentalisation makes the tissue unappealing. On the right hand side of the image you can see circular exit holes, where mature beetles have left the timber to breed.

Injecting trees with poisons, such a termite controlling pesticides or fungicides has also been regarded as hit and miss by arborists for around forty years. In England, I had plenty of opportunity to see how damaging the injection sites can be on trees infected with Dutch elm disease and bleeding canker disease of horse chestnut.

Oak tree 3
Image 5: Larvae of bark boring beetles tunnel into and feed on dead wood, naturally recycling it.  Circular exit holes can be seen on the right hand side.

If your pest controller has suggested this method, remember it is not possible to accurately dose a tree by injecting chemicals, it is hit and miss. Furthermore, the injection sites themselves are often responsible for creating new wounds – entry points for pests and disease.

It may be possible to extend the safe, useful life of your tree, but only an expert examination, involving a resistograph to test the cord wood the bole for significant decay or termite activity can determine if it is safe enough to merit retention. The structural timber must be sound enough to withstand storms.

At this point you should also consider what might be damaged if your tree does collapse suddenly in a storm and if any property damage will be covered by insurance.

To sum up:

* bark boring beetle are warning. Here, poor pruning is the primary cause of decay;
* pruning to a branch collar is the most acceptable technique;
* chemical treatment by painting wounds or trunk injection is an avoidable risk;
* investigating for structural weakness by resistograph is essential to decide if you can extend the life of the tree or if it must be felled;
* consider the expense of collateral damage to your or neighbouring property if the tree collapses;

A physical inspection is the next step. I advise that a decision needs to be made before spring, which is when a new, leafy canopy will put additional stress and strain on your tree.

Pin oak, Quercus palustris
Pin oak, Quercus palustris, has good autumn colour and is better suited to warm temperate Sydney than English oak.


Your task is now to contact Arboriculture Australia, our peak body, to seek expert help. Their members include the most enthusiastic, up to date practitioners. Never contact an amateur tree lopper, of which there are dozens, because your tree’s safe, useful life expectancy has already been seriously damaged by amateur attention.


Jerry Coleby-Williams*
27th June 2020

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* Jerry Coleby-Williams was professionally trained by the Royal Botanic Gardens, Kew, in horticultural and botanical sciences, botanic garden and business management, landscape design, horticultural estate management and arboriculture (1978 – 82).

Jerry established the first arboricultural service in the London Borough of Lewisham (1983 – 85). He has managed urban forests in the London Borough of Islington and the London Borough of Wandsworth using in-house and contract staff. In particular, he helped repair and restore the urban forest in Wandsworth following extensive destruction caused by the Great Storm in 1987.

Between 1992 and 2003, Jerry managed the tree collection of the Royal Botanic Gardens, Sydney, The Sydney Domain and Government House – Australia’s oldest tree collection. He founded and equipped the first professional arboricultural team and plans of management for these trees. During independent Market Testing conducted by the NSW government in 1999 – 2000, the arboriculture team was found to be more effective, more efficient and more cost effective than competition in the marketplace.


One Comment Add yours

  1. cathyc says:

    I will never be lucky enough to live with such a magnificent tree as an oak, but I expect that the same advice holds true for pruning small trees and bushes too – I hope that I can be kinder to our trees by following your advice.

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