So-called Cactus Warriors wade into a sea of wheel cactus in Central Victoria. (Supplied: Lee Mead, Tarrangower Cactus Control Group)
A crack team of ecowarriors is on a seek-and-destroy mission.
They alight from a convoy of all-terrain passenger vehicles and head into the scrub armed with poison and other tools of destruction.
The target of this raid is an unwelcome, spiky invader with the potential to turn an entire landscape into a prickly wasteland.
Native to Mexico, wheel cactus (Opuntia robusta) was likely introduced to Australia, innocently enough, as a hardy ornamental garden plant. It escaped domestication and is now listed among Australia's worst weeds of national significance.
The so-called Cactus Warriors from Maldon in Central Victoria (officially the Tarrangower Cactus Control Group) have waged a campaign against this noxious weed for two decades.
Control Group volunteers Asha Bannon and Jan Elliott attacking a wheel cactus infestation. (Supplied: Lee Mead, Tarrangower Cactus Control Group)
There have been some local successes. But one of the traits of these weeds is their ability to bounce back after even the shortest period of neglect.
"We have had huge success with awareness raising [about wheel cactus and other cactus varieties]," the group's president Lee Mead said.
"We guesstimate that 70 per cent of our local landholders now control their infestations.
"It's because of the other 30 per cent, who are mostly absentee landowners, that the problem persists.
"It will never go away because there are still cactus plants there and it's just so invasive."
Wheel cactus is just one of several varieties of exotic cactus that thrive in Australian conditions.
Other Opuntia varieties to establish themselves in Australia include common prickly pear (Opuntia stricta) and Hudson's pear (Opuntia Cylindropuntia pallida).
Left unchecked, invasive species like wheel cactus (pictured) and prickly pear can take over a landscape. (Supplied: Lee Mead, Tarrangower Cactus Control Group)
The attributes that made it attractive for gardeners — it grows easily in comparatively hostile conditions and is devilishly hard to kill — make it a menace for farmers and environmentalists.
While smaller plants can be dug out of the ground, the most practical way to destroy larger infestations is to directly inject herbicides such as glyphosate into every cactus lobe.
It is labour-intensive work.
Weeds of national significance
In 1999 the Australian government identified 20 weed species as "weeds of national significance".
The list was created with input from the science community. In 2012 an additional 12 weeds of national significance were added.
Each is extraordinarily well equipped to survive and thrive in Australian conditions and together they represent the greatest threats to agriculture and the natural ecosystem, according to Ali Bajwa, a senior lecturer in weed science and agronomy at La Trobe University.
Ali Bajwa with a gazania plant removed from Arno Bay in South Australia. (Supplied: Ali Bajwa)
"In Australia we have a variety of climatic conditions often not very conducive to plant growth, but a lot of these species probably benefit from that stress-tolerance ability as well," Dr Bajwa said.
"Some of the common traits are: they are really furious competitors, so they compete with native plants for space, light, water, nutrients.
"They are often massive seed producers or have other plant structures that help them to establish very quickly.
"If a species producing thousands of seeds and every life cycle has that ability to establish quickly, that's a massive benefit.
"Some of the other traits include stress-tolerance ability, so a lot of our problematic weeds or invasive plants, they naturally have the ability to tolerate harsh climates."
Serrated tussock growing near Bathurst in New South Wales. (Supplied: Ali Bajwa)
The Australian Centre for Invasive Species Solutions estimates noxious weeds (including weeds of national significance) cost Australia about $5 billion per year, a figure which includes agricultural and environmental damage, as well as the expense of controlling them.
"Weed management is Australia's most expensive land management problem – for both the environment and agriculture," Invasive Species Council policy director Carol Booth said.
"Recent research found that the cost of weed management made up more than three-quarters [81 per cent] of the total cost for recovering Australia's threatened species," Dr Booth said.
Dr Bajwa said prevention and early intervention were generally the most cost-effective approach for managing noxious weeds.
However, with most of these weeds of national significance, the horse has already bolted.
"Unfortunately, many of these species are widespread. The threshold to declare them as weeds of national significance requires them to be a problem in more than one state or at least having major impacts in any region," Dr Bajwa explained.
"When we talk about 'eradicating' weed species, I'm not a big fan of that terminology for widespread weeds. Once a weed is there, it's there permanently. We cannot remove them completely; however, with integrated weed management there are success stories.
"Things like biological control where you use their natural predators, along with appropriate chemical herbicides, you can start winning the war."
