10 of Australia's Weeds of National Significance from our gardens
Australia has 32 plants officially listed as Weeds of National Significance. This is a federal government classification that identifies species causing the most harm across the country. The bar for inclusion is high.
Weed invasion is the leading pressure on threatened species in Australia. Many of the worst began as garden introductions.
The ten species here are widely distributed and likely to surprise many gardeners who may have encountered them without knowing what they are looking at. All of them are now causing damage costing billions of dollars and immeasurable ecological loss.
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1. Boneseed and bitou bush (Chrysanthemoides monilifera)
Weed of National SignificanceSouth Africa · ~1900s
Boneseed and bitou bush are both native to South Africa and together they represent one of the most destructive coastal weed problems in southern Australia. Bitou bush dominates along the NSW coast, where it now infests an estimated 46% of the coastline between the Queensland border and Nowra. It is one of the most complete ecological takeovers of any habitat type in the country.
Bitou bush (Chrysanthemoides monilifera subsp. rotundata) infests an estimated 46% of the NSW coastline between the Queensland border and Nowra, displacing native coastal scrub communities.
Both plants produce yellow daisy flowers followed by black berries that are spread by birds. The loss of native coastal scrub beneath bitou bush infestations is complete as it does not coexist with native vegetation.
Why it was introduced
Bitou bush most likely arrived in dry ship ballast dumped near Newcastle around 1900 to 1908. From 1946 to 1968 it was then deliberately planted along NSW coastal dunes as an erosion control measure following sand mining.
Native alternative
Coastal wattles, coastal grevilleas and westringia provide equivalent erosion control and screening without any invasive risk. Lomandra and native sedges stabilise dune faces effectively.
Restored coastal scrub of native wattles, grevilleas and westringia supports honeyeaters and other small birds that bitou bush thickets exclude entirely.
2. Serrated tussock (Nassella trichotoma)
Weed of National SignificanceSouth America · early 1900s
Serrated tussock is one of the most ecologically damaging weeds and one of the hardest to identify. It looks almost exactly like several native tussock grasses. But the distinction matters enormously. A single serrated tussock plant can produce 100,000 seeds per year. In parts of the ACT and tablelands NSW, it has replaced more than half of the original native grassland cover.
Serrated tussock (Nassella trichotoma) is almost indistinguishable from native tussock grasses and one of the most productive seed-setters of any weed in Australia.
Harry Rose from South West Rocks, Australia, CC BY 2.0, via Wikimedia Commons
The key identification feature is the leaf tip: serrated tussock leaves are very fine, curl inward and have a distinctive serrated edge that catches on skin and clothing.
Why it is so destructive
Serrated tussock has almost no nutritional value for livestock or native animals. Once it dominates a grassland, it creates a monoculture that produces no meaningful food web. It is also extremely difficult to eradicate as seeds remain viable in the soil for several years.
Native alternative
Kangaroo grass (Themeda triandra) and wallaby grass (Rytidosperma species) are the appropriate native tussock grasses for temperate grassland restoration. Both are visually similar to serrated tussock when young but support richer insect and bird communities.
Kangaroo grass (Themeda triandra) — the native tussock grass to plant in place of serrated tussock, supporting a far richer insect and bird community once established.
3. Scotch broom (Cytisus scoparius)
Weed of National SignificanceEurope · early 1800s
Scotch broom invades highland and subalpine environments, including the margins of native alpine ash forest and subalpine woodland. These are some of the most ecologically significant and least resilient habitats in southeastern Australia.
Scotch broom (Cytisus scoparius) — the bright yellow pea flowers make it visually appealing in spring, which contributed to its spread as a garden and roadside plant across cool temperate Australia.
It was widely used in highland revegetation and soil stabilisation programs from the mid-twentieth century and planted in gardens for its vivid spring flowers. A single plant can produce 12,000 seeds annually, with seeds remaining viable in the soil for up to thirty years. It also fixes atmospheric nitrogen, which alters soil chemistry and favours further broom establishment over native species.
