A weed is a plant growing where it is not wanted. A flowering plant that is welcome in a garden may be considered a weed on the roadside or in a bush reserve.
Of New Zealand’s 26,500 vascular plant species, only 9% (about 2,500) are native. At least 1,800 introduced plant species have become naturalised, which means they grow in the wild – many of them climb over, smother or outgrow native species. They are sometimes called introduced weeds, adventives, exotics or alien plants. Weeds not only threaten native vegetation – if they become dominant they can alter ecosystems and displace animals adapted to living in native bush.
From the time of first settlement, Māori and European farmers and gardeners brought useful or ornamental plants to New Zealand. Over 75% of bush weeds have come from gardens. Called garden escapees, many weeds have entered the bush as wind- or water-borne seeds, bulbs or broken fragments. Others have been spread as dumped waste or by stock, vehicles and birds. Reserves and picnic areas near cities or roads, or places visited by large numbers of tourists, are particularly susceptible to weedy invaders. People dump garden waste in these areas, or may carry seeds on muddy footwear.
In warm northern New Zealand, tropical or subtropical plants can escape from gardens and become aggressive weeds, thriving in frost-free areas and spreading until they reach their southern limit. The north, especially Auckland, is afflicted with many weeds that are rare or unheard of in frostier upland or cooler parts of the country.
Weeds are plants that people judge to be bad. Whether a plant is a weed can be a matter of opinion. Some people were upset when the native tutu was included in Common weeds of New Zealand (1998). A farmer would consider tutu growing in a field a weed as it can poison stock. But the same plant growing on reserve land is protected as a valuable part of the natural vegetation.
Weeds make little headway into dense, mature bush. But they can thrive in scrubland, at the edges of bush, in clearings, in areas damaged by possums, deer, goats, pigs and farm stock, and in regenerating bush. Weedy grasses, thistles and wind-blown seeds often take root on ground newly exposed by slips, changed river courses, forest fires and road cuttings.
Monitoring and eradicating weeds is difficult and expensive. The Department of Conservation lists about 300 weeds as serious problems in national parks and reserves. The department monitors all its reserves for newly arrived weeds, paying special attention to areas where lots of people walk.
City and regional councils tackle weeds in their reserves, and increasingly, volunteers are clearing small areas. For example, the New Zealand Trust for Conservation Volunteers and Weedbusters rid reserves of bridal creeper, ivy, Chilean flame creeper, sweet briar, pine, holly, hawthorn and sycamore.
This many-stemmed shrub or small tree has sweetly-scented, tapering heads of mauve flowers which attract butterflies, particularly red admirals. Originally from Japan and China, the plant arrived in New Zealand via Europe. It was first noticed growing wild in the 1940s. It spreads to disturbed sites and up shingle river beds into remote, mountainous places such as the Urewera and Remutaka ranges, where it supplants native shrubs. It grows abundantly in the North Island and in the northern half of the South Island.
Radiata pine trees were first noticed growing wild in New Zealand in 1904, their wind-blown seeds having escaped from plantations. Today, wilding (uncultivated) pines grow above the bushline and in disturbed areas such as on slips and burnt sites, where they overgrow and displace native species. Wilding pines disfigure landscapes in many parts of the country, but are a serious nuisance in Urewera Forest Park, on the east coast north of Napier, in the Marlborough Sounds and in Central Otago. By 2016 wilding pines were thought to covered a similar land area to plantation forests. Controlling them is labour-intensive. Saplings must be pulled out by hand, or small trees felled at the base and the stump pasted with herbicide.
Contorta pine is a serious problem on the Volcanic Plateau, and is controlled by the Department of Conservation and the New Zealand army.
Tree privet grows up to 14 metres high and can survive for 100 years. Originally from China and Korea, it was first noticed growing wild in New Zealand in 1958. It is a problem in the Auckland, Waikato and coastal Bay of Plenty regions, where it has displaced many native canopy trees. In the north, some areas of native bush have been reduced to tree privet forest, with its own seedlings providing the only new growth. It has spread as far south as the West Coast. Its pollen can induce severe asthma.
