Why all-purpose fertiliser harms your Australian native plants
Standard garden fertilisers are often formulated for exotic plants like roses, lawns, vegetables and the foreign species that dominate many of our garden centres. For these plants, they work brilliantly.
The problem is that Australian natives did not evolve in anything like the same conditions and feeding them as though they did causes real harm. Here is what to look out for when buying or applying fertiliser to your native garden.
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Why Australian natives are different to begin with
Australia's soils are among the oldest and most nutrient-depleted on earth. Over millions of years of weathering, leaching and biological activity, they have been stripped of the minerals that do fertilise gardens in younger, more geologically active parts of the world.
Our soils contain just one to five parts per million of phosphorus compared to the fifty to two hundred parts per million typical in parts of Europe and North America.
Native plants adapted to these depleted soil conditions in fundamental ways. The Proteaceae family, which includes grevilleas, banksias, hakeas and waratahs, developed specialised root structures called proteoid roots or cluster roots: dense, hair-like mats that dramatically increase the surface area available for nutrient absorption.
These roots are so efficient at extracting phosphorus from lean soil, they cannot regulate their own uptake when phosphorus levels rise. This means that when you apply a standard fertiliser, the plant absorbs far more phosphorus than it can process and that excess becomes toxic.
The phosphorus problem
The symptoms of phosphorus toxicity develop gradually, often over weeks or months and they look very similar to several other conditions, which means we are often tempted to apply more fertiliser which only exacerbates the issue further.
What phosphorus toxicity looks like
| Symptom | What you see | Why it happens |
|---|---|---|
| Leaf tip burn | Brown or black necrosis at the tips and margins of older leaves | Excess phosphorus accumulates in leaf tissue and causes cell death at the margins. |
| Yellowing between leaf veins | Green veins with yellowing tissue between them on young leaves | High phosphorus binds iron and manganese in the soil, making them chemically unavailable to roots. The plant develops iron deficiency symptoms even when iron is present. |
| Older leaf drop | Progressive loss of leaves from the base of stems upward | The plant sheds phosphorus-saturated older leaves in an attempt to reduce its toxic load. |
| Branch dieback | Stems dying back progressively from tips toward the trunk | This is advanced toxicity. At this stage root damage is significant and recovery is uncertain. |
| Sudden collapse | A plant that appeared healthy declines and dies within weeks or months | Once proteoid roots are severely damaged they cannot recover. The plant may look viable above ground long after the root system has been compromised. |
Phosphorus toxicity is often irreversible once it reaches the branch dieback stage. The best outcome once symptoms are advanced is removing the source of phosphorus and hoping the plant has enough root integrity left to recover.
Which natives are most at risk
Grevilleas, banksias, hakeas, waratahs, isopogons and telopeas are the most phosphorus-sensitive and can be killed by a single application of standard fertiliser. Wattles and many eucalypts are considerably more tolerant of phosphorus, though they still perform better without it. When in doubt, assume sensitivity and choose a low-phosphorus native-specific product.
The nitrogen problem
In wild conditions where nutrients are scarce, flowering heavily is a survival strategy to ensure seed production before the next drought, fire or flood. When you feed a native with a high-nitrogen fertiliser, the plant interprets the sudden nutrient abundance as a signal to grow rather than flower, pushing out a flush of soft foliage at the expense of the flower buds that were developing. The display you were hoping to encourage will be suppressed by the fertiliser.
High-nitrogen growth also tends to be structurally weaker than growth produced under leaner conditions, more susceptible to wind damage, pest attack and fungal disease. The dense, twiggy, resilient framework of many Australian plants is better built slowly in nutrient-poor conditions.
The hidden sources of phosphorus
Liquid or pelletised synthetic fertiliser is the most obvious risk, but there are several other common products and practices that introduce phosphorus into native garden soils in ways that are easy to overlook.