Native alternative
Native pea-flowered shrubs including Pultenaea and Daviesia species provide similar spring flower display in cool temperate gardens without any invasive risk. For highland revegetation, snow gum (Eucalyptus pauciflora) and mountain ribbongum (E. viminalis) are the appropriate structural species.
Running postman (Kennedia prostrata) — a hardy native pea-flowered groundcover that gives the spring colour of broom without the invasive risk.
4. Blackberry (Rubus fruticosus aggregate)
Weed of National SignificanceEurope · mid-1800s
Blackberry is arguably the most widely recognised weed on this list. It infests more than 8.8 million hectares of land across Australia, primarily in the temperate southeast. It is the most widespread environmental weed in Victoria and one of the most costly to manage nationally.
Blackberry (Rubus fruticosus aggregate) infests more than 8.8 million hectares across Australia, making it one of the most geographically extensive environmental weeds on the continent.
Blackberry forms dense thickets that harbour foxes and rabbits, exclude native understorey plants and prevent native tree regeneration. It spreads by both seed (dispersed by birds and foxes) and by tip-rooting, where arching canes touch the soil and establish new plants.
In the garden
Established blackberry cannot be controlled by cutting alone. The root crown must be removed or the plant will reshoot. Use loppers to cut back all canes first, then dig out the root crown with a sharp spade. Wear thorn-proof gloves as the canes are aggressively spined and will cause serious injury without adequate protection.
What Weed of National Significance means
The WONS classification is given to plants that are invasive or have the potential to become so, have significant impact on the economy, environment or community; and are a nationally significant problem that require coordinated action. It is reviewed and updated periodically by the Australian Weeds Committee.
5. Sweet briar (Rosa rubiginosa)
Weed of National SignificanceEurope · mid-1800s
Sweet briar is the WONS entry that is most surprising. It looks exactly like a classic cottage rose and for much of Australian gardening history was actively encouraged. It is native to Europe and western Asia and was introduced both deliberately as an ornamental and inadvertently in contaminated seed and packing material in the nineteenth century.
Sweet briar (Rosa rubiginosa) — the pink flowers and red autumn hips make it visually appealing, which is why it was planted widely across temperate Australia before its invasive potential was understood.
In temperate grassland and grassy woodland — already among Australia's most threatened ecological communities — sweet briar forms impenetrable thickets that exclude native grasses and forbs. The hips are eaten and spread by birds and foxes, allowing it to establish across uncleared land rapidly.
In the garden
Remove by digging out the entire root system as sweet briar reshoots aggressively from any remaining root tissue. Cut back all growth first with loppers or secateurs, then use a sharp spade to expose and remove the root crown. Thorn-proof gloves are essential.
Native alternative
Native pink-flowered shrubs including correas, boronias and native rose (Boronia serrulata) provide equivalent flower colour and garden interest in temperate settings without any invasive risk.
Native rose (Boronia serrulata) — the one species that genuinely carries the common name, and a fragrant native substitute for sweet briar.
6. St John's wort (Hypericum perforatum)
Weed of National SignificanceEurope · 1880s
The plant most Australians associate with a gentle herbal supplement is, in ecological terms, one of the country's worst declared weeds. It causes hundreds of millions of dollars in agricultural losses annually.
St John's wort (Hypericum perforatum) — sold in pharmacies as a herbal remedy while simultaneously listed as a Weed of National Significance causing significant agricultural and ecological harm.
It produces vivid yellow five-petalled flowers and spreads prolifically by seed across disturbed ground, roadsides, pasture and native grassland. It is toxic to livestock and causes photosensitisation in animals that consume it, leading to significant production losses.
In the garden
Small infestations can be removed by hand-pulling or digging before seed set, using a hori hori to extract the tap root cleanly.
Native alternative
Native yellow-flowered groundcovers and perennials including yellow buttons (Chrysocephalum apiculatum) and bulbine lily (Bulbine bulbosa) provide equivalent low-growing yellow flower display in temperate gardens without any invasive risk.
