Flora of New Zealand Series

PREFACE

New Zealand has a rich weed flora. It is primarily the flora of the lowlands, of waste places, roadsides, farms, gardens, and other disturbed and modified places where indigenous plants have been displaced as a result of human activities. Naturalised species evolved elsewhere, and many have brought with them growth forms, life histories, and flower colours that are foreign to this country. Generally they have been viewed as weeds rather than wildflowers and not accepted as an integral part of the New Zealand flora. Flora writers have usually treated them separately from indigenous species, either as supplementary lists in Floras on indigenous species or as distinct volumes.

The Preface to Volume 3 of the Flora of New Zealand series discussed the reasons behind the segregation of naturalised and indigenous species and the text of that volume moved towards integrating indigenous and naturalised species; genera and families with only indigenous species were keyed out (these had been treated in detail in Volume 2), and all species in genera with both indigenous and naturalised species were described. This Volume describes the naturalised pteridophytes, gymnosperms and dicotyledons, the groups treated by Volume 1 for indigenous species; as with Volume 3, it is a bridging Flora in that it keys out all indigenous families, keys out all genera for families with naturalised species, and describes indigenous species when they are congeneric with naturalised species. This Flora is the first comprehensive treatment of the naturalised plants in these groups since H. H. Allan's Handbook of the Naturalised Flora of New Zealand (1940). In New Zealand, the naturalised flora has also been known as the adventive, alien, exotic, and introduced flora.

Naturalised plants have either arrived accidentally in New Zealand, or have been deliberately introduced for various purposes but have later escaped from cultivation. In a few cases there have been deliberate attempts to naturalise species, as for example in the use of Alnus viridis and Salix purpurea for erosion control and Coronilla varia and Lotus pedunculatus for stabilising road cuttings. Many early accidental introductions were of widespread temperate weeds which accompanied settlers or arrived as stowaways with crop seed, cargo and ballast. Many of these species did not become established and in some cases, Acicarpha tribuloides, Hypochoeris tweediei, and Petunia parviflora for example, are known only from Kirk's original gatherings from ballast in Wellington.

For species which have not been deliberately introduced, in a few cases it is not always easy to determine whether they should be treated as part of the naturalised or the indigenous flora. Webb, D. A., Watsonia 15 : 231-236 (1985), suggested several criteria for determining status with reference to the British flora and all of these might also be applied to New Zealand examples. Fossil evidence may show that a species was established in New Zealand before Polynesian settlement, but the absence of fossils is less helpful. Historical information may be helpful; for some horticultural species the actual date of introduction may be known and for weedy species evidence may come from the absence of the species from early Floras or species lists for regions in which the plant is now common. The recent rapid spread of some species indicates naturalised status although this is not always the case as some indigenous species such as Epilobium nummulariifolium and Hydrocotyle heteromeria have recently successfully colonised artificial habitats. In spite of this, habitat may also provide evidence of status, in particular when species are known from only modified areas. Geographic distribution may also be a useful indicator, as, when a species is not known to be indigenous to regions close to New Zealand, it is unlikely to be indigenous here. An example is Hibiscus trionum which was included as indigenous in Volume 1 but is considered to be naturalised in many regions including eastern Australia - it is treated as naturalised in this Volume. Also, when a species is known to be naturalised in many parts of New Zealand but is considered to be indigenous to other parts, this may raise doubts about its status. Genetic differences between small populations established in natural habitats and populations of the same species occurring as a weed, indicate that the species may be indigenous or at least have an indigenous component. The reproductive system may provide clues to status as for example with dioecious species such as Myriophyllum aquaticum which is represented in New Zealand by female plants only. The likely means of introduction may help to decide status. The Australian Pittosporum undulatum is established as a small isolated population near Kaitaia; the sticky fruits may have been naturally introduced to New Zealand by birds and so the species should probably be considered part of the indigenous flora. Two other criteria which might indicate that species are not indigenous are the absence of specialised insect pollinators or predators, and the possession of a habit or other characters not normally found in indigenous New Zealand plants. All these criteria considered together will clearly determine the status of most species but doubt will always remain for some, especially species of disturbed sites which are known to be indigenous in eastern Australia, for example Veronica plebeia and Gypsophila australis.

