A weed may be defined as any plant or vegetation that interferes with the objectives of farming or forestry, such as growing crops, grazing animals or cultivating forest plantations.
A weed may also be defined as any plant growing where it is not wanted. For example, a plant may be valuable or useful in a garden, or on a farm or plantation – but if the same plant is growing where it reduces the value of agricultural produce or spoils aesthetic or environmental values, then it is considered a weed. However, some plants are weeds regardless of where they grow.
The term ‘pest plant’ (or ‘plant pest’) is commonly used by New Zealand’s central and regional government agencies.
Internationally, the term ‘invasive alien plant’ is often used. This is because weeds have usually been introduced, accidentally or deliberately, from other countries. This is true of New Zealand, where only a handful of native species are considered weeds.
Very few plants were regarded as weeds by European botanists when they arrived in New Zealand – the ancestors of Māori must have brought these with them from Polynesia. They included punawaru (Sigesbeckia orientalis), kohiriki or cobblers’ pegs (Bidens pilosa), small-leaved nightshade (Solanum americanum), horned oxalis (Oxalis corniculata) and bladder hibiscus (Hibiscus trionum).
Māori methods of horticulture involved burning the land to clear it, and growing crops for no more than two years before allowing the area to revert to bush. This meant that invaders such as bracken (Pteridium aquilinum) did not have time to become a problem. The land was then left for many years before being cleared and cultivated again.
Māori vocabulary scarcely acknowledged the existence of a ‘weed’. There were too few fast-growing annual plants for them to need such a word.
On Captain James Cook’s second voyage to New Zealand, crews from the Resolution and Adventure planted vegetable gardens in Queen Charlotte Sound, in 1773. Some of the plants, notably cabbage, turnip and radish, spread or were deliberately spread. By the 1840s, European vegetables and the weeds that came with them were found in many parts of the country.
Charles Darwin, visiting New Zealand aboard the Beagle in 1835, was dismayed to see some unwelcome colonisers. His journal entry for 24 December notes:
‘In many places I noticed several sorts of weeds, which, like the rats, I was forced to own as countrymen. A leek has overrun whole districts, and will prove very troublesome, but it was imported as a favour by a French vessel. The common dock is also widely disseminated, and will, I fear, for ever remain a proof of the rascality of an Englishman, who sold the seeds for those of the tobacco plant.’
Some weeds grew and spread more vigorously than they had in England. Many were introduced accidentally as impurities in agricultural and garden seed, or as seeds in packing material, bedding, and imported hay and straw.
Settlers also brought plants for their use – culinary and medicinal herbs, hedging plants, and trees for timber and firewood. Several of these were useful initially, and widely spread by farmers and growers. Too late, some were found to cause more harm than good when they escaped from gardens and plantations and became weeds in other environments. These include wilding pines (Pinus spp.), used in forestry and now a pest in tussock grassland, and sweet briar (Rubus rubiginosa), once a rootstock for grafted roses.
Gorse is one of New Zealand’s most notorious weeds. It was deliberately introduced, often sold as seed or seedlings in the 1800s, and planted for stock fodder or as hedgerows. At that time timber was the only available fencing material and gorse was a cheap alternative. But gorse quickly spread to cover hillsides and pasture.
Other plants grown for hedging included broom (Cytisus scoparius), elaeagnus (Elaeagnus x reflexa), hakea (Hakea spp.), boxthorn (Lycium ferocissimum) and barberry (Berberis glaucocarpa), all of which are now regarded as weeds. Hedgerows of gorse, barberry or boxthorn still persist today in some parts of the country.
Some thistles too may have been deliberately introduced, distributed and planted by Scottish settlers as heraldic or Scotch thistles. Cirsium vulgare has kept the name Scotch thistle, and is now a common pasture weed everywhere.
Although sometimes regarded as a weed, mānuka is the source of mānuka honey, which is quickly becoming popular. It has an antibacterial property found only in honey produced from Leptospermum plants. Some farmers are now being encouraged by beekeepers to preserve areas of mānuka scrub.
Mānuka (Leptospermum scoparium) has been the scourge of generations of hill-country farmers, spreading quickly across new pasture. Tutu (Coriaria spp.) poisoned stock (and unlucky circus elephants on at least three occasions) in earlier days, and honey made from it is poisonous. Bracken or rauaruhe (Pteridium esculentum) was a problem in newly cleared land.
Native weeds that pose local problems include mātātā or ring fern (Paesia scaberula), matagouri (Discaria toumatou), and tauhinu (Ozothamnus leptophyllus).
Weeds can become established and persist in grazed pasture. Some, such as scrubweeds, thistles and dock reduce the pasture available to stock.
Others may directly harm livestock, by:
Among the most prevalent thistles are Scotch thistle, nodding thistle (Carduus nutans) and Californian thistle, some of which have been common in pastures since before 1900. Others (for instance, nodding thistle) have become a problem only in the last 50–60 years.
Mouse-ear hawkweed (Hieracium pilosella) is allelopathic – it produces toxins that inhibit the growth of other plants. Mouse-ear hawkweed spread rapidly in the 1950s, and is a serious problem in the high-country farmland of the South Island.
