The adventive flora, as distinct from the native (indigenous) and the cultivated plant floras, consists of all plant species which are not native of New Zealand and grow spontaneously outside of cultivation; examples of adventive species are wheat growing along a railway track, gorse in a pasture, or hemlock in a pasture. It is a significant flora, not only because some of its members play an important role in the vegetation by forming distinct communities alone or with native species, but also because it contains the majority of our most troublesome weeds. There are now few landscapes at lower altitudes in which adventive species are not prominent; some species are so widely distributed and are in such harmony with their adopted countryside as to have every appearance of their being natives here, and are sometimes mistaken for them.
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).
Although many adventive species have little or limited significance as weeds, being spectacular rather than serious, for example, the Californian poppy (Eschscholtzia californica), the adventive flora has definite economic importance because of the troublesome weeds it contains. It is not inappropriate then to consider briefly the ways by which adventive species entered New Zealand initially. The means of introduction fall into two categories:
Deliberate Introductions: Introductions of domesticated plants for particular purposes have yielded a number of escapes from cultivation, the main contributors being: (a) Agriculture: From pastures have come the common grasses and clovers of roadsides; from crops have come comfrey (Symphytum spp.), linseed (Linum usitatissimum), and lucerne (Medicago sativa); and from hedges have come broom (Cytisus scoparius), gorse (Ulex europaeus), hakea (Hakea spp.), and kangaroo acacia (Acacia armata). (b) Horticulture: Over 25 per cent of this flora has been derived from this source; from ornamental subjects have come Japanese honeysuckle (Lonicera japonica), the ornamental grass Pennisetum macrourum, sweet briar (Rosa rubiginosa), and watsonia (Watsonia bulbillifera); from culinary subjects have come blackberry (Rubus spp.), wild parsley (Carum petroselinum), and wild parsnip (Pastinaca sativa); and from herbal subjects have come hemlock (Conium maculatum) and thyme (Thymus vulgaris). (c) Forestry: From this source have come lodgepole pine (Pinus contorta var. latifolia) and Pinus radiata. (d) Aquaria, fish raising, water gardening: From these have come Canadian pondweed (Elodea canadensis), oxygen weed (Lagarosiphon major), and water hyacinth (Eichhornia crassipes). (e) Cage-bird seed: From this source come broomcorn millet (Panicum miliaceum), Guizotia abyssinica, and Indian hemp or marihuana (Cannabis sativa).
Unintentional or Accidental Introductions: The greater proportion of the species in the adventive flora were accidental introductions due to: (a) Impurities in agriculture seeds. It is estimated that ±55 per cent of the total had this origin. Even in recent years with modern seed-cleaning techniques, impurities still arrive, as in French lucerne, hedge bedstraw (Galium mollugo) and yellow star thistle (Centaurea solstitialis); in carrot seed, barnyard grass (Echinochloa crus-galli), and (Trifolium cernuum); in linen-flax seed, cut-leaved mustard (Sinapis dissecta). (b) Ballast. This was a fruitful source in sailing days; Paspalum dilatatum and yellow weed (Galinsoga parviflora) are examples. (c) Impurities in grain imported for flour or stock feed. The Mexican poppy (Argemone mexicana) and saffron thistle (Carthamus lanatus) are being repeatedly introduced, and recently three European weeds of the cabbage family, coming in via bulk wheat for flour, have been appearing about ports, railway sidings, and flourmills. (d) Twitch-like stems in dirt about the roots of commercial plants such as field horsetail (Equisetum arvense), a spore-producing ally of the ferns, in the roots of Iris sp. from Japan, and gout weed (Aegopodium podagraria), with imported ornamental shrubs. (e) Impurities in horticultural seeds. Several species have appeared including a European plantain (Plantago arenaria) in night-scented stock beds. (f) In packing of merchandise. The horned poppy (Glaucium flavum), common about the shores of Wellington Harbour, came in with the Patent Slip machinery in the 1870s, and others have come in with plate glass, whisky, crockery, and other goods. (g) In packing of military equipment. A ripgut grass (Bromus rubens) came in with the American Armed Forces in the Second World War and established itself near Paekakariki. (h) Impurities in cage-bird seed. One noteworthy newcomer in recent times was the large, spiny-fruited, poisonous thorn apple (Datura ferox) at Geraldine. (i) Attached to merchandise. A number of tropical weeds, including the troublesome khaki weed (Alternanthera repens) came to North Auckland with imported fertiliser; an Argentine evening primrose (Oerothera longiflora) established itself about a hardwood sleeper dump at Christchurch, as did the American ragweed (Ambrosia artemisiifolia), which came in with American-manufactured railway lines. (j) Attached to or associated with imported animals. Kangaroo grass (Themeda triandra) was introduced to the Wairau Valley, Marlborough, with Merino sheep from Australia; and recently, at Oamaru, pods of snail clover (Medicago scutellata) came in with an Australian sheep crate.
The possible means of introduction are legion. How many species have been brought in attached to clothing, in trouser cuffs, or in mud on the footwear of overseas travellers is not known, but, from mud brushed off boots used in New Caledonia, over 350 plants have been grown, representing 35 species, with lesser numbers from footwear used in Curacao, Fiji, and Chile.
The expansion of New Zealand's overseas trade and increased importing of a wide range of products from many different regions of the world indicate that, if past history is any criterion, the species of adventive flora will steadily increase, with the further possibility of the arrival of some troublesome weeds amongst future introductions.
by Arthur John Healy, M.AGR.SC., Assistant Director, Botanical Division, Department of Scientific and Industrial Research, Lincoln, Canterbury.