Garden plants, in all their glory, now dominate our ‘worst weeds’ lists, making up a staggering 30% of all our noxious weeds and accounting for seven of our eighteen worst environmental weeds. Of the new weeds emerging in recent years, two-thirds are garden escapees, and in many national parks they pose the main management problem. Garden plants behave so badly partly because there are so many of them. In Queensland alone some four thousand foreign species are grown, more than all of Australia’s food, pasture and timber plants combined. Millions of them grow in gardens close to bushland. They represent one of Australia’s most pressing problems, but one of the least acknowledged.

Those seven garden plants on our worst-weeds list behave like invaders from Mars. Rubber vine (Cryptostegia grandiflora) loops up trees on long, latex-oozing stems, smothering whole forests of riverine rainforest in the monsoon tropics. Brought in from southern Madagascar in the 1870s, it is perhaps Australia’s worst vegetable scourge, having claimed 350,000 square km of Queensland. Blue thunbergia (Thunbergia grandiflora) from India is invading Wet Tropics rainforests around Cairns in much the same way, overpowering trees up to 40 metres tall, sprouting from bloated, 70-kilogram tubers. In Central Australia, Athel pine (Tamarix aphylla) has claimed hundreds of kilometres of outback riverbanks, deposing the native river red gum and changing the hydrology forever. In the Northern Territory, Jerusalem thorn (Parkinsonia aculeata) forms dense colonies of impenetrable bush, several kilometres wide; and in northern and eastern Australia, water hyacinth (Eichhornia crassipes) smothers lakes, rivers and dams, wiping out aquatic life and impeding boats. In southern Australia, bridal creeper (Myrsiphyllum asparagoides) and boneseed (Chrysanthemoides monilifera) now dominate the understorey in many eucalypt woodlands, replacing a wealth of native shrubs, grasses, orchids and lilies.

Apart from these seven there is lantana, the prickly claimant to 4 million hectares of eastern Australia; the camphor laurel and privet that have conquered northern NSW; the blue periwinkle that lines hundreds of kilometres of waterways in Victoria; the prickly pears; and all the other colourful garden escapes in our forests, on our beaches, and in wetlands, proliferating, flowering, seeding, transforming the land forever.

Australia’s backyard gardeners, unfortunately, have no sense of how bad it all is. Weeds? They are those little plants that sprout among the garden shrubs, they think, surely not the shrubs themselves? The truth seems inconceivable – that English ivy, arum lily and Spanish heath are themselves the weeds. As an environmental consultant working around Brisbane, I have come to loathe these colourful invaders. Recently I visited a beautiful woodland where young asparagus ferns – the advance troops – are staking out their positions among the kangaroo grass clumps on the hills. I know only too well the unfolding scenario – the proliferation of asparagus, the arrival of umbrella trees, then jacarandas, and mother of millions. I pulled out an asparagus fern in disgust and walked on, trying to forget.

Our cities are fertile breeding grounds for such weeds. A plethora of plants is grown in bushier outer suburbs where gardening is so popular. When unruly ones are pruned back, the clippings that won’t fit into the bin are often heaved into nearby bushland, to set down roots or shed vast quantities of seeds. It is ironic that the plants most likely to go feral – those that misbehave in gardens – are the very ones most often dumped in this way. If the forests fringing our cities were healthy this might not matter so much, but they are already degraded by pollution, dumping, runoff, fires, erosion, clearing and logging.

Most native plants are adapted to infertile soils and they dislike all the soapy seepage and dog do. Nutrient-loving invaders quickly take their place, along gullies especially, where rushing storm waters strip back foliage and pollute the earth. Birds and bats make matters worse by devouring brightly coloured berries and spreading the seeds. They are especially troublesome during droughts, when the only fruits around are likely to be the privet and asparagus fern growing in over-watered yards. Shrubs with bright berries make popular garden subjects, and not surprisingly, many of our worst weeds – such as bitou bush, camphor laurel, asparagus ferns, privets, sweet pittosporum and so on and so on – are fruit-bearers.

