10 Australian native plants with a hidden dark side

1. Gympie gympie (Dendrocnide moroides)

Stinging tree · Rainforest · QLD and northern NSW

Gympie gympie is considered the most painful plant in the world and the experience of being stung by one is not something its victims tend to describe lightly. Found in the Queensland rainforest, it has reportedly caused horses to throw themselves off cliffs and at least one WWII soldier who mistakenly used its leaves as toilet paper to have never recovered psychologically. It appears as a straightforward large-leafed shrub but is a wildly deceptive Australian plant that offers no warning of what it is capable of.

Steve Fitzgerald, CC BY-SA 4.0, via Wikimedia Commons

The hidden secret

The entire plant is covered in silica-tipped hollow hairs so fine they are nearly invisible to the naked eye. On contact, the tips break off and inject a neurotoxic peptide directly into the skin. The initial sensation is described as being burned with acid and electrocuted. The hairs remain embedded in the skin and the toxin is reactivated by cold water, touch, or even changes in atmospheric pressure. Victims have reported episodes of intense pain months and years after the original sting.

2. Poison peas (Gastrolobium species)

Gastrolobium · WA shrublands · Southwestern WA

The poison peas of southwestern Western Australia look exactly like the kind of thing you might plant in a native garden without a second thought. They are small shrubs with pea-shaped flowers in yellow, orange or red. They are also, chemically speaking, one of the more remarkable things growing in Australian soil.

jeans_Photos, CC BY 2.0, via Wikimedia Commons

The hidden secret

The poison peas did something that took human chemists until 1080 to figure out: they synthesised sodium fluoroacetate and deployed it as a defence against herbivores. The compound (better known as 1080), used extensively in pest control, occurs naturally in around 40 Gastrolobium species. Native marsupials that evolved alongside these plants developed tolerance to fluoroacetate over millions of years. Introduced animals have no such tolerance so a sheep grazing on Gastrolobium calycinum will die from heart failure within hours. This is one of the most striking examples of chemical survival strategy in the Australian flora.

3. Zamia palms (Macrozamia species)

Macrozamia · Widespread · Eastern and southwestern Australia

Cycads are among the oldest living plant groups on earth. They predate the dinosaurs and have been part of the Australian landscape for tens of millions of years. The plants themselves are slow-growing and popular in Australian gardens. However, the seeds in the bright orange-red cones are toxic enough to kill livestock.

MargaretRDonald, CC BY-SA 4.0, via Wikimedia Commons

The hidden secret

The seeds contain cycasin, a compound that causes severe liver damage, neurological degeneration and a progressive condition known as zamia staggers in cattle. Indigenous Australians across the east and southwest developed a detoxification process involving leaching the seeds in running water for days, or fermenting and roasting them in a specific sequence. Get the process wrong and the result is poisoning. Get it right and the seeds are a significant food source.

4. Milky mangrove (Excoecaria agallocha)

Blind-your-eye mangrove · Coastal · Northern and eastern Australia

The milky mangrove is an unremarkable-looking plant with small oval leaves and greyish bark that gives no obvious warning of what the inner tissues contain. It is a component of mangrove systems across a wide stretch of northern coastline and entirely innocuous unless damaged. In some parts of northern Australia it goes by a more direct common name: blind-your-eye mangrove.

Vengolis, CC BY-SA 4.0, via Wikimedia Commons

The hidden secret

The milky white latex sap that oozes from any cut or broken surface causes severe chemical burns on contact with skin and if it reaches the eyes, temporary or permanent blindness. Fishermen working in mangrove systems along the northern coast have a long history of learning about this plant the hard way. The same compound made the plant useful: Aboriginal peoples of northern Australia used the toxic latex to stun fish in tidal pools, a technique that required careful handling to avoid self-poisoning.

5. Gymea lily (Doryanthes excelsa)

Giant lily · Coastal heathland · NSW

The gymea lily is one of the most dramatic plants in the Australian flora. It has a rosette of strap-like leaves up to two metres long, from which a flower spike rises to five or six metres bearing a crown of deep crimson flowers. It is spectacular, unmistakable and, in the right setting, genuinely breathtaking.

