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Indumentum Rot under the Collar

By Sue McIntyre

Nov, 2024

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Being driven down the main street of Yass recently, I had time to notice a handsome ironbark eucalypt, many decades old, in the prime of life, and perfectly healthy. The next moment, we passed two more of the same species, similar in size. Unlike the first, both of these were well on the way to meeting their maker. One had lost half its limbs, the other, half its foliage. What was happening to them?

In the third instant, I spotted pale patches at the base of each of the two sick trees, pictured below. There were grass clippings that had been heaped up against the base of their trunks, while the healthy tree had none.

It is never easy to prove the cause of death of a tree, but it is no secret that piling mulch, grass clippings or soil around their base can kill them. We are reminded of this regularly by the presenters of Gardening Australia, and the trio of trees in Yass is a striking illustration of this. While mulching trees can be of great benefit to them, we are warned not to place any soil or mulch close to the trunk.

Chopping down a healthy tree of comparable size in such a public place, would likely lead to protests. But these slows deaths, from a well-understood cause, seem to have been ignored. It is puzzling that the arborist removed the large dead limbs, without seeing to the removal of the mulch. Or perhaps they did at the time, but the practice continued.

Why is it so easy to kill a tree this way? Immediately under the bark is a very thin layer of tissue, known as the phloem. The cells in the phloem are responsible for moving sugar and nutrients up and down between leaves and roots, as is required to keep the tree alive and growing. The moisture that lingers under the pile of soil or mulch encourages fungi which eat into the bark, destroying the phloem and effectively starving the tree. Roots are not as vulnerable, as the phloem runs down their centre, and is well protected.

The dying trees in Yass brought to mind the recent changes to road grading techniques that can be observed on our local roads. You may have noticed more aggressive building of drains and banking up of soil along the sides. It has resulted in wider roads and improved drainage. It has also resulted in soil being piled up against trunks, with many tree deaths to follow, as well as increased danger from falling dead limbs. Are the grader operators unaware of this, or indifferent?

These trees could be saved by raking away the soil from the base, exposing the trunk down to its original soil level. If it were done promptly, taking care not to damage the bark in the process, some of these buried trees could be saved. Theoretically we could request the council to rectify the damage, but if there are any trees that you care about on your road, some direct action may produce better results.

http://www.gang-gang-gundaroo.com/

This article was previously published in The Gundaroo Gazette Volume 245.1 August 2024

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