Start by asking whether it is needed
Fertiliser is the first thing people reach for when a tree looks unhappy, and it is usually the wrong answer.
An established tree in a garden bed with reasonable soil finds what it needs. Poor colour and thin growth are far more often caused by compacted soil, root damage from a recent build, waterlogging, drought stress, or a tree being planted in a position it does not suit.
Feeding a tree with a root problem does nothing except cost money. Feeding a tree that is drought stressed can make it worse, by pushing growth it cannot support.
When it is justified
Four situations genuinely warrant it. A soil test showing a specific deficiency. A tree in a confined or heavily modified site - a nature strip, a courtyard, a raised planter - where there is no natural nutrient cycling. A newly planted tree in its establishment years. And a fruit tree being asked to produce a crop every season.
Reading the label
The three numbers are nitrogen, phosphorus and potassium as percentages by weight.
Nitrogen drives leafy green growth. Too much produces soft sappy shoots that are prone to breakage and attractive to sucking insects. Phosphorus supports root development and flowering. Potassium supports general vigour, water regulation and stress tolerance.
A balanced slow-release product in the range of 10-4-6 suits most ornamental trees. Fruit trees generally want more potassium. And Australian natives want almost no phosphorus at all.
The native phosphorus problem
This is the mistake that kills more Australian garden trees than any other.
Banksias, grevilleas, hakeas and their relatives evolved on ancient, deeply weathered, phosphorus-poor soils. They take phosphorus up so efficiently that standard garden fertiliser causes toxicity - blackening foliage, dieback and often death.
Use a native-specific low-phosphorus product on those species, and if you are not sure what you are feeding, use one anyway.
How to apply it
Broadcast a slow-release granular product evenly over the root zone, from about a metre out from the trunk to one and a half times the canopy radius. Feeder roots sit in a broad shallow plate in the top 300mm of soil, not in a ball around the stem, so putting fertiliser against the trunk misses them entirely and can scorch the bark.
Water it in thoroughly. Un-watered granules do nothing until it rains and can burn surface roots in the meantime.
Timing: early spring, just before the growth flush. Avoid feeding in late autumn, which pushes soft growth at the wrong time, and avoid feeding a tree that has just been heavily pruned or is visibly stressed.
What usually works better
Mulch and water. A 75 to 100mm layer of coarse organic mulch over the root zone, kept 100mm clear of the trunk, moderates soil temperature, holds moisture, suppresses competing grass and releases nutrients slowly as it breaks down.
On the Sunshine Coast, chip from your own tree work is free and does the job. If a tree is genuinely struggling, ring (07) 5408 6029 and get the cause diagnosed before buying anything.
Frequently asked questions
Does my tree need fertilising?
Probably not. Established trees in reasonable soil find what they need. Fertiliser is justified when a soil test shows a deficiency, when a tree is growing in a confined or heavily modified site such as a nature strip or a courtyard, or when a young tree is being established. Yellow leaves on a mature tree are more often a water or root problem than a nutrient one.
What do the numbers on a fertiliser bag mean?
NPK - nitrogen, phosphorus and potassium, as percentages by weight. Nitrogen drives leafy growth, phosphorus supports roots and flowering, potassium supports general vigour and stress tolerance. A 10-4-6 product is 10 per cent nitrogen, 4 per cent phosphorus and 6 per cent potassium, with the rest being carrier and trace elements.
Why do Australian natives need low phosphorus fertiliser?
Many Australian natives, especially the Proteaceae family - banksias, grevilleas, hakeas - evolved on ancient phosphorus-poor soils and take up phosphorus so efficiently that normal garden fertiliser poisons them. Use a native-specific low-phosphorus product on those species, and on anything you are unsure about.
Where should fertiliser go?
Over the root zone, which extends well beyond the canopy edge - out to one and a half times the drip line is a reasonable target. Not against the trunk. Most feeder roots are in the top 300mm of soil in a broad plate, not in a ball around the stem.