Two species of Nassella are also a problem in pasture – nassella tussock (Nassella trichotoma) and Chilean needle grass (Nassella neesiana). Other Nassella species are also potential problems.
The most widespread is Nassella tussock, a perennial tussock native to South America which can form dense mats on pastures. It is unpalatable to sheep and cattle. A mature plant can produce 120,000 seeds a year, which can remain in the soil for a decade or more. Seeds are borne by wind and water, and carried on animals, humans and machinery. Plants are found in the drier parts of New Zealand. However, prolonged efforts by Nassella Tussock Boards, and more recently, regional councils, have limited its spread.
In pasture, any plants other than sown grass species and clovers tend to be regarded as weeds because they may be less productive. However, organic farmers regard a ‘mixed herb ley’ of a mixture of species as healthier for stock. They often consider so-called weeds such as dock, plantains and dandelions to be useful components of pasture.
Many weeds that grow among crops and in gardens were introduced either accidentally or deliberately by early British settlers. They include groundsel (Senecio vulgaris), chickweeds (Stellaria media and Cerastium spp.), fathen (Chenopodium album), nightshade (Solanum spp.), amaranthus (Amaranthus spp.) and willow weed (Persicaria persicaria) – plants that germinate readily, and grow, flower and fruit quickly to produce copious seeds.
Sometimes weeds contaminate harvested crops. For example, black nightshade berries may be collected along with peas. Weeds can also host destructive pests such as aphids, caterpillars, fungal rots and viruses.
In gardens, weeds are mainly an unsightly nuisance, but they also compete for water, nutrients and space. The more troublesome and difficult to eradicate are couch grass (Elymus repens), which has underground rhizomes or lateral roots, bindweed or Calystegia spp., also with rhizomes, and Oxalis, which multiplies rapidly with many small bulbs that are difficult to eradicate.
Presumably as a result of climate change, weed species from warmer regions are gradually spreading southwards. They include the summer grasses (Digitaria, Setaria and Panicum spp.), paspalum (Paspalum dilatatum) and kikuyu grass (Pennisetum clandestinum).
It is impossible to say which is New Zealand’s worst pastoral weed. But in terms of lost production, land covered and money spent on control, it’s easy to argue that gorse is the worst. Scrub weeds like gorse, broom and others waste useful pasture space. But gorse can also be an excellent nursery for native seedlings.
In forestry, weeds cause the most problems in the first two years of tree growth. They can reduce the growth rate of young trees and sometimes distort their growth patterns. In some forests pampas grass (Cortaderia selloana) has become established and is now widely regarded as a weed. Herbicides are used extensively at the start of new forestry projects, and mowing and other mechanical methods of weed control are also used on less rugged terrain.
Among the most serious introduced aquatic weeds are the oxygen weeds egeria (Egeria densa) and hydrilla (Hydrilla verticillata). These form dense floating mats which block pump- and power-station intakes, clog drains and irrigation races, interfere with recreational activities, and impair water flows in rivers and streams. They grow in water where fertiliser runoff from farmland has raised nutrient levels.
Managing water weeds is costly. Methods include building screens to prevent turbines from clogging, cutting and harvesting weeds in large open waterways, dredging, using mechanical diggers in drains and irrigation races, and applying herbicides.
About 75% of land weeds and over 50% of freshwater weeds were originally grown in gardens or home aquariums. The places with most weeds are often close to towns. On average, eight species of garden plant each year become naturalised in the wild. They threaten the survival of native plants and the long-term existence of some native animal species. Weeds that seriously threaten New Zealand’s native species, ecosystems and conservation lands are old man's beard (Clematis vitalba), wild ginger (Hedychium gardnerianum and Hedychium flavescens), pampas grass (Cortaderia selloana and Cortaderia jubata) and contorta pine (Pinus contorta).
The Department of Conservation (DOC) manages about 8 million hectares of native forests, tussocklands, alpine areas, wetlands, dunelands, estuaries, lakes and islands – about 30% of New Zealand's land area. DOC is responsible for preserving and protecting these areas, including managing invasive weeds. The cost is considerable – DOC spends over $14 million a year on weed control, and regional and district councils also spend large sums.
The first legislation recognising weeds as a problem was enacted by various provincial councils in the 1850s. These were acts or ordinances against individual species or groups of species such as gorse, watercress or thistles. In 1900 the Noxious Weeds Act was passed, followed by several similar acts and the Nassella Tussock Act 1946. In 1950 a new Noxious Weeds Act became law, and successive acts were introduced to meet the changing patterns of the agricultural industry. These were superseded by the Noxious Plants Act 1978.
The Noxious Plants Act has been replaced by the Biosecurity Act 1993, which moved the main responsibility for weed legislation away from central to regional government. Regional councils and unitary authorities must now develop pest management strategies every five years. These apply to current and potential animal and plant threats and to pest control, and may be the responsibility of the land owner or the council, or both, depending on the location and pest species.
The National Plant Pest Accord is an agreement between the Nursery and Garden Industry Association and central and regional government, listing potential problem plants that cannot legally be propagated, distributed or sold anywhere in the country.
