Botanical Relationships of Weeds. Interpreted in the broadest sense, our weeds are representative of the major subdivisions of the plant kingdom. A minor element numerically in our weed flora, the Cryptogams or spore-bearing plants include the stoneworts (Chara) of watery places, the lichens (Xanthoria) of tiles on roofs, the liverworts (Marchantia) of glasshouses, the mosses of lawns, and the troublesome bracken fern (Pteridium esculentum) of hill-country pastures. The wide range of weedy herbs, shrubs, climbers, and trees belong to the Phanerogams or seed-bearing plants

New Zealand's weeds are disposed in more than 100 plant families, some well-known ones, and their weedy representatives including the antirrhinum family, Scrophulariaceae – foxglove (Digitalis pur-purea), speedwell (Veronica spp.); beetroot family, Chenopodiaceae – fathen (Chenopodium album); cabbage family, Cruciferae – hedge mustard (Sisymbrium officinale), wild turnip (Brassica campestris); carnation family, Caryophyllaceae – chickweed (Stellaria media), pearlwort (Sagina procumbens); carrot family, Umbelliferae – fennel (Foeniculum vulgare), hemlock (Conium maculatum); daisy family, Compositae – Californian thistle (Cirsium arvense), ragwort (Senecio jacobaea); forget-me-not family, Boraginaceae – viper's bugloss (Echium vulgare); grass family, Gramineae – nassella tussock (Nassella trichotoma), twitch or couch (Agropyron repens); iris family, Iridaceae – Cape tulip (Homeria collina), montbretia (Crocosmia x crocosmiiflora); mint family, Labiatae – horehound (Marrubium vulgare), pennyroyal (Mentha pulegium); pea family, Papilionaceae – broom (Cytisus scoparius), tares (Vicia spp.); potato family, Solanaceae – black nightshade (Solanum nigrum), thorn apple (Datura stramonium); rose family, Rosaceae – blackberry (Rubus spp.), sweet briar (Rosa rubiginosa).

Origin of New Zealand Weeds. Most of our weeds have been introduced, either accidentally or intentionally, from overseas and are native of countries in both hemispheres. The native flora has its “black sheep” which, few in number, are nevertheless significant weeds, prominent being bracken fern (Pteridium esculentum), hard or pig fern (Paesia scaberula), kanuka (Leptospermum ericoides), manuka (L. scoparium, piripiri – corrupted to bidibidi – hutawai (Acaena spp.), rushes (Juncus spp.), sedges (Carex spp.), tauhinu (Cassinia spp.), and tutu – corrupted to toot (Coriaria spp.). The native weed element includes, too, the shrubs and trees, variable in number of species according to locality, which make up the “secondary growth” of deteriorating hill-country pasture in higher rainfall districts. Other native species are less significant weeds; several aquatics, e.g., pond weed (Potamogeton spp.), are troublesome in stock-water dams, wild Irishman or matagouri (q.v.) (Discaria toumatou) and New Zealand cotton plant (Celmisia coriacea and C. spectabilis) occur in modified tussock grassland, and some mat-forming perennials, e.g., Cotula spp., Hydrocotyle spp., Pratia spp., are troublesome in lawns and playing-field turfs. An Australian plant, Carex longifolia, commonly known as sedge, is causing serious concern in Northland by spreading over dairy land. It grows in dense tufts up to 3 ft tall, its leaves being greyish-green in colour, and up to 5 mm in width.

Duration and Forms of Weeds. Many weeds, especially those of cultivated land, waste places, and modified tussock grassland, are annuals, that is, plants completing their life cycle from germination in under 12 months, familiar examples being fathen (Cheno-podium album) and chickweed (Stellaria media). Such significant weeds as hemlock (Conium maculatum) and spear thistle – misnamed Scotch thistle (Cirsium vulgare) – are biennials, completing their life cycle in less than two years, but not flowering in the first year. In some seasons and some localities annuals may act as biennials, or biennials act as annuals. Annuals and biennials are herbaceous, although in some species the stems tend to be woody, especially near the base, e.g., Bathurst bur (Xanthium spinosum).

Perennial weeds, that is, plants which live longer than two years and flower several times, are diverse in form and size and include many troublesome weeds. They range from herbs, e.g., buttercups (Ranunculus spp.), daisy (Bellis perennis), through semiwoody climbers, e.g., Japanese honeysuckle (Lonicera japonica), to shrubs, e.g., gorse (Ulex europaeus) and trees, e.g., elderberry (Sambucus nigra) and lodgepole pine (Pinus contorta var. latifolia).

Broomrape (Orobanche minor) and clover dodder (Cuscuta epithymum) are total parasites, deriving all their nutrients from a range of host plants, including vegetables, ornamentals, agricultural crops, and weeds.