The first plants to become naturalised in New Zealand were those brought here by Polynesian settlers. Two examples of such early accidental introductions are Sigesbeckia orientalis and Bidens pilosa, both Asteraceae. B. pilosa is a widespread weed in Polynesia (Merrill, E. D., Chronica Botanica 10 : 334-345 (1946)) and its spiny fruits easily adhere to clothing; it was collected by Solander on Cook's first voyage to New Zealand but not reported until 1832. Aleurites moluccana, candlenut, is an example of a species which was probably deliberately introduced to the Kermadec Islands by Polynesian settlers and is now naturalised there (Sykes, W. R., Kermadec Islands Flora: An Annotated Checklist (1977)).

The earliest published record of a species treated in this Flora may be that for Pinus pinaster, included in a list of British Horticultural plants as P. novazaelandica  (Loudon, J. C., Hortus Britannicus (1830)), presumably, but not certainly, based on seed collected from wild New Zealand plants. The earliest fully acceptable records are those in botanical reports of exploring expeditions. Richard, A., Essai d'une Flore de la Nouvelle Zélande (1832), recorded Raphanus sativus and Plantago major among others and in the following 22 years, A. Cunningham, E. Raoul, and A. Gray recorded such widespread species as Anagallis arvensis, Daucus carota, Raphanus raphanistrum, and Urtica urens from New Zealand.

Hooker, J. D., Fl. N.Z. 2 (1852-1855), included an appendix of European and other plants introduced into and by then naturalised in New Zealand. He listed 49 species for the groups covered by this Flora, and some additional species which he regarded as indigenous are now considered to be naturalised here. Some of the species recorded by Hooker are unsubstantiated by specimens and have not been recorded since, but his Flora recorded for the first time several well known naturalised plants, for example Spergula arvensis, Taraxacum officinale, Euphorbia helioscopia, and Plantago lanceolata.

Following European settlement there was a rapid increase in the number of naturalised species with T. Kirk, J. B. and J. F. Armstrong, T. F. Cheeseman and G. M. Thomson providing many first records. Kirk contributed substantially to the list of 135 naturalised plants in Hooker, J. D., Handbk N.Z. Fl. (1867), and included naturalised species as an integral part of the text in his own Stud. Fl. N.Z. (1899). By the turn of the century 454 of the species, or taxa treated as species, accepted in Volume 4 had been recorded. In the early part of this century the works of Cheeseman, with naturalised species listed as appendices in both editions of his Man. N.Z. Fl. (1906, 1925), Thomson, especially in his Naturalisation of Animals and Plants in New Zealand (1922), and H. H. Allan, have been particularly important. Allan's Handbook of the Naturalised Flora of New Zealand (1940) contained 441 species treated in full in the groups covered by this Flora, and an additional 347 species of more minor or doubtful occurrence in an appendix. Since then by far the greatest contribution has been made by A. J. Healy who recorded many new species and wrote extensively on the biology and history of naturalised plants.

For the naturalised plants accepted in this Flora (including infraspecific taxa) the following made the most significant contributions in providing first records for New Zealand: Healy (18%), Kirk (16/pecent/), Allan (9%), Cheeseman (6%), and J. D. Hooker (4%). In preparation of this Volume 326 (22%) new records were published in the series of preliminary checklists or are in the Flora itself.

