Native plants that are weeds outside their local range in Australia
One common myth in Australian gardening is that choosing a native plant is always the responsible choice. It is mostly a great one, but there are some exceptions.
A small group of Australian natives are significant environmental weeds within Australia, because they've been planted and spread aggressively outside their home range. Several are among the most damaging environmental weeds in the Australian regions they have invaded. Here are some of the worst offenders.
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Natives that are considered weeds outside their local range in Australia
1. Sweet Pittosporum (Pittosporum undulatum)
Sweet pittosporum is one of the most damaging environmental weeds in southern Australia. Native to moist gullies and rainforest margins along the east coast, it has spread far beyond that range into the dry sclerophyll woodlands of western Victoria, the heathlands of South Australia and the coastal bushland of south-western WA. It has been carried almost entirely by introduced European blackbirds, which eat the orange fruit and deposit seeds throughout the surrounding bush.
It grows rapidly, creates dense shade that suppresses virtually all understorey plants and produces chemicals in its leaf litter that further inhibit competition from other species. Even within its native range around Sydney, it has expanded aggressively into habitat types it did not previously occupy, aided by nutrient-rich urban runoff and the suppression of bushfire. It is listed in the Global Invasive Species Database and is a declared weed in WA.
2. Sydney Golden Wattle (Acacia longifolia)
A beautiful fast-growing wattle with bright yellow flower spikes in late winter and spring, Acacia longifolia is native to the coast and ranges of NSW and Victoria. It has been planted extensively across Australia as a sand stabiliser and ornamental. In WA and SA, it has escaped cultivation and invaded coastal scrub and heathland, outcompeting local species and altering soil chemistry through nitrogen fixation. The enriched soils that result are unsuitable for the low-nutrient-adapted native flora of WA. It forms dense monocultures that are extremely difficult to remove.
Krzysztof Ziarnek, Kenraiz, CC BY-SA 4.0, via Wikimedia Commons
It is also listed among the 100 worst invasive species in the world and has invaded vast areas of coastal Portugal, which gives some indication of how serious a plant this is when conditions suit it.
3. Port Jackson Wattle (Acacia saligna)
The near-inverse of Acacia longifolia, Port Jackson Wattle is native to WA and invasive in the eastern states. It is a striking, fast-growing species with blue-green weeping foliage and masses of bright yellow flowers in spring. It was planted widely across southern Australia for erosion control and revegetation and has naturalised aggressively in SA and parts of Victoria.
Like Acacia longifolia, it fixes nitrogen and alters soil chemistry in ways that disadvantage locally adapted flora. In South Africa, where it was introduced under the same rationale of dune stabilisation, it is now a Category 1 declared invasive species. It is a measure of how seriously land managers elsewhere treat the consequences of misplaced Australian natives.
4. Silky Oak (Grevillea robusta)
Grevillea robusta is fast-growing with striking fern-like foliage and brilliant orange flower clusters and has been widely planted as a street tree, farm shelter belt and garden ornamental across Australia. Native to subtropical rainforest margins in south-east Queensland and northern NSW, it has naturalised extensively in the Sydney basin and Blue Mountains, where it invades dry sclerophyll forest and woodland.
The concern is not just the displacement of local vegetation but genetic contamination. It hybridises with local grevillea species, altering the genetics of populations that have evolved in isolation for thousands of years. That process is essentially irreversible. It is also a known contact allergen as the pollen and sap cause skin reactions in sensitive individuals, so wearing gloves when handling it is advisable.
5. Coast Tea Tree (Leptospermum laevigatum)
Coast tea tree is a tough salt-tolerant shrub that was planted across southern Australia to stabilise coastal dunes. In WA, it is now a high priority environmental weed listed in the state's Environmental Weed Strategy, invading coastal heathland and forming dense thickets that exclude the mostly endemic WA native flora. In SA and south-west Victoria it has behaved similarly.
It produces chemicals in its leaf litter that reduce the germination of companion species, effectively sterilising the ground beneath it. The lesson here is a familiar one in Australian environmental history: a plant chosen for its toughness and fast establishment in difficult coastal conditions turns out to be difficult to stop for exactly the same reasons.
6. Cape Leeuwin Wattle (Paraserianthes lophantha)
In the nineteenth century, Ferdinand von Mueller — then government botanist of Victoria — handed packets of Cape Leeuwin Wattle seeds to explorers heading into the interior. He said that the fast-growing trees would mark their campsites and provide cattle browse and shelter for establishing other plants. The trees grew, seeded and spread.
It is now an environmental weed in Victoria, NSW, Tasmania and South Australia, where it invades coastal scrub, disturbed bushland and stream banks, growing to eight metres in a single season under good conditions and producing large quantities of seed that remain viable in the soil for decades. Native to a small section of the south-west WA coast, it is now a weed across four Australian states.
7. Broad-leaved Paperbark (Melaleuca quinquenervia)
Broad-leaved paperbark is a beautiful tree with cream bottlebrush flowers, papery peeling bark and excellent tolerance of waterlogged soils. It is native to coastal Queensland and north-east NSW. Beyond its natural range, it has naturalised into wetland margins and riparian vegetation, where its tolerance of wet ground gives it a significant competitive advantage over local species less adapted.
Its behaviour in Florida, where it was deliberately introduced to drain wetlands in the early twentieth century and has since invaded more than 200,000 hectares of the Everglades, offers a stark illustration of what this species is capable of when its ecological constraints are removed.
8. White Oak Grevillea (Grevillea baileyana)
White oak grevillea is a handsome rainforest tree native to the wet tropics of far north Queensland, where it is an ecologically valuable species. In south-east Queensland, where it has been widely planted as a street tree and garden specimen for its attractive foliage and fast growth, it has escaped into local bushland and is listed on the Brisbane City Council environmental weed register.
Tatiana Gerus from Brisbane, Australia, CC BY-SA 2.0, via Wikimedia Commons
The distance between its home range and the areas it has invaded is only a few hundred kilometres. South-east Queensland has its own distinct flora and ecology, and unfortunately this tree from the wet tropics does not belong.
How to check before you plant: The Atlas of Living Australia (ala.org.au) allows you to search any plant species and see its recorded natural distribution. Your local council's environmental weed list is also worth checking as most are available online and updated regularly.
But what about native weeds versus exotic ones?
It is a reasonable question: if a plant is going to escape the garden and cause problems, is it better or worse for it to be an Australian native rather than an exotic species? The instinctive answer is that a native must be preferable because it belongs to the landscape in a broad sense.
But ecologists regard native weeds outside their range as a distinct and (in some ways) more insidious problem than exotic ones.
Visibility
Exotic weeds are obviously foreign and land managers, ecologists and community volunteers can identify them, target them and make the case for removing them. A native plant that has escaped its range can be harder to argue against and harder for us to perceive as a problem.
Genetic pollution
An exotic weed cannot interbreed with Australian native species. A native plant outside its range sometimes can and when it does, it corrupts the locally adapted genetics of species that have evolved in isolation for thousands of years. This is considered one of the most serious and irreversible threats to the genetic integrity of Australian plant populations.
Ecological advantage
Native plants outside their range can exploit relationships with soil fungi, with birds that disperse their seeds and with the fire regimes of their new home that local species have no defences against. They arrive with the ecological toolkit of an Australian plant in an Australian landscape, which makes them far more effective invaders.
None of this means that choosing native plants is the wrong approach. It means that it's important to be mindful of the impacts our choices can have.
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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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