ABSTRACT
An unknown number of alien plant species have been introduced to Australia, both accidentally and deliberately. A recent publication on Australian plants of horticultural significance
lists some 30,000 plant names as being available from 450 nurseries in all states and territories of Australia. This listing includes not just plant species that have been deliberately introduced, but also native plants that are used in horticulture, together with some synonyms and cultivar names. Until more accurate estimates are available, we may assume that the number of alien plant species introduced to Australia at least equals the number of native higher plant species, which are currently estimated to be about 25,000. Not all of these alien species have become naturalized, however. An equally recent listing of the naturalized alien flora of Australia
gives a total of 2681 plant species that are known to be naturalized and to have voucher specimens lodged in Australian herbaria. In other words, about 10 to 15% of the total Australian flora is alien and naturalized. Some of these alien and naturalized plant species affect, or are perceived to affect, human activities in some way and may be regarded as weeds. This chapter considers some of these 2681 alien naturalized plant species and their impacts on Australian ecosystems, but
the coverage is not limited to the smaller proportion of naturalized aliens that are generally regarded as weeds.