Early botanists feared from their observations of the diversity and rapid increase of naturalised plants in New Zealand that the indigenous vegetation would be threatened. This fear was well expressed by Darwin in The Origin of Species (1859), but as outlined in the Preface to Volume 3 has proven largely unfounded. In part the success in New Zealand of introduced species must be attributed to the paucity of indigenous species, particularly annuals, which will rapidly occupy continuously disturbed habitats. This contrasts with the situation in Australia where there are many weedy indigenous species (Kloot, P. M., J. Adelaide Bot. Gard. 10 : 99-111 (1987)). Also, New Zealand's equable climate and wide range of latitudes have allowed a great diversity of species to become established.

Over the last 150 years plants from all continents except Antarctica have found their way to New Zealand and become naturalised. In many cases weedy species have not come directly from their area of endemism to New Zealand, but have come indirectly via other countries, notably Britain and Australia, where they are also naturalised. For all regions, both typical weedy species and cultivation escapes are represented although most truly weedy species are from northern temperate areas, with other regions such as southern Africa and East Asia mainly represented by cultivation escapes. This pattern reflects both the sort of contact New Zealand has experienced with these regions and the fact that successful invaders are more likely to come from areas with similar soils and climates (Baker, H. G., in Mooney, H. A. and Drake, J. A. (Eds) Ecology of Biological Invasions of North America and Hawaii (1986)). Those areas with which New Zealand has been most involved in trade or immigration, and those with similar agricultural practices, have supplied the greatest number of weedy species. Some early introductions of weeds and of colourful horticultural species which have become naturalised reflect immigration ship routes to New Zealand coming as they do from Macaronesia, South Africa, Australia, and to a lesser extent southern South America. Europe, the Mediterranean regions, western Asia and Macaronesia have supplied the greatest number of species (50%), with typical examples being Capsella bursa-pastoris, Aphanes arvensis, and Acer pseudoplatanus. Southern and eastern Africa and Madagascar are the source of many species (6%), such as Senecio skirrhodon, most Aizoaceae, Selaginella kraussiana, and Oxalis pes-caprae. A number of distinctive species such as Hypericum mutilum, Eschscholzia californica, and Pinus contorta are included among those (10%) from North America, while from Central and South America (10%) there are many cultivation escapes and a few weedy species, including Tropaeolum majus, Berberis darwinii, Calandrinia compressa and Conyza bonariensis. Although most of those (8%) from East Asia and Himalaya are cultivation escapes, for example Ligustrum sinense and Duchesnea indica, there are also a few weedy species such as Datura ferox. Australia, particularly eastern areas, has provided a large number of species (8%) well suited to New Zealand conditions, for example Senecio bipinnatisectus, Hakea sericea and species of Racosperma and Eucalyptus. A few (4%) species have much wider distributions or are more or less cosmopolitan and several naturalised ferns, for example Osmunda regalis, fall into this category. A further 4% have obscure origins or are classed as cultivated hybrids.

Even within a single genus a diversity of origins may be represented by species established here; in addition to the indigenous species, Lepidium is now represented in New Zealand by species from Eurasia, northern, tropical and southern Africa, North and South America, and Australia. By contrast all 25 species of Trifolium naturalised in New Zealand come from the Eurasian-Mediterranean region.

To some extent the region of origin may be reflected in the distribution within New Zealand. Species requiring an oceanic environment are often concentrated in coastal sites - this is seen with the many species from Macaronesia, such as Aeonium haworthii and Chamaecytisus palmensis - and those with a tropical origin may be confined to the northern North Island or Kermadec Islands. In fact, a number of tropical species, for example Annona cherimola and Ipomoea alba are naturalised within New Zealand only on the Kermadec Islands. However, in many cases distribution within New Zealand is determined by the pattern of human disturbance rather than by ecological factors.

