Weeds in NZ

History

A brief history of the introduction of weedy species to New Zealand may help to explain why we now have the weeds we do, and help us avoid repeating past mistakes.

The Maori introduced six crop species (kumara, taro, hue gourd, aute or paper mulberry, yam and ti pore, the tropical cabbage tree) to NZ2, but apparently brought no weed species. Esler3 points out that the New Zealand flora had few annual species to act as weeds and that bracken was perhaps the most serious perennial weed species.

European explorers brought the first weed seeds to New Zealand, probably as contaminants of potatoes or vegetable seeds. According to Harris4, 'The introduction of potatoes to New Zealand is generally attributed to the French explorer de Surville who planted potatoes at Doubtless Bay in 1769'. Harris attributes further introductions to du Fresne's expedition in 1772 and to the Adventure (sister-ship to Cook's Resolution) in 1773. Beaglehole5 reported that the crew of the Adventure 'planted vegetable gardens', while the Resolution brought the first sheep, possibly carrying seeds in their wool. The sheep themselves died, perhaps poisoned by tutu.

With the rapid expansion of agriculture following European settlement in the 1800s, many crop and pasture seeds were introduced, mostly from Europe, and with them came weed seeds. Many of our weeds are of European origin although some have arrived, directly or indirectly from America, Asia and Australia. John Hampton (pers. comm.) reports that 

'Although seed certification was introduced in 1927/28 for cereals and 1929/30 for herbage species, initial emphasis was on varietal purity. Imported seed lots were probably never cleaned of weed seeds (not possible completely for herbage anyway). They were usually sown as received.'

Although many weeds were introduced as contaminants of crop or pasture seed, many, including gorse, were deliberately introduced. Charles Darwin6, visiting New Zealand in December 1835, noted the presence of gorse hedges, and observed:

'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.'

When provincial governments were set up in 1850, noxious weed legislation was thought necessary to help stem the spread of new weeds. In the early part of the 20th century Cockayne7 advised farmers against buying and sowing cheap seed, and the same author8 listed 'extraneous seeds found in commercial samples of agricultural seed on the New Zealand market', including barley grass, barnyard grass, Bathurst burr, and Cape weed.

Since then, new plant species have continued to become established in the wild. Heenan et al.9 and Webb et al.10 list the recently naturalised species. According to DoC11 half of the 4,000 plant species growing wild in New Zealand have been introduced to this country. At least another 17,000 introduced plant species are present in gardens or under cultivation. Many of these could become naturalised and some will certainly become weed problems in the future. DoC estimