Exotic weeds are slowly choking Australia to death and costing the nation A$4 billion (HK$23.4 billion) a year in lost agricultural output, a government-funded report says. While the environmental impact of introduced animals such as rabbits, pigs and goats has been well documented, the effect of introduced plant species has received less attention. The plants, most of which were deliberately introduced as ornamental garden species, have taken a stranglehold on Australia's grazing and arable land, as well as coastal areas, jungles and deserts. They range from the humble blackberry bush, introduced from England, to exotic types of water lily brought in from South America. Aquatic species are smothering lakes, rivers and estuaries, overwhelming efforts to control what scientists have dubbed 'green death'. A government-funded report found that A$1 in every $7 of agricultural output was lost because of weeds. Nearly one-fifth of the economic cost was being borne by consumers, who each year pay A$729 million more for their fruit, vegetables and meat, the study found. Of the more than 28,000 plant species introduced to Australia, over 2,500 have become naturalised. Two-thirds escaped from urban gardens and parks. 'We have, as a nation, made mistakes. We have allowed in plants that in hindsight should never have been considered,' Conservation Minister Ian Macdonald said. 'I think the research shows that it's really too hard to attribute blame and say who should pay. I think in the end result, governments of all levels should have to pick up the bill for what is really a major national problem.' While the government was spending A$80 million a year on combating the problem, much of the anti-weed war was being fought by unpaid conservation volunteers. Eradicating the invasive species with herbicides is often not feasible because spraying would kill native vegetation or pollute the waterways. Many weed species have to be painstakingly tackled by hand - a hazardous task in areas inhabited by ticks, leeches, snakes and crocodiles. The authors of the report, from the University of New England in New South Wales, said the A$4 billion was an underestimate of the true cost of the effect of weeds.