This Flora treats 132 families with naturalised taxa and, in addition, keys out a further 47 families which have only indigenous representatives. In total, 617 genera with naturalised taxa are accepted and of these 82 also have indigenous species; an additional 167 genera with indigenous species only are keyed out. In total 1470 naturalised species or species equivalents (hybrids or cultivars treated as species) are accepted (1419 dicotyledons, 28 gymnosperms, 23 pteridophytes); of these 1243 are considered sufficiently established to be given full treatments and the remaining 227, known from a few casual or only early collections, are given reduced (ζ) entries. Indigenous and naturalised species are described when they occur in the same genus and this accounts for 397 descriptions of indigenous species. The total number of naturalised species just exceeds the number of indigenous species, 1457, accepted for the same groups in Volume 1; but research on the indigenous flora since then has identified many new species so that there are still, in fact, fewer naturalised than indigenous species although many of the latter remain unnamed.

The largest families according to number of naturalised species or species equivalents are: Asteraceae (215), Fabaceae (121), Rosaceae (99), Brassicaceae (74), Caryophyllaceae (54), Lamiaceae (54), Scrophulariaceae (52), and Solanaceae (50). The largest genera ranked by number of naturalised species or equivalents only are: Trifolium (25), Rubus (24), Eucalyptus (20), Solanum (19), Salix (16), Senecio (16), Veronica (16), Oxalis (15), Euphorbia (14), and Rosa (14). When indigenous species are included in the total the largest genera are: Ranunculus (53), Epilobium (42), Myosotis (40), Senecio (34), and Rubus (29); whereas the largest genera which contain no indigenous species are: Trifolium, Eucalyptus, Salix, Veronica and Rosa.

In habit, naturalised species range from small herbaceous ephemerals through to large trees. Among those with very short life cycles are garden and pasture weeds such as Erophila verna, Veronica verna and Stuartina muelleri which flower within a few weeks of germinating; in total 30% of the species or species equivalents are annual. Biennials and monocarpic perennials are less common (5%) but are well represented in some families such as the Brassicaceae and Apiaceae. Macaronesian species such as Echium pininana and Melanoselinum decipiens are good examples of monocarpic perennials. Perennial herbs are well represented (32%) with typical examples being Hypericum perforatum, Oenanthe pimpinelloides, Ranunculus repens, Rorippa nasturtium-aquaticum, Equisetum arvense, and Prunella vulgaris. Climbing plants, both herbaceous and woody, make up 6% of the Flora and are best represented by genera such as Clematis, Passiflora, and Jasminum in which most species have this habit. Shrubs (17%) and trees (10%) have not often become fully naturalised although some species such as Racosperma dealbatum, Chrysanthemoides monilifera, Alnus glutinosa and Pinus pinaster are a significant component of the vegetation in some areas. Many naturalised tree species have been collected wild only as seedlings or saplings.

Some members of the naturalised flora are specialised in other ways. Aquatic species, for example Alternanthera philoxeroides, Mentha × piperita and Ceratophyllum demersum, may be important because of their potential for blocking waterways. Of the parasitic species only broom rape, Orobanche minor, is widespread, but Cassytha pubescens and Cuscuta epithymum are also established locally. There are no true epiphytes in the flora but Ficus rubiginosa often starts life as an epiphyte. On the other hand succulents are well represented, particularly as members of the families Crassulaceae and Aizoaceae from southern Africa, Macaronesia and Mexico.

In New Zealand, naturalised plants have invaded all plant habitats. Some, such as Anagallis arvensis and Lamium amplexicaule, are mostly found on bare sites or in habitats with sparse vegetation. Many are well established in ruderal habitats, gardens, roadsides, and along railway lines; examples are Achillea millefolium, Silene gallica and Parentucellia viscosa. Fields and other habitats with more established or secondary vegetation also contain many naturalised species, particularly where the indigenous or cultivated species are under some stress. Lawns, for example, may be invaded by Soliva sessilis, Bellis perennis, Dichondra micrantha and Veronica arvensis, while regenerating indigenous vegetation may have Leycesteria formosa, Ribes sanguineum and Clematis vitalba associated with it. Very few naturalised species will invade stable indigenous vegetation except where it occurs on naturally open sites such as stabilised sand dunes. Mycelis muralis and Lonicera japonica are examples of species which may establish in indigenous scrubland or forest and Rumex acetosella and Hieracium praealtum will invade relatively undisturbed alpine areas.