Native bulbine lilies fill the same low, sunny niche that St John's wort exploits, without the invasive risk.
7. Alligator weed (Alternanthera philoxeroides)
Weed of National SignificanceSouth America · 1946
Alligator weed is one of the world's most problematic aquatic weeds and is considered among the most difficult to eradicate. It was most likely introduced to Australia in ship ballast dumped at the Newcastle docks, where it was first recorded in 1946. It grows both in water and on land, forming dense floating mats. A single fragment of stem is capable of establishing a new plant.
Alligator weed (Alternanthera philoxeroides) — forms dense floating mats on waterways and establishes on land with equal ease; a single stem fragment is sufficient to start a new infestation.
It has invaded the Hawkesbury-Nepean, Macleay, Clarence and Richmond river systems in NSW and parts of the Queensland coast and the Murray-Darling basin. On water surfaces it blocks light, depletes oxygen and eliminates habitat for native aquatic animals. On land it suppresses native riparian vegetation.
In the garden
If found in a garden pond or water feature, remove all plant material carefully to avoid fragmentation, bag securely and contact your council for disposal advice. You can report infestations to your state biosecurity authority.
Water snowflake (Nymphoides indica) — one of many native aquatic plants that belong in a pond or water feature in place of alligator weed.
8. Mimosa (Mimosa pigra)
Weed of National SignificanceTropical America · late 1800s
Mimosa is the most ecologically destructive weed in Australia's tropical north. A single shrub can reach four metres in height within two years and produce 9,000 seeds annually. Those seeds remain viable in the soil for up to twenty-three years. In the Northern Territory, mimosa has invaded more than 800 square kilometres of the Adelaide River floodplain alone.
Mimosa (Mimosa pigra) — has invaded more than 800 square kilometres of the Adelaide River floodplain in the NT alone, converting open wetland into impenetrable thorny thicket.
Kakadu National Park, Fogg Dam Conservation Reserve and the Mary River floodplain are all significantly affected. The loss of open floodplain habitat has had cascading effects on magpie geese, brolgas, waterbirds and the native plant communities that depend on periodic flooding and fire. Mimosa is resistant to both fire and flooding — the two ecological processes that maintain tropical floodplain biodiversity — which gives it an enormous competitive advantage over native vegetation.
In the garden
If found on a property in NT or QLD, report it to the relevant state biosecurity authority immediately — it is subject to mandatory control requirements in both jurisdictions. Do not attempt removal without guidance from local authorities, as small-scale disturbance can accelerate seed dispersal.
9. Hawkweeds (Hieracium and Pilosella species)
Weed of National SignificanceEurope
Hawkweeds are among the least known of the Weeds of National Significance to most Australians. Several species in the Hieracium and Pilosella genera have invaded alpine and subalpine zones in Victoria and New South Wales, where they pose a direct threat to some of our most ecologically unique and sensitive plant communities.
Mouse-ear hawkweed (Pilosella officinarum) — one of the hawkweed species currently threatening alpine and subalpine plant communities in the NSW and Victorian high country.
In New Zealand, where hawkweeds arrived earlier, they have dominated large areas of high-country grassland, reducing floristic diversity to a fraction of its original level. Australian authorities are attempting to prevent the same outcome, but eradication is extremely challenging in remote and difficult terrain.
In the garden
Hawkweeds are not sold in Australian nurseries and are not a garden plant. If you walk in alpine areas, clean boots, clothing and gear before and after visits. Report any sightings in alpine zones to the relevant state authority or the Hawkweed Hotline maintained by the NSW Department of Primary Industries.
10. Parkinsonia (Parkinsonia aculeata)
Weed of National SignificanceCentral America · 1800s
Parkinsonia has become one of the most serious weed threats across arid and semi-arid Australia. It was introduced in the nineteenth century as an ornamental tree and shade plant for outback station gardens and as a fodder and hedge plant. Its drought tolerance, rapid growth and prolific seed production made it seem ideal for harsh inland conditions. The consequences are now visible across millions of hectares of arid floodplain and pastoral land.
