Our continent has a wretched history of invasive species. Ecosystems have succumbed to exotic animals such as foxes, rabbits and carp, while at the same time being beset by strange, virile plants. "Whenever I travel, I'm always amazed by the hold that exotic species have obtained over this land," says Tim Low of the Invasive Species Council. Two centuries ago, when British fleets brought the first bedraggled convicts to Australia, they brought foreign flora as well - including the infamous prickly pear. Shipped across from South America, the cactus slowly became recognised as one of the most invasive weeds ever imported into the country. By the 1920s, the pear was no longer a garden ornamental or a ferocious homestead hedge; it had stretched its spiky pads, grown into dense, tangled structures, and covered 25 million hectares of Queensland and New South Wales. Last spring, a federally funded weed-awareness campaign was launched to educate people about the plants we purchase, as a third of weeds in Australia are said to originate in our backyards. "When you talk about weed invasion, it is often very pretty plants escaping out of our gardens that cause the problem," says Low. Weeds, we're told, cost the economy around $3.3 billion a year in control efforts and lost production. At the same time, significant resources have been spent determining what constitutes a weed in the first place. In 1999, after recruiting specialists to study and rank Australia's most troublesome plants, commonwealth ministers announced the inaugural list of 'Weeds of National Significance', with species including pond apple, bridal creeper and lantana labelled especially problematic. On the federal government's weed website, each plant's information is accompanied by a floral mug shot.