In most of the national parks surrounding our cities, ornamental weeds are a major management problem — usually the major problem. This holds true for Ferntree Gully in Melbourne, the Blue Mountains outside Sydney, Mount Glorious near Brisbane and Belair in Adelaide. In the Blue Mountains, naturalist AG Hamilton first witnessed the problem back in the 1880s: ‘All sorts of plants, commonly looked on as garden flowers, escape into the bush here, become weeds, and beat the natives out of the field’.

Around Sydney, where a unique heathland flora – a national treasure – evolved on infertile sandstone, the soil enrichment has been measured. Annemarie Clements, in a study ‘Suburban development and resultant changes in the vegetation of the bushland of the northern Sydney region’ (1983), found that the soil phosphorus level in bushland near housing, now weed infested, was 50 parts per million higher than it should be. And the ground was wetter. Extra water was running off roofs, roads, courtyards and paths, and overflowing from swimming pools and septic systems.

Such problems, although worse around cities, fester everywhere. Country towns and even remote farms can serve as beachheads for vast invasions. One example is Athel pine, not considered a weed until the remarkable wet year of 1974 when floods carried its seeds from homesteads around Alice Springs hundreds of kilometres along the Todd, Ross and Palmer rivers. After one bout of rain it emerged from obscurity as one of Australia’s most baneful weeds.

Seashores suffer more than most habitats because we love to live by the sea, and because dunes, pummeled by wind, waves and wandering feet, are easily invaded. George Batianoff and Andrew Franks of the Queensland Herbarium surveyed Queensland beaches (a pleasant-sounding task) and found 105 species of invading ornamentals (not a pleasant conclusion). Weeds (including non-ornamentals) now account for about half of all plant species sprouting along the Queensland coast. The authors also saw plenty of signs of gardeners dumping plants. Their subsequent report, ‘Invasion of sandy beachfronts by ornamental plant species in Queensland’ (1997), makes fascinating reading.

Small rainforest remnants also fare badly. Wingham Scrub, a precious nine-hectare remnant of floodplain rainforest near Taree, was subsiding under garden vines until the National Trust raced to the rescue in 1980. Cat’s claw (Macfadyena unguis-cati), with its brilliant yellow flowers, was trailing nets of stems over the ground and rising high into the canopy, draping rainforest trees under so many stems their crowns tore apart. One giant stinging tree was holding up 560 vines, an intolerable burden. Madeira vine (Anredera cordifolia) groped through the mid-storey at a metre a week, sprouting hundreds of aerial tubers, and a third creeper, wandering jew, carpeted the forest floor, denying seedlings a chance to sprout.

In other rainforests, Dutchman’s pipe (Aristolochia elegans) is killing off one of Australia’s largest, rarest, and most celebrated butterflies – the Richmond birdwing (Ornithoptera richmondia). This butterfly lays its eggs on the alien vines, mistaking them for its proper foodplants (which are closely related) and dooming its caterpillars to an early death. Even in Australia’s alpine national parks, garden plants are multiplying. Some of these weeds have spread from ski resort gardens, with more than 30 species sprouting around Thredbo.

There are many more of these weeds than we think, partly because our tastes in gardening have changed so much. Few of such earlier garden subjects as lantana, gorse (Ulex europeus), purple-top (Verbena bonariensis), sweet briar, mullein, pink periwinkle and evening primrose (Oenothera) would earn a place in gardens today. We are locked into an absurd cycle of introducing new garden plants (new weeds-to-be) whenever older ones fall out of favour. And the problems are getting worse. More and more Australians are opting to live near bushland beside rivers, beaches, rainforests and national parks. New garden plants keep entering the country and our established ones are escaping.

Australia urgently needs a new gardening ethos. We should accept that gardening, within a kilometre or so of bushland, entails an ecological responsibility. Weedy species should not be grown. Lists of problem plants can be obtained from our local council or from Greening Australia. Waste from gardens should never be dumped near bushland, drains or creeks. Berry-bearing plants should be avoided unless they are local natives. New garden plants should be treated less like exciting new products invented to brighten our lives, and more like wild organisms harbouring the drive to escape. Native plants are one alternative, so long as we realise that some of these, when taken to new regions, can also take off as weeds. Benign azaleas and roses are better than weedy wattles.

Tim Low 
is the author of Feral Future (1999), 
an account of how exotic species are homogenising Australia and the world. 
It is published by Penguin, rrp $24.95 (plus GST).