Aquatic weeds are of two types, the first free-floating on the water surface, e.g., water hyacinth (Eichhornia crassipes), the second with crowns or tubers anchored in mud often several feet below the surface, e.g., Cape pond weed (Aponogeton distachyum) and oxygen weed (Lagarosiphon major).

Where Weeds Grow. Significant definable sites of growth or habitats include cultivated land, sown pastures, modified native tussock grassland, artificially afforested land, aquatic places (rivers, streams, irrigation channels, drains, dams), coastal sands, waste land (roadsides, railway reserves, river beds, vacant urban sections), and areas about industrial establishments. Some weeds are troublesome in a number of habitats, as gorse in sown pasture, modified low-tussock grassland, and waste land generally; others, as aquatic weeds, are much more specific in their requirements and may be restricted to a particular habitat. The common weeds of cultivated land, which are often found on bare or partially bare waste land too, are rarely long persistent in sown pastures on better class land because of an increasing density of the sward and greater competition. Conversely, pastures on poorer soils tend to be less dense and permanent, and the decreasing competition allows the entry of weeds, including some normally occurring in newly sown pastures. The sunny or northerly aspect (slope) on hill country generally bears a less dense vegetation than does the shady or southerly, and the weeds occurring on the two aspects are often different; for instance, on sunny slopes in modified tussock grassland in low-rainfall districts in Canterbury, there is often a predominance of short-lived annual weeds with the occasional hardy perennial, whereas on the shady aspect with a denser sward there are fewer weeds in terms of species and individuals, and these are usually longer lived annuals with more perennial species.

Conditions Favouring Weeds. The primary requirement for weeds, as with humans, is living space. In any vegetative cover, therefore, whether in domestic gardens, agricultural crops, artificial forest plantations, native forests, or pastures, bare ground, either localised or generalised, permits entry and establishment of weeds, provided seed is present in the soil or available in the vicinity for dispersal to the particular sites. Associated needs are for light, moisture, air, and subsequently, nutrients for continued growth. In satisfying these basic needs, which are, incidentally, those of useful plants, the demands of weeds may reach a level at which useful plants are starved or smothered to the point of death.

Any factor, therefore, which promotes deterioration of vegetative cover and results in bare ground facilitates the establishment of weeds; such factors include hard or over-grazing by livestock or feral animals, burning, cultivation, and flooding or drought.

Geographical Distribution of Weeds. Some weeds will tolerate, and be troublesome over, a wide range of climatic and soil conditions. They may occur, often abundantly, from the Auckland Province to Southland, as, for example, black nightshade (Solanum nigrum), docks (Rumex spp.), fathen (Chenopodium album), gorse (Ulex europaeus), rushes (Juncus spp.), and Poa annua. Other weeds may have a similar geographical range, but be absent or not troublesome over considerable areas; ragwort (Senecio jacobaea), for instance, is not significant in the low-rainfall districts of Marlborough, Canterbury, and Otago. Weeds of tropical or subtropical origin are significant in the warmer northern part of the North Island, some appearing again in parts of the Nelson Province and down the west coast of the South Island, e.g., Bermuda buttercup (Oxalis pescaprae), hakea (Hakea spp.), nut grass (Cyperus rotundus), and woolly nightshade (Solanum auriculatum). Certain significant weeds are localised in occurrence, e.g., Cape tulip (Homeria collina), nassella tussock (Nassella trichotoma), and common reed (Phragmites communis), and it is difficult to determine whether this is due to some limiting factor, such as soil or climate, or to chance, in that the seed has not been introduced into other localities.

The weed flora of low-rainfall areas in Marlborough, Canterbury, and Otago is characterised by short-lived annuals and hardy perennial species, e.g., St. John's wort (Hypericum perforatum), spring speedwell (Veronica verna), sweet brier (Rosa rubiginosa), and upright cinquefoil (Potentilla recta), whereas high-rainfall areas in both islands show a different range of species, including longer-lived annuals, herbaceous perennials, and some shrubs and trees, both the native “secondary growth” element and introduced species, e.g., barberry (Berberis aristata) and spiderwort or Himalayan honeysuckle (Leycesteria formosa).

Altitudinal Range of Weeds. Many plants generally regarded as weeds have proved their ability to grow at altitudes from sea level to over 2,000 ft, but it is only at lower altitudes up to 1,500 ft that most introduced species are troublesome. Some particular plants are regarded as weeds at lower altitudes, e.g., catsear (Hypochaeris radicata), but at high altitudes the same plants are in fact useful, providing grazing or assisting in the conservation of the soil. Again, some weeds are restricted to lower altitudes, either because suitable sites for growth are lacking, e.g., weeds of cultivated land, or because climatic conditions are too rigorous for survival, e.g., weeds of tropical or subtropical origin.

