One of Australia's worst weeds, Paterson's
curse was introduced during the mid 19th
century as a garden plant.
Image: Jon Dodd

The number of plant species now growing in Australia has more than doubled since European settlement in the 1780s due to new plants introduced from overseas, according to a new publication from Australian weed scientists launched on 29 August 2008.

And thousands of them are just 'weeds in waiting', say the researchers.

Launching the Introduced flora of Australia and its weed status at the National Botanic Gardens in Canberra on 29 August, Tasmanian Senator Christine Milne said the 28,000 species brought in from overseas in merely 200 years was many more than the number of native species.

'Scientists estimate that the original pre-European rate of plant introductions was as little as 1-5 species per century, a rate which the native flora was able to cope with', Senator Milne said.

'However, in the last 200 years the average rate of introduction rocketed to 14,000 per century. In many places this has simply overwhelmed the adaptive capacity of Australian ecosystems', Senator Milne said.

'If we are to maintain our current level of biodiversity in Australia and build resilience in ecosystems, then we must have a war on weeds', she said.

Compiled by Rod Randall of the WA Department of Agriculture and Food and the Cooperative Research Centre for Australian Weed Management (Weeds CRC), the recently completed project set out to list all introduced plant species now found in Australia as well as their weed status here or overseas.

Mr Randall said that many weedy plants were introduced for agriculture, especially for pasture, and later abandoned to go weedy when they failed to perform.

'However, most of our worst weeds come from parks and gardens', Mr Randall said. Examples include Paterson's curse, blackberry, willows, bridal creeper, gorse, lantana and soursob.

The 'Introduced flora of Australia and its weed status' lists precisely 2739 foreign species that have become weedy, and a further 5907 that are here, not yet weedy, but have a history of becoming weeds overseas.

'Australia has such a diversity of climates we can be sure than many of these 'weeds in waiting' will eventually find their way to a site that suits them - and then they will simply explode in numbers', Mr Randall says.  'We are pretty adept at moving plants and seeds around, on purpose or by accident, which gives weeds the chance they need to spread and try their luck in new locations.'

Climate change is also working in their favour, he says, as changes in local conditions stress existing plants, and open up opportunities for tougher invaders.

'Some of these 'weeds in waiting' may find that just staying put works for them', says Mr Randall, 'especially if the local changes to rainfall and temperature suit them. It could be that their time is coming.'

Gardeners and plant retailers will be able to use the 'Introduced flora of Australia and its weed status' to see immediately whether a plant is known to be weedy outside its native range, and can then choose to avoid planting or selling it.

'This is basic 'weed risk' information that people have lacked up till now', says Mr Randall.

'If gardeners and sellers over the last 200 years had only known what we know now about which plants can become highly invasive, we might have avoided much of the degraded landscapes, lost biodiversity and $4 billion per year agricultural loss now caused by weeds', he said.

Mr Randall points out that the document lists over 20,000 non-weedy foreign plants for gardeners to choose from, in addition to the 11,000 native plants now cultivated. The total of over 30,000 species and cultivars should be a big enough palette for us, he says, and obviate the need to grow known weeds.

Users will need to know the correct scientific name of plants to search the system, since common names are too unreliable and vary too much, Mr Randall says.

Now available free from the Weeds CRC web site, the Flora can also be searched online via the University of Queensland web site.