This tree has large woolly leaves and grows up to 10 metres high. It becomes stag-headed after 25 years. Originally from Brazil, woolly nightshade escaped from New Zealand gardens in the 1880s and is now a common weed in the country’s north, but less so further south. It rapidly invades and out-competes native plants at forest margins and in damaged forest. Rainwater dripping off their leaves poisons anything growing beneath them.
Each year a single gorse bush will produce 500–1,000 seeds (and occasionally up to 20,000) per square metre of ground. Seeds stay viable in the soil for over 100 years. As early as 1859, laws were passed to prevent its spread or sale. But by 1893, gorse was common throughout the country, and in 1900 it was declared a noxious weed.
Missionaries brought gorse, a prickly, yellow-flowered shrub, to Northland in the 1830s for farm hedging. Today, gorse is troublesome and expensive to control on farmland, scrubland and in damaged forest. It often thrives where possums or farm stock have opened up the forest canopy and let light in. On the positive side, gorse adds nitrogen to the soil, stabilises erosion-prone slopes, and is a good nurse plant for emerging native trees. Around Wellington it takes native trees 30–40 years to overgrow gorse, but around Dunedin it takes up to 60 years.
This hollow-stemmed shrub grows up to 3 metres high. Birds and water help spread its seeds, and it grows at the margins of bush, in open river beds and forest clearings. Himalayan honeysuckle stops native plants from regenerating and replaces native pioneer plants.
A large, vigorous woody climber, banana passionfruit scrambles into tree canopies and stifles its host. A single plant can grow to cover the area of a house. The plant has three-fingered leaves, tendrils, long pink hanging flowers the year round, and pendulous, banana-shaped fruit in winter. The fruit turns yellow when ripe.
Banana passionfruit grows at forest edges in the North Island and in warmer parts of the South Island. Its seeds are spread by possums, wild pigs and birds. It is partly held in check by garden snails, which ring-bark the lower stems. Originally from Brazil, it was first noticed growing wild in New Zealand in 1958. Banana passionfruit is an aggressive weed on many Pacific islands.
The first British settlers brought blackberry to New Zealand, and more plants have come from Australia and the USA. The plant quickly spread, and 19th-century farmers declared it the most harmful weed in New Zealand. It was jokingly said that there were only two blackberry plants on the West Coast – one running along each side of the railway line.
At least 19 varieties of blackberry grow throughout New Zealand. Its roots can spread for many metres, making mature plants difficult to eradicate. Fruit-eating birds such as blackbirds and quail spread seeds along the roadside, in bush margins, clearings and in regenerating forest.
In 1925 the New Zealand government vainly offered a £10,000 reward (about $800,000 in 2006) for a method to stamp out blackberry. Goats were sometimes used to trample it down – mostly without success. It was only when hormone sprays were invented in the 1940s that farmers could control blackberry. It still remains a serious pest in scrubland and forest reserves in Northland, Nelson, the West Coast and elsewhere.
Also known as snakefeather, this is a scrambling, shade-tolerant climber with feathery leaves and a tuberous root. Introduced from Africa in the late 1940s, the plant has spread in frost-free areas of New Zealand as far south as Banks Peninsula. It smothers native shrubs and saplings, and stops native seedlings from regenerating. The plant is still spreading in the warmer parts of the country.
German ivy is a yellow-flowered scrambler with stems up to 5 metres long. It may extend along the margins of bush, smothering shrubs and preventing native plants from regenerating. It is a problem in native plant reserves and cutover forest in the central North Island, Wellington, Nelson and elsewhere.
Japanese honeysuckle, a vigorous, woody scrambler, has long wiry stems. The plant has escaped from gardens and is abundant in Auckland City and northern Hawke’s Bay. It is a problem in native forest reserves, where it smothers trees and shrubs. In the USA this weed can overrun entire forests.
Jasmine is a fragrant, evergreen climber that twines up trees and shrubs. It suffocates native plants along forest margins and in broken bush, from Gisborne northwards. South of Gisborne, jasmine is a well-behaved garden plant. Charles Darwin saw it (as well as gorse and honeysuckle) growing in missionaries’ gardens in Northland in 1835, but it was not reported growing wild until 1980.