Dispersal of Weeds. Dispersal of fruits and seeds from the parent plant influences the invasive capacity and sometimes the significance of weeds and may influence weed-control programmes. Fruits may be modified to ensure dispersal by particular agencies, important ones being:

1. Wind: Fruits or seeds modified for wind dispersal by parachute-like structures, as in dandelion (Taraxacum officinale), ragwort (Senecio jacobea), willow herb (Epilobium spp.), are often carried long distances; whole plants, i.e., tumble-weeds, as saltwort (Salsola kali) of coastal sands and tumbling mustard (Sisymbrium altissimum) of Central Otago roadsides, are rolled along dropping seed as they go, or, again, it may be a portion only of the plant – the seed heads of nassella tussock (Nassella trichotoma) are easily carried for distances of several miles. Non-modified fruits or seeds of many weeds are dispersed for varying distances about the parent plants.

2. Water: Rivers, streams, drains, irrigation channels, water races, and the sea carry for varying distances a wide range of weeds, not only the fruits and seeds, but also complete plants, portions of twitchlike plants, bulbs, and corms. The efficiency of such agencies is shown by the weeds which come up following floods.

3. Animals, including birds: These disperse weeds in three ways – by eating succulent or dry fruits and casting out viable seeds in the dung, e.g., blackberry (Rubus spp.), clovers (Trifolium spp.), and sweet brier (Rosa rubiginosa); by dry fruits with hooks, hairs, or spines catching in hair, wool, or feathers and being dislodged at a distance from the parent plants, e.g., many weedy grasses, horehound (Marrubium vulgare), and piripiri (Acaena spp.); and by carrying fruits or seeds mixed in mud in hooves.

4. Humans: Man in the course of his diverse activities disperses fruits, seeds, and portions of living plants, such as bulbs, corms, and rhizomes, through the agency of motor vehicles, railway equipment, agricultural and construction machinery, the packing of merchandise and produce generally. Seeds and the like are often attached to personal clothing and are carried in mud on footwear; many home gardeners have derived their oxalis (Oxalis spp.) either from bulbs in dirt around the roots of ornamental plants or in mud carried on their own footwear.

Reproduction of Weeds. Reproduction of weeds is normally by seeds (spores in ferns), but some weeds reproduce, in addition, by vegetative means which involve specialised or modified portions of the plant body. Species which reproduce in this way are often very troublesome weeds, as the following examples indicate – by underground stems or rhizomes, e.g., Californian thistle (Cirsium arvense), greater bind-weed – erroneously called convolvulus (Calystegia sepium), and twitch or couch (Agropyron repens); by above-ground rooting stems or stolons, e.g., Bermuda grass or Indian doob (Cynodon dactylon), Kikuyu grass (Pennisetum clandestinum), and pennyroyal (Mentha pulegium); by corms, e.g., Cape tulip (Homeria collina), montbretia (Crocosmia x crocosmiiflora), and watsonia (Watsonia bulbillifera); by bulbs, e.g., oxalis (Oxalis spp.) and wild garlic or wild onion (Allium spp.); by tubers, e.g., climbing dock (Rumex sagittatus) and nut grass (Cyperus rotundus).

Control and Eradication. Briefly, control is limiting the spread of weeds, e.g., slashing a bush of gorse to prevent seeding, but leaving the crown intact from which regrowth can take place. Continued control measures may result in the ultimate eradication of a weed or weeds.

Measures for the control and eradication of weeds fall into four categories:

1. Mechanical: Includes digging, grubbing, hoeing, slashing, mowing, bulldozing, ploughing, or other types of cultivation.

2. Firing: Burning off of weeds, either on a restricted or an extensive scale.

3. Chemical: Involves the application of one or other of an already wide but ever-increasing range of chemicals, some of which cause the death of many species of weeds and also useful plants. There are also selective herbicides which exercise control or eradicate one or more weed species while causing no permanent damage to useful crop plants.

4. Biological: Has been achieved by several methods, including (a) liberation and establishment of insect parasites which are specific to particular weeds, e.g., seed weevil (Apion ulicis) on gorse, stem gall fly on mist flower or Mexican devil, and Chrysolina beetles on St. John's wort; (b) planting of trees to smother gorse or other woody weeds, or use of vigorous crops to smother herbaceous weeds; (c) use of farm management techniques to induce improved growth of pastures and crops giving increased competition; and (d) crushing out bracken fern, etc., by large mobs of cattle or sheep, or eating out blackberry by goats.