The number of plant species now growing in Australia has more than doubled since European settlement in the 1780s, largely due to new plants introduced for gardeners, according to a new publication from Australia's weed scientists. And thousands of them are just 'weeds in waiting', say the researchers.

The 'Introduced Flora of Australia', now available on-line as a printable document and as a searchable database, was compiled by Rod Randall of the WA Department of Agriculture and Food and the Cooperative Research Centre for Australian Weed Management (Weeds CRC).

Begun in 2001, the recently completed project lists all 26,242 introduced plant species in Australia. This number exceeds the likely number of native plant species in Australia.

'Still feeling foreigners in a strange land, nineteenth and twentieth century Australians set up whole societies to import plants and animals from the old world and elsewhere', says Mr Randall.

'This was partly to reproduce the look and feel of their original countries, especially England and Europe, to allow them to feel more 'at home'. Many also thought that the native flora was not attractive or appropriate for gardens.'

Most of our worst weeds come from this era, through the demands of gardeners, Mr Randall said. Examples include Paterson's curse, blackberry, willows, bridal creeper, gorse, lantana and soursob.

'Many other plants were introduced for agriculture, especially for pasture, and simply abandoned to go weedy when they failed to perform', he said.

The 'Introduced Flora of Australia' 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' to see immediately whether a plant is known to be weedy somewhere in the world, and can 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/year loss now caused by weeds.'

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 from the CRC web site, the Flora can also be searched on-line via the at University of Queensland web site .