The earliest methods of weed control were hand-weeding or burning. Weeding by hand is adequate for managing small areas of crop or pasture, but larger areas require faster methods. Large areas of hill-country scrub, especially mānuka, were initially cleared by hand, but later, machinery was used to crush scrub before burning it.
Organic farmers manage weeds with steam, burners or mechanical weeding.
The chemical control of weeds has always been seen as an attractive solution because it is relatively cheap and easy.
The most dramatic changes in herbicide use came after the Second World War, when the phenoxy herbicides 2,4-D and MCPA were developed. They were effective against broadleaf weeds in lawns, pasture and cereal crops like wheat, barley and maize. These chemicals are still used today to kill thistles and ragwort in pastures, and as ingredients in solutions to combat weeds in cereal crops. The herbicide 2,4,5-T was applied to gorse and other scrub until the late 1980s. Its withdrawal followed public concern over its use as an ingredient in Agent Orange, a defoliant used extensively in Vietnam. This was associated with birth deformities, cancer and many other illnesses thought to be caused by dioxin, a toxic and carcinogenic by-product of its manufacture. In the early 2000s, controversy still surrounded the Dow AgroSciences plant in New Plymouth, where 2,4,5-T and other phenoxy herbicides were made. Local residents claimed ongoing health problems resulting from contamination.
The main drawback of using sodium chlorate as a herbicide was its tendency to catch fire when exposed to heat. Overalls or work clothes soaked in the chemical would burst into flames if dried in front of a fire even after being washed. Today, common salt (sodium chloride) is added to reduce inflammability).
Many new products were developed in the 1960s and 1970s. Among them was Roundup, or glyphosate, probably the most widely used of all the herbicides. Although not selective, it kills the whole of a treated plant – the roots as well as the above-ground parts. When organo-silicone was added to Roundup, its uptake by scrub weeds like gorse was improved.
Developing and testing new herbicides is expensive. Few new ones now appear on the market, and many older brands are no longer available. Herbicides are likely to continue being used for many years to come.
Millions of dollars have been spent killing scrub weeds, but unless there is adequate follow-up with fertiliser, pasture seed, fencing and good grazing, the cleared areas will quickly revert to weed. Gorse in particular regrows rapidly from seeds in the soil and from stumps.
Weeds can become resistant to chemical herbicides. This occurs when a small number of individual plants develop a genetic makeup that means they are not killed by the herbicide. When the susceptible plants have been killed, the resistant ones multiply and, if herbicide application continues, come to dominate the population.
The first species in New Zealand known to become herbicide-resistant, in the late 1970s, were the crop weeds fathen and willow weed. Resistance was discovered in the 1980s in two pasture weeds repeatedly treated with 2,4-D or MCPA – nodding thistle and giant buttercup. New Zealand has fewer cases of herbicide resistance than many other countries, but new cases appear every few years.
Weeds in New Zealand known to have developed resistance to herbicides (by 2007) are:
Biological control (biocontrol) uses the natural enemies of a weed to keep it in check, and is seen by many as the long-term solution to weed problems. This method has been effective in reducing, but not eliminating, ragwort and St John’s wort, which are poisonous to farm stock.
The spread of nodding thistle may also have been deterred by biocontrol agents. Those released include the nodding thistle crown weevil (Trichosirocalus sp.), the nodding thistle gall fly (Urophora solstitialis), and the nodding thistle receptacle weevil (Rhinocyllus conicus). Attempts are being made to find biocontrol agents for many weeds, including gorse, broom, Californian thistle, Chilean needle grass and nassella tussock.
It is difficult to measure the costs caused by weeds. An immense amount of time is spent by home gardeners removing weeds from their gardens and lawns. In the agricultural, horticultural or forestry sectors, costs include lost production as well as weed control. An estimate by Monsanto (an agricultural biotechnology corporation) in 1984 put the overall cost of weeds in New Zealand at $393 million, equivalent to $1.03 billion in 2007 terms.
In 2003 it was estimated that giant buttercup (Ranunculus acris) cost the dairy industry $156 million in one year in lost production. Californian thistle (Cirsium arvense) was calculated in 1999 at costing about $10 million annually.
With the climate expected to become warmer in the future, weeds currently restricted to the north of New Zealand will spread further south. Those that prefer a temperate climate may find future conditions less suitable and die off in some areas. Plants like gorse may spread more rampantly into areas that now suffer heavy frosts, such as Central Otago and parts of Hawke’s Bay. Higher incidence of droughts and floods, which lead to increased areas of bare ground, will see greater numbers of annual weeds like thistles., which can thrive in such conditions.
Some weeds have the potential to become serious weeds, but so far have limited distribution, although climate change could hasten their spread. These include several grasses and sedges, Japanese knotweed (Fallopia spp. – already a major problem in Britain), field horsetail (Equisetum arvense – a serious problem in several countries), and a number of aquatic weeds.
Flora of New Zealand. Vols 3, 4, 5. Wellington: Government Printer, 1988–2000.
Roy, Bruce, and others. An illustrated guide to common weeds of New Zealand. 2nd ed. Lincoln: New Zealand Plant Protection Society, 2004.