The hidden secret

The leaves and roots contain saponins, compounds that disrupt the surface tension of water and interfere with the gill function of fish. Aboriginal peoples of the Sydney Basin used the lily crushed in waterholes to stun fish for collection. The fish were not rendered inedible as the saponins dissipate quickly. The same saponins cause skin irritation in some people handling the plant, so always wear thick, long gloves.

6. Native nightshades (Solanum species)

Solanum · Widespread · Across Australia

The native nightshades occupy an uncomfortable middle ground: some are edible when ripe, some are toxic regardless of ripeness, and several look virtually identical to each other. The genus Solanum in Australia includes around 100 species, one of the more remarkable examples of how a single plant genus can evolve in radically different directions under Australian conditions. The same identification challenges apply to many toxic ornamental plants found in Australian gardens.

The hidden secret

The toxin in the dangerous species is solanine, the same compound that makes green potatoes harmful. In sufficient quantities, it causes gastrointestinal distress, neurological symptoms, and in serious cases respiratory failure. Experienced botanists occasionally find the Australian Solanum species difficult to distinguish from each other. The rule with any unfamiliar Solanum berry is straightforward: do not eat it unless you have identified the species with certainty.

7. Moreton Bay chestnut (Castanospermum australe)

Black bean · Rainforest · QLD and northern NSW

The Moreton Bay chestnut is a large, beautiful rainforest tree widely planted as a street tree in subtropical Queensland. The timber, known commercially as black bean, is one of the finest cabinet timbers produced by the Australian rainforest. It is also one of the most pharmaceutically interesting trees in the country.

The hidden secret

The seeds have the same shape, similar size and weight in the hand as European chestnuts. Early European settlers in Queensland ate them and became seriously ill. The seeds contain castanospermine, which causes severe gastrointestinal damage in sufficient doses. Here is where it gets interesting: the same compound has been studied as a potential antiviral agent. Castanospermine inhibits the same enzymes that certain viruses use to replicate, which makes a lethal seed from a Queensland rainforest tree a subject of genuine pharmaceutical interest.

8. Macadamia (Macadamia integrifolia)

Queensland nut · Subtropical rainforest · QLD and northern NSW

Macadamia is Australia's only native to have become a significant global food crop. It is considered bulletproof as one of the most reliable and rewarding trees for subtropical gardens. The nuts are genuinely delicious and the whole picture is one of uncomplicated success. Except for one thing.

The hidden secret

Macadamia nuts are acutely toxic to dogs. The toxin causes weakness, hyperthermia, vomiting, tremors and, in severe cases, temporary paralysis of the hindquarters. The symptoms typically appear within 12 hours and resolve within 48 hours without treatment, but the experience is alarming and the cause is often not immediately identified. The irony of Australia's most celebrated native food crop being dangerous to one of the most common companion animals on the continent is not lost on anyone who has had to explain it to a vet.

9. Poison peach (Trema tomentosa)

Trema · Woodland and disturbed ground · Eastern and northern Australia

Poison peach looks like a perfectly ordinary small tree. It grows in disturbed ground, roadsides and forest margins across a wide range of eastern and northern Australia. It does not have the notorious reputation of gympie gympie or the cultural familiarity of nightshades, which is part of the problem.

Mark Marathon, CC BY-SA 4.0, via Wikimedia Commons

The hidden secret

Horses grazing in paddocks where it grows have died from a condition known as Kimberley horse disease. The toxin targets the liver, causing progressive and irreversible damage. By the time neurological symptoms appear, the liver damage is typically too advanced to reverse. The berries have also caused poisoning in children.

10. Coast banksia (Banksia integrifolia)

Coast banksia · Coastal and warm temperate · Eastern Australia

Coast banksia is one of the most reliably garden-worthy natives in the Australian flora. It is tough, fast-growing, spectacular in flower and extraordinarily attractive to honeyeaters and native bees. It is also, as it turns out, running a very small bar.

The hidden secret

The flower spikes of banksia species produce large volumes of nectar that ferments rapidly in warm conditions. Possums and lorikeets feeding heavily on banksia flowers frequently exhibit the classic signs of intoxication, losing coordination and becoming uncharacteristically docile. The flowers are perfectly safe to grow and the plant remains one of the best garden choices on the east coast. It just occasionally turns the local wildlife into a slightly undignified version of itself.