If you care about Australia’s natural environment, you may be shocked at the devastation caused by exotic garden plants.
In Potential Environmental Weeds in Australia, Steve Csurches and R. Edwards summarise the problem:
"Non-indigenous invasive plants are an insidious, widespread and poorly recognised threat to Australia’s native plant communities and associated wildlife. At least 2,200 species of non-native plants have naturalised in Australia . . . Since additional plant species are imported each year, the invasion process is by no means complete . . . Some species that could have been eliminated quite cheaply when first detected, have now spread over many thousands of hectares . . . "
In Feral Future: The Untold Story of Australia’s Exotic Invaders, biologist Tim Low goes to the heart of the problem:
"We are locked into an absurd cycle of introducing new garden plants (new weeds-to-be) whenever older ones become weedy and fall out of favour. And the problems are getting worse."
Australia’s most damaging weeds
A team of CSIRO scientists, commissioned by WWF-Australia, has researched the impact of invasive garden plants on natural ecosystems and Australian agricultural land.
The report on their findings reveals that garden plants account for 94 per cent of the 27,000 introduced plant species in Australia, and are by far the biggest source of weeds, totalling 70 per cent of Australia’s combined agricultural, noxious and natural ecosystem weeds. They contribute to the $4 billion annual cost of weeds to agriculture.
The report identifies over one thousand species that nurseries continue to promote, including:
- 20 per cent of the weeds impacting on rare or threatened native plant species;
- 25 per cent of the Weeds of National Significance; and
- 25 per cent of the invasive plants on the World’s Worst Invasive Alien Species list.
Our hit list
Based on our personal observations, research and reflections, we feel particularly concerned about the following species:
Araucaria heterophylla Norfolk Island Pine. It is almost impossible to avoid this species as it continues its relentless spread across coastal New South Wales. Norfolk Island Pine develops into a very tall tree that makes an inescapable visual impact – even when seen at a distance – destroying the visual unity and identity of the natural landscape. If allowed to develop in the company of eucalypts, that are always dominant (or co-dominant) wherever they occur, Norfolk Island Pine may become the locally dominant species, shading out native trees as its canopy spreads out above them.
Although once considered non-invasive, Norfolk Island Pine has in recent years shown signs of genetic mutation, with seedlings appearing in native bushland. This is an example of the ‘lag effect’ whereby an introduced 'sleeper' that seems for many years to be benign, suddenly starts to set seed to become an environmental weed.
Asparagus aethiopicus (Protasparagus densiflorus) Asparagus 'Fern' and Protasparagus setaceus (Protasparagus plumosus) Climbing Asparagus 'Fern'. In eastern and southern Australia, Asparagus 'Fern' has rampaged through bushland, replacing many native species of lilies, orchids and shrubs. Introduced to Lord Howe Island as an ornamental plant, it has now spread across the island, where it has since been declared a noxious weed. At the time of writing (2010), it was still widely available from nurseries and garden centres in mainland New South Wales.
Cinnamomum camphora Camphor Laurel, provides food and shelter for several bird species including Indian Mynah and Pied Currawong, thus contributing to their plague proportions and the consequent decline in small bird numbers. It also secretes soil toxins that stunt nearby plants, and its fallen leaves and branches are toxic to native frogs and fish. It spreads rapidly and in southern Queensland is fast invading remnant stands of Eucalyptus tereticornis Forest Red Gum, an important food tree for koalas.
Cotoneaster species. Cotoneasters are fast-growing, winter-berrying shrubs and small trees that sustain the Pied Currawong during winter when other food sources are scarce, thereby contributing to the plight of small endangered birds. These invasive garden plants are rapidly spreading into native ecosystems, with the potential to destroy them.
xCupressocyparis leylandii Leyland Cypress, is an exceptionally fast-growing hybrid between species of two different genera, Cupressus macrocarpa Monterey Cypress and Chamaecyparis nootkatensis Nootka Cypress. Several of its cultivated varieties are marketed by nurseries and garden centres in Australia. In 2003, the British Government passed a Bill that restricts the planting of the 'Curse of Leylandii', that kills other plants, degrades the soil, creates heavy shade and devalues property. In Australia, one of its parents, Monterey Cypress invades native woodland and dry coastal vegetation.