Parkinsonia (Parkinsonia aculeata) — introduced as a shade and fodder tree for outback stations, now infesting millions of hectares of arid floodplain across the NT, QLD, WA and SA.
Parkinsonia seeds germinate prolifically after flooding events, when floodwaters carry seeds across entire catchments and deposit them across freshly disturbed floodplain soil. It also provides harbour for feral animals including pigs, which further accelerate its spread.
In the garden
If found on a property in the NT, QLD, WA or SA, contact your state biosecurity authority for control guidance — it is subject to mandatory management requirements across most of its range. Young trees can be removed using a sharp spade and loppers, but mature trees require professional equipment.
What you can do in your own garden
The garden is one of the primary pathways through which invasive plants enter the landscape, which means it is also one of the places where individual action makes a measurable difference. The principles are simple and the table below sets out the practical dos and don'ts for keeping a garden from becoming a source of the next infestation.
| Do | Don't |
|---|---|
| Check plants against your state weed list before buying or accepting cuttings, especially older ornamentals | Assume a plant is safe because a nursery is selling it — several WONS species were sold ornamentally for decades |
| Remove the entire root crown when clearing a weed, since most on this list reshoot from remaining root tissue | Cut back and walk away — partial removal stimulates vigorous regrowth in blackberry, sweet briar and broom |
| Bag and bin seed-bearing or fragmenting material as general waste | Compost weed material or put it in green waste, where seeds and fragments survive and spread |
| Confirm identification with a local weed authority when a plant resembles a native, as with serrated tussock | Remove a suspected weed on a guess — you may be pulling a native tussock or lily instead |
| Clean boots, tools and gear after walking in alpine or bushland areas to avoid carrying seed home | Move soil, mulch or machinery between sites without checking it for weed seed and root fragments |
| Replace a removed weed with a regional native that fills the same garden role | Leave cleared ground bare, which simply invites the same or another weed to recolonise |
| Report notifiable species such as mimosa, parkinsonia and alligator weed to your state biosecurity authority | Attempt removal of a notifiable declared weed yourself before seeking guidance — disturbance can worsen spread |
The good news
The scale and cost of the weed problem is daunting, but the record of coordinated control shows that targeted effort produces recovery. Bitou bush is the clearest example: sustained control along the NSW coast has allowed native coastal scrub to regenerate. Biological control has delivered some of the most durable wins. The alligator weed flea beetle (Agasicles hygrophila) suppresses aquatic infestations to a fraction of their former vigour in warmer catchments. The sustained release of seed-feeding beetles against parkinsonia and mimosa has slowed the rate of spread that once looked unstoppable.
The alligator weed flea beetle (Agasicles hygrophila) suppresses aquatic infestations to a fraction of their former vigour.
Stephen Thorpe, CC BY 4.0, via Wikimedia Commons
Revegetation is the other half of the story. Once a weed is removed, replanting the cleared ground with regional natives is what prevents reinvasion. Landcare groups, catchment authorities and Traditional Owner ranger programs have restored river frontages, grassy woodlands and dune systems by combining weed removal with dense replanting. The returning insects, birds and understorey diversity confirm the approach works.
Bushland regeneration combining weed removal with dense native replanting is what turns a cleared site back into functioning habitat.
None of this eradicates the problem, but it demonstrates that a weed-dominated landscape can be turned back toward a functioning one, and that the garden-scale version of the same work — remove, replace, maintain — is a genuine contribution.
Native plant communities that have not been invaded retain ecological functions — soil stability, water filtration, carbon storage, food webs — that weed-dominated landscapes cannot replicate.
What gardeners can do is straightforward: do not plant WONS species or other known invasives, remove any that are already present and replace them with native plants appropriate to your region and climate.
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