Compost and soil improvers
Many bagged composts marketed as premium or enriched, have elevated phosphorus levels from the animal manures or bone meal used in their production. Mushroom compost is particularly high in phosphorus and is also alkaline, which creates a double problem for native plants. If you are adding compost to a native garden bed, choose a product specifically formulated for native soils.
If you've already applied standard fertiliser
If you have recently applied a standard fertiliser to phosphorus-sensitive natives and the plants are not yet showing symptoms, water the area deeply and repeatedly over the following week to leach as much phosphorus through the soil profile as possible. Apply a thick layer of fresh organic mulch to help stabilise the soil biology. Do not apply any further fertiliser. Monitor the plants closely over the following month.
Blood and bone
Blood and bone is widely used by gardeners who want a natural, organic product, but it is extremely high in phosphorus and should be kept well away from phosphorus-sensitive native species regardless of how it is sourced or labelled.
Slow-release granules
A standard slow-release fertiliser applied to a grevillea or banksia will cause phosphorus toxicity more slowly than a liquid fertiliser, but the outcome is the same. Only slow-release fertilisers specifically formulated with low phosphorus levels are appropriate for native plants.
Lawn fertiliser runoff
Lawn fertilisers are among the highest-phosphorus products in the domestic garden. If your native garden beds are downslope or adjacent to a fertilised lawn, phosphorus-laden water can move through the soil profile into the native root zone with every rain event or irrigation cycle. This is a slow, cumulative process that is very difficult to reverse once it has begun.
Reading a fertiliser label
The three numbers on any fertiliser label represent the ratio of nitrogen (N), phosphorus (P) and potassium (K). A standard general-purpose fertiliser might read 12:5:7, meaning five per cent of its content is phosphorus. For phosphorus-sensitive Australian natives, look for a product where the P number is below two. Some native-specific fertilisers will list their phosphorus content as a fraction, such as 0.5% or even lower. If the label does not show an NPK ratio, check the guaranteed analysis panel on the back. If phosphorus is not listed separately or the information is not available, do not use the product on sensitive natives.
Do my Australian natives need fertiliser at all?
The vast majority perform perfectly well, flowering generously, growing at a healthy rate and living long lives, with nothing more than good soil preparation at planting, a layer of appropriate mulch, occasional soil conditioner you can make at home and adequate water through establishment.
But there are circumstances where a native-specific fertiliser provides a genuine benefit:
- Establishing young plants in their first season
- Plants growing in containers and pots where nutrients leach out with every watering
- Plants recovering from stress or damage
- Plants in very sandy or heavily leached soils can all benefit from careful, targeted feeding
In these situations a fertiliser formulated specifically for Australian native plants is the right tool.
When and how to feed natives safely
| Situation | Feed? | Guidance |
|---|---|---|
| Established plants in garden beds | Rarely | A well-mulched native garden feeds itself as the mulch breaks down. If the plants are stressed or damaged, an appropriate amount can be beneficial. |
| Young plants in first season | Lightly | If growth seems very slow, a single light application of native-specific slow-release fertiliser in early spring is reasonable. Water in well and do not repeat too regularly. |
| Plants in containers | Yes, regularly | Container plants lose nutrients with every watering. A low-phosphorus native fertiliser applied through the growing season keeps them performing well. |
| Plants recovering from stress | Not immediately | A stressed native best helped first by water, mulch and reduced competition from weeds. Feed lightly only once new healthy growth has appeared. |
| Plants in very sandy or leached soils | Yes, carefully | Sandy soils hold few nutrients and leach quickly. A slow-release native fertiliser applied in autumn gives plants a nutrient buffer going into winter and spring. |
The simple rule
Check the phosphorus level before applying any product to a native garden bed. If the P number on the label is above two, put it back on the shelf.
Australian native plants are not difficult to feed once you understand what they actually need. Most of the difficulty lies in unlearning the habits that work perfectly well in exotic gardens and feel like the right approach.
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A Guide to Australian Native Gardening
How to plan, plant and care for a thriving native garden, whatever your experience level.
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