Lantana camara, Lantana montevidensis and their hybrids. Although these plants are notorious throughout Australia, and Lantana camara is listed by the federal government as one of twenty Weeds of National Significance, several nurseries and garden centres continue to sell them.
Phoenix canariensis Canary Island Date Palm, bears huge leaves with long sharp basal spines that are dangerous to handle when fallen. Seed dispersal by birds and adaptation to Australian conditions has resulted in seedlings springing up in gardens, and in rainforests and other eco-systems throughout Australia. In Western Australia, Date Palms are considered a serious pest. In eastern Australia, the demise of this species may be only a matter of time, as individual specimens are said to be dying from an unpreventable and incurable fungal disease.
Pinus radiata Radiata Pine. At least ten species of exotic pines, particularly Radiata Pine (also called Monterey Pine, after its natural habitat in Monterey, Calfornia), are invading forests and heathlands throughout New South Wales and elsewhere in Australia. Radiata and other pines have now become major weeds, threatening the collapse of ecosystems, biodiversity and regional identity.
Populus species. Poplars are suckering trees, four species of which have naturalised in temperate regions of Australia. Those brought into Belair National Park in South Australia early in the twentieth century, continue to infest the park despite a natives-only policy since 1923. Populus nigra Black Poplar and Populus alba White Poplar have formed dense suckering stands in Western Australia, and now threaten riverside vegetation in south-eastern Australia. Additional species were being imported as recently as 1996 and, as far as we’re aware, are still being imported.
Salix species Willow. Listed among twenty ‘Weeds of National Significance’ by the federal government in 1999, yet still available from many garden centres and nurseries. Often planted near water, detached branches float downstream and take root wherever they happen to lodge, developing quickly, accelerating bank erosion, slowing water flow, crowding out immature native plants and increasing sedimentation and flooding.
Syagrus romanzoffianum Cocos Palm, is a cheap, faster-growing alternative to native Archontophoenix cunninghamiana Bangalow Palm, and is popular with property developers. It is fast adapting to Australian conditions and invading coastal rainforest and other native forests.
"If we cannot stop this McDonaldisation of the biosphere, we stand to lose a substantial part of global biodiversity." Gabor Lovei, Senior Scientist, University of Aarhus, Denmark
Exotic plants and new diseases
The introduction of new diseases by exotic plants, represents a further, under-reported and largely unrecognised hazard. In a June 2004 New Scientist article, ‘Felled by Fungus’, staff writer Stephanie Pain reports:
"Our passion for exotic plants is creating nasty diseases that have the power to alter natural landscapes forever . . . . In their natural habitats and hosts, phytophthoras [parasitic fungi that feed on roots] do little harm. Transport them to new places and introduce them to species they've never encountered before and they can turn nasty. Worse, the mass movement of plants is responsible for a malevolent form of matchmaking, bringing together species of pathogen that would never normally meet, and paving the way for the evolution of new and potentially dangerous diseases . . .
"Today, almost everyone's small plot is filled with exotics, a trend fuelled by TV makeover shows and gardening magazines. This obsession with the exotic has created a multibillion-dollar global industry, and the risk of importing lethal pathogens is far higher.
"Perhaps worst of all, Phytophthora cinnamomi [Cinnamon fungus] is wrecking ecosystems of international importance in the southern hemisphere. In south-west Australia, the fungus has infested about 20 per cent of jarrah forest and around 60 per cent of the montane shrublands and sandy heaths - hotspots of biodiversity with thousands of endemic species. More than 20 per cent of the 9000 native species that grow there are susceptible, and some are at risk of extinction."
How you can help solve the problem
You can help the environment, by favouring plants indigenous to your locality or bio-region, and by avoiding:
- imported plants.
- species that successfully compete with native species, for nutrients, sunlight and water.
- species that have established — or have the potential to establish — self-propagating populations in native vegetation outside their natural range.
- species that prevent or reduce the establishment of native species.
- species that change geomorphological processes, fire regimes or hydrological cycles.
- species that change the nutrient content of the soil.
- plants that need fungicides and/or insecticides to sustain them.
"Australians need to change their way of thinking about foreign plants. We should see in every species a potential weed; unless we have evidence to show it won’t spread, a presumption of guilt should be applied."
Tim Low, Feral Future: The Untold Story of Australia’s Exotic Invaders