An impenetrable wall of wheel cactus. (Supplied: Lee Mead, Tarrangower Cactus Control Group)
Ten of the worst weeds
There are 32 Australian weeds of national significance. These are among the worst, according to Dr Bajwa.
CACTUS (Opuntia spp.) — several varieties are considered weeds of national significance, including wheel cactus (Opuntia robusta), common prickly pear (Opuntia stricta), drooping prickly pear (Opuntia monacantha) and tiger pear (Opuntia aurantiaca). Extremely hardy and capable of inflicting nasty wounds with its spines.
Wheel cactus (Opuntia robusta). (Supplied: Lee Mead, Tarrangower Cactus Control Group)
BLACKBERRY (Rubus fruticosus) — while many like the taste of its fruit, this thorny shrub spreads quickly, is difficult to kill, and its prickly thickets provide shelter for invasive animals like rabbits and foxes.
Blackberry (Rubus fruticosus). (Supplied: Invasive Species Council)
GORSE (Ulex europaeus) — A thick, woody shrub that is a major agricultural weed in Tasmania and Victoria. Originally introduced as a hedge it now dominates south-eastern landscapes.
Gorse (Ulex europaeus). (ABC Rural: Luke Radford)
SERRATED TUSSOCK (Nassella trichotoma) — this wiry, tufted grass has little nutritional value for stock and quickly dominates grasslands at the expense of native species. Each plant can produce up to 100,000 seeds.
Serrated tussock (Nassella trichotoma). (Supplied: Ali Bajwa)
ALLIGATOR WEED (Alternanthera philoxeroides) — native to South America, alligator weed forms large dense mats on water surfaces, clogging waterways and killing native flora and fauna.
Alligator weed (Alternanthera philoxeroides). (Supplied: CSIRO)
LANTANA (Lantana camara) — a sprawling shrub with brightly coloured flowers. Most types are toxic to livestock and poisoning is common. Spreads by seeding and layering into dense thickets.
Lantana (Lantana camara). (Supplied: Invasive Species Council)
PARKINSONIA (Parkinsonia aculeata) — introduced as an ornamental plant, it is a fast grower and has spread to 800,000 hectares of Australian land.
Parkinsonia (Parkinsonia aculeata). (Wikimedia Commons: Stan Shebs, Parkinsonia aculeata, CC BY-SA 3.0)
PARTHENIUM (Parthenium hysterophorus) — also known as bitter weed and carrot grass, it is toxic to animals, including humans, and capable of producing more than 15,000 seeds per plant.
Parthenium (Parthenium hysterophorus). (Supplied: Ali Bajwa)
GAMBA GRASS (Andropogon gayanus) — native to African savannahs, gamba grass was introduced into Australia as pasture. It has outcompeted native grasses and is a major grassfire risk.
Gamba grass (Andropogon gayanus) grows quickly in dense thickets. (Supplied: CSIRO)
WATER HYACINTH (Eichhornio crassipes) — also known as water orchid and Nile lily, it is native to the Amazon basin in South America. It grows on still or slow-flowing water, and is highly invasive, blocking waterways and making it unfit for drinking.
Water hyacinth (Eichhornio crassipes). (Supplied: United States Department of Agriculture)
And four more of our worst weeds
In addition to the 32 designated weeds of national significance, there are other plants which are damaging within specific environments.
BUFFEL GRASS (Cenchrus ciliaris) — described as a weed of potential national significance. Prevalent in northern Australia and high concentrates are a significant fire risk. Buffel grass is currently being considered for addition to the list of nominated weeds of national significance.
Buffel grass seedheads in Hawaii. (Flickr: Forest and Kim Starr, CC BY 2.0)
ANNUAL RYEGRASS (Lolium rigidum) — while useful as forage pasture, is the number-one problem weed for grain growers and can take over native grasslands. Resistant to many herbicides, costing the grains industry more than $93 million annually.
Annual ryegrass (Lolium rigidum). (Supplied: Ali Bajwa)
GAZANIA (Gazania ssp.) — a flowering ornamental plant that has escaped gardens and now dominates roadsides in Victoria and South Australia. Still sold in some plant nurseries despite being an invasive weed.
Gazania is an invasive weed. (Supplied: Department of Environment and Water)
PATERSON'S CURSE (Echium plantagineum) — known as Salvation Jane in South Australia, it was introduced in the 19th century as a garden plant. Its wide leaves smother native plants and it is potentially toxic to livestock.
Paterson's Curse (Echium plantagineum) is a noxious weed in Australia. (ABC Central West: Luke Wong)