XVI International Botanical Congess

The belief seems widespread that most new exotic weed outbreaks come from seeds moved around unintentionally. The reality is different: an Australian study found that 46% of 233 major weeds had been introduced intentionally, while only 18% were found to have been introduced accidentally, the remaining 36% having arrived by unknown means. Most intentional introductions into Australia are as ornamentals, and examples also abound in north American of exotic weeds introduced by the garden seed trade. Agricultural species often become weeds because previously beneficial species remain behind as pests after changes in land use - the most important weed of Australian grain-growing regions is rye-grass, which was an extremely valuable species when grazing was predominant. The answer to the question in the title is therefore simple: We move them. Thus, we can change things for the better.