Most tree care advice online was written for the northern hemisphere, or at best for the east coast. Applied to a jarrah in a Perth backyard, a lot of it is somewhere between useless and actively harmful.
WA's south west is a distinct floristic region with species that evolved in ancient, nutrient-poor sands under a Mediterranean climate. They are adapted to summer drought, periodic fire and phosphorus levels that would starve almost anything else. They do not respond to intervention the way an imported liquidambar does.
Marri (Corymbia calophylla)
The big one with the rough bark and the enormous honky nuts. Marri is magnificent and it is also the species most likely to drop a limb on your car.
What it needs
Marri is prone to summer limb drop, where a long horizontal limb fails on a still, hot afternoon with no wind involved at all. Nobody fully agrees on the mechanism but the risk factors are well known: long lateral limbs, heavy end weight, and extended heat. The management is end-weight reduction, taking the mass off the outer thirds of the long limbs so there is less leverage on the union.
Prune marri in the cooler months. Fresh cuts in summer heat attract borers, and a stressed marri with fresh wounds is exactly what a longicorn beetle is looking for.
What to watch
Marri canker shows as sunken, weeping lesions on the trunk and branches. Established trees often live with it for decades, but heavy infection combined with drought stress is a decline pattern worth having assessed.
Jarrah (Eucalyptus marginata)
Slower, denser and generally better behaved than marri, but with its own problem.
What it needs
Very little, and that is the point. Jarrah is adapted to poor soils and it does not want fertiliser. Phosphorus in particular is toxic to many WA natives at levels a standard lawn fertiliser delivers happily. If you have jarrah in the yard, keep the lawn feed away from the root zone and use a native-specific low-phosphorus product if you use anything at all.
What to watch
Jarrah dieback, caused by the soil-borne water mould Phytophthora cinnamomi, is the big one. It moves in soil and water, which means it moves on tyres, boots and imported fill. Symptoms are a thinning canopy from the top down and a general decline over a few seasons. There is no cure, though phosphite injection can slow it significantly in a valuable tree.
Practical implication: if you are bringing in fill or a machine, care about where it came from.
Tuart (Eucalyptus gomphocephala)
The giant of the coastal plain, restricted almost entirely to a narrow limestone belt from Jurien Bay to Busselton. Many are protected, and rightly so, because tuart is in significant decline as a species.
What it needs
Space and to be left alone. Tuart forms enormous, heavy structure and responds badly to being reduced. Where a tuart is genuinely too big for its position, the answer is rarely a hard reduction; it is either careful deadwooding and end-weight management, or removal with council approval.
What to watch
Crown decline with progressive canopy thinning, epicormic shoots erupting along the trunk and limbs, and increased insect activity. Tuart decline is complex and linked to changed hydrology, fire regime and pest pressure. An established tuart in reasonable health is worth investing in keeping.
The rules that apply to all three
Do not fertilise them like exotics
WA natives evolved in some of the most nutrient-poor soils on earth. Standard fertiliser, and phosphorus especially, causes toxicity, weak fast growth and root damage. If in doubt, do nothing. They have managed without you for a long time.
Do not raise or lower the soil level
Adding fill over the root zone during landscaping or a build suffocates the fine feeder roots that live in the top 300mm. This is the single most common way an established native is killed by accident, and the tree often does not show it for two or three years, by which point everyone has forgotten the retaining wall went in.
Do not overwater
These species are adapted to a dry summer. Reticulation running daily onto the root zone of a jarrah creates exactly the warm, wet soil conditions Phytophthora thrives in. Established natives in Perth generally need no supplementary watering at all.
Never top them
Eucalypts respond to topping with vigorous epicormic regrowth from dormant buds under the bark. Those new stems attach only to the outer wood, not the core, and they grow fast and heavy. A topped eucalypt in five years is measurably more dangerous than the tree that was topped.
Prune to AS 4373, in the cooler months
Correct collar cuts, no flush cuts into the trunk, no stubs, and nothing over about 30 percent of the live canopy in one go. Autumn through early spring for most WA natives, so the tree can seal before summer borer pressure.
When to get someone qualified in
Native tree decline is usually multi-factorial and it is slow, which makes it easy to miss until it is not recoverable. If a mature marri, jarrah or tuart on your property is thinning at the top, throwing epicormic shoots, or has had construction anywhere near its root zone in the last few years, it is worth an assessment.
We can usually tell you in twenty minutes whether you have a tree in decline or a tree having a bad summer. Free, across the Perth metro.
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