The adventive flora may well have had its beginnings with the arrival of the Maori voyagers from A.D. 1150 onwards, with the probable introduction of “stowaway” plants in dirt about the roots of their food and fibre plants. The formal history of the adventive flora begins, however, with the recording of a Mediterranean canary grass (Phalaris canariensis) in 1786 by George Forster, a botanist on Captain Cook's second voyage of discovery. Whalers, sealers, and timber seekers were responsible for the introduction of further species, the number rising sharply with the onset and expansion of organised colonisation. With this came the widespread modification (at times complete destruction) of the aboriginal vegetation and the introduction of a range of agricultural and horticultural plants, accompanied by a wide variety of impurities. The two necessary prerequisites for the successful establishment of an adventive flora were being realised – provision of bare ground or living space by the disturbance of existing vegetation and the introduction of a range of non-native species. Contrary to the views expressed last century by prominent overseas botanists that adventive species were superior to the native species and vegetation which they would in time displace, the true position is that adventive species did not enter into and establish themselves in primitive vegetation. Before this could take place, some disturbance or modification by man and his agents (firing, cultivation, grazing, etc.) was necessary.

Although in this article only ferns and flowering plants are considered, the adventive flora in the broadest sense covers a wide spectrum of the plant kingdom, including species of bacteria, fungi, lichens, and mosses. This flora has built up steadily to the extent that, over a period of nearly 180 years, the flowering plants and ferns growing outside of cultivation number more than 1,500 species.

Most major geographical regions of the world are represented in the adventive flora; Australia has contributed kangaroo wattle (Acacia armata); South Africa, Cape weed (Cryptostemma calendula); tropical Africa, woolly nightshade (Solanum auriculatum); North Africa, apple of Sodom (Solanum sodomaeum); South America, nassella tussock (Nassella trichotoma) and Onehunga weed (Soliva valdiviana); Central America, mist flower or Mexican devil (Eupatorium adenophorum) and Oxalis (Oxalis latifolia); North America, Canadian fleabane (Erigeron canadensis); Europe, foxglove (Digitalis purpurea); Asia Minor, Grecian thistle (Chamaepeuce afra); Western Asia, horehound (Marrubium vulgare); India, Cotoneaster simonsii; Eastern Asia, Manchurian wild rice (Zizania latifolia) and strawberry-raspberry (Rubus illecebrosus); and the Pacific region, nutgrass (Cyperus rotundus).