Introduced from Europe as a garden plant, old man’s beard was first noticed growing wild in New Zealand in 1940. Also known as traveller’s joy, this woody, deciduous climber scrambles up and over shrubs and trees, its thick stems sometimes growing 30 metres long. Masses of fluffy seeds are released in autumn, and plants can sprout from fragments of stem.
This aggressive weed takes hold in forest gaps and along the margins of forest and scrub reserves, especially on recently disturbed sites. It overwhelms its host trees, notably in the southern half of the North Island, Nelson, Marlborough, Buller, Kaikōura and Canterbury, and especially near Taihape, where a single plant can cover an area the size of a tennis court. Old man’s beard grows throughout both islands except in Westland and Fiordland, and on Stewart Island and the Chatham Islands. Property owners are expected to remove it.
A relative of the kūmara, morning glory is abundant in many tropical countries and was first reported growing wild in New Zealand in 1950. It is a vigorous purple-flowered climber with stems, sometimes hundreds of metres long, that swarm up native trees. It does best in sunny, frost-free places, and blooms year round in northern New Zealand. Most plants never produce seeds, but in 1996 one Bay of Plenty specimen had large amounts of seed, and seedlings nearby.
A kiwifruit vine can grow 20 metres high and cover 1,000 square metres in 30 years. The plant has escaped cultivation to grow wild along bush margins in the Bay of Plenty, where it chokes native trees. The species can also be found in the Marlborough Sounds.
Wild ginger or ginger lily is originally from India and was first grown in New Zealand gardens in the 1920s. It grows to head-height, with a thick-rooted mat of stubborn rhizomes a metre deep. It has escaped from gardens to overrun forest reserves and displace native plants in Northland, Auckland, Waikato, Coromandel and Buller. It is also a prolific weed in Hawaiian forests.
Ragwort was probably brought to New Zealand accidentally with pasture grasses. It was first recorded growing wild in Dunedin in 1894 and rapidly spread throughout the country, especially where cattle and horses were grazed. Ragwort also grows on the Kermadec and Chatham islands. It was declared a noxious weed in 1900.
In 1947 the New Zealand Weed Control Society was established to help educate people about weeds. It later became the New Zealand Weed and Pest Control Society, and today it is the New Zealand Plant Protection Society. They publish papers and books on weeds and their control.
The plant grows especially well in areas of high rainfall and along bush margins. Although shade-tolerant, ragwort does not make headway into closed bush. If horses or cattle eat it, they may suffer cirrhosis of the liver, and die. Sheep are less prone to being poisoned by it when grazing, and help control the plant on some farms. In 1928, caterpillars of the cinnabar moth were introduced from Europe to control ragwort, with some success.
This primitive creeping plant has slender, many-branched stems that form mats on the damp floor of bush margins or in regenerating forest. It grows so densely that it chokes smaller ground-growing plants. Clubmoss will grow from tiny stem fragments which, along with its spores, are easily spread on boots, by stock or machinery. Originally from South Africa, it was first noticed in New Zealand in 1919. Three species have been identified.
Also known as wandering Jew, this low-growing, brittle, dark-leaved plant puts down roots at intervals along its creeping stems. It smothers other low-growing plants. A native of Brazil, it was first noticed growing wild in New Zealand 1915. It prefers cool, moist, shady places, usually invading bush margins and damaged or regenerating bush.
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Esler, Alan E. Wild plants in Auckland. Auckland: Auckland University Press, 2004.
Owen, S. J. Ecological weeds on conservation department land in New Zealand: a database. Wellington: Department of Conservation, 1996.
Plant me instead: plants to use in place of common and invasive environmental weeds in the lower North Island. Wellington: Dept. of Conservation, Wellington Conservancy, 2005.
Roy, Bruce, and others. An illustrated guide to common weeds of New Zealand. 2nd ed. Lincoln: New Zealand Plant Protection Society, 2004.
Thompson, G. M. The naturalisation of animals and plants in New Zealand. London: Cambridge University Press, 1922.