Although there is a tendency to regard all naturalised plants as troublesome weeds, many are in fact useful and might be better regarded as wildflowers. Garden escapes in particular add a great deal of colour to roadsides and coastal areas and, as is the case with Lupinus polyphyllus at Mt Cook, are often thought by visitors to New Zealand to be indigenous plants. Other wild plants are culinary herbs ( Origanum vulgare, Petroselinum crispum), medicinal plants ( Papaver somniferum, Digitalis purpurea), sources of honey for beekeepers ( Echium vulgare, Trifolium repens, Thymus vulgaris), fodder plants ( Medicago arborea, Brassica napus), or have edible leaves ( Atriplex hortensis, Rorippa nasturtium-aquaticum) or fruits ( Malus × domestica, Physalis peruviana). At the other extreme are widespread weeds such as Cytisus scoparius, Rubus fruticosus agg., and Cirsium arvense, which because of their invasive habits have been very deleterious to farming and forestry, poisonous species such as Heracleum mantegazzianum, Datura stramonium and Lantana camara, which affect both livestock and people, and other troublesome species such as Solanum nigrum, the fruit of which is often accidentally harvested and canned with peas. Most of the more useful wild plants are cultivation escapes whereas many of those that are problem weeds are those introduced unintentionally by various means. However, there are many notable exceptions to this generalisation; plants originally introduced to New Zealand for hedging, fodder, or as ornamentals have often become naturalised to the extent of being agricultural pests. Examples are Ulex europaeus, Berberis darwinii, Brassica rapa, Rosa rubiginosa and Erica lusitanica.

The reproductive biology of naturalised species is important in understanding their establishment and dispersal as wild plants. Most are self-compatible and have hermaphrodite flowers, or are even autogamous or cleistogamous, and so can produce seeds freely even when a single plant invades a new site. However, some species are self-incompatible or dioecious and this may limit their spread. The means of dispersal of seeds or fruits is also important in determining the spread of introduced species. Disseminules are carried by water in a few coastal and aquatic species such as Cakile edentula and Callitriche stagnalis, but by far the most common means of active abiotic dispersal is wind. Wind dispersal may be as dust (fern spores, Eucalyptus spp.), by balloons ( Trifolium fragiferum), by plumes and parachutes ( Gomphocarpus fruticosus, Epilobium ciliatum, and many Asteraceae), by wings ( Acer pseudoplatanus, Nemesia floribunda), as tumbleweeds ( Salsola ruthenica, Sisymbrium officinale) or from wind-ballasts ( Campanula, Papaver). There are several examples where the plant itself provides the propulsion for dispersal; perhaps the best known examples are Impatiens spp. and the explosively dehiscent pods characteristic of many Fabaceae, but the most spectacular example must be Ecballium elaterium, the squirting cucumber, the seeds of which are expelled from the fruit at the point of attachment to the pedicel with the whole fruit propelled like a deflating balloon. Gravity plays a part in the dispersal of all disseminules, but in many weedy species is the only means. This is true of many annual species such as Chenopodium album, Tropaeolum majus, and Brassica spp., which merely seed under the parent plant, and some tree genera, for example Quercus and Juglans.

Biotic dispersal is also common in the naturalised flora. Dispersal by invertebrates is rare, but the seeds of Viola spp. have an elaiosome and are taken by ants in New Zealand. Some Fabaceae, particularly those indigenous to Australia, have a fleshy aril which is utilised by ants and in New Zealand seeds of Cytisus scoparius may be taken by ants following initial explosive dispersal from pods (P. A. Williams, pers. comm.). Both indigenous and introduced birds are common dispersers of fleshy fruits. This is of significance for the naturalised flora in that it accounts for the spread of some species to sites far from parent plants. Ilex aquifolium, Cotoneaster spp., Taxus baccata and Hedera helix are good examples and the first three in particular are not infrequently found invading indigenous scrub and forest far from human habitations. Fleshy fruits adapted for dispersal by mammals are not common and generally lack their dispersal agents or any effective substitute - as indeed they often do in their country of origin where the megafauna is now often extinct. The kiwifruit ( Actinidia deliciosa) displays the characteristics typical of mammal-dispersed fruits: dull colouring, large size, and no hard rind. In contrast, dispersal by adhesion to the outside of mammals is relatively common, perhaps because the mechanisms are often not specialised for particular animals. Naturalised species with this means of dispersal may be regarded very unfavourably by farmers as the disseminules decrease the quality of fleeces and pelts or may even, through their ability to penetrate skin, endanger animal health. Species found in sheep fleeces include Medicago polymorpha, Calotis lappulacea, Galium aparine, Marrubium vulgare, and Arctium lappa. Many of the species with adhesion mechanisms are not above using human clothing or hair as a means of dispersal.

Incidental dispersal with mammals and birds is also common and is the means by which many species originally came to New Zealand. Humans are undoubtedly the main agents for this form of dissemination within New Zealand as weed seeds or fruits are unintentionally moved about along with crop seed, domesticated animals, farm machinery, nursery plants, with soil or gravel used in road works, and by many other means. In most such cases the disseminules are not specifically adapted to such dispersal. Even cars provide a means of dispersal for some species; Wace, N., Proc. Ecol. Soc. Australia 10 : 167-186 (1977), has described the dispersal of many species also naturalised in New Zealand by cars in Australia and there is every reason to believe that this situation also prevails here.

Although most species in the naturalised flora are dependent on spores or seed as their main means of reproduction, many also reproduce vegetatively and a few are entirely dependent on vegetative reproduction for their survival in the wild in New Zealand. It has been argued that only species which produce viable seed should be considered fully naturalised; this can be little consolation to those who have Oxalis latifolia or O. incarnata, tristylous species each represented in New Zealand by one self-incompatible stylar morph, established by bulbils in their gardens. Salix fragilis is widespread and well-established but is represented in N.Z. almost entirely by one male clone. Similarly, Petasites fragrans is known only from male plants and although local in New Zealand is widespread in Europe where female plants are also absent. The ability of a species to spread vegetatively as well as by seed frequently adds to its potential as a weed especially when, as in Calystegia silvatica, Taraxacum officinale, and Salpichroa origanifolia, stolons, roots or bulbils resist at least mechanical attempts at weed control.

The naturalised flora will continue to change. Some species are lost from the flora as a result of changing agricultural practices, as for example with Agrostemma githago, and Scandix pectin-veneris which was widespread in New Zealand around the turn of the century but has not been collected since 1930. Such losses are more than compensated for by additions to the flora. Because of New Zealand's stringent import control procedures at points of entry and because many common temperate weeds are already established here, most additions to the flora are escapes from horticulture. Species which are easily cultivated here, and in particular seed freely or tend to run wild in gardens, constitute a vast reservoir of potential wild plants and this is being added to by a continuous stream of deliberate plant introductions. Greater attention should be afforded the taxonomy and classification of the horticultural flora.

Naturalised plants have become an important component of the New Zealand flora in their number, frequency and degree of dominance, especially in lowland communities. Now, after over a century of relative neglect in ecological studies, naturalised species are receiving more emphasis and their importance to such research is likely to increase rather than diminish. Some species should be welcomed, now that they are here, as useful additions to our flora, others at least add colour to disturbed sites in cities and on rural roadsides. However, others must remain unwelcome, at least in many circumstances, because of their invasive habits, or poisonous or other undesirable properties. Whatever their characteristics naturalised plants now form an integral part of the New Zealand flora and will always remain an important part of our environment.

Lincoln, 15 December 1987

C. J. Webb

W. R. Sykes

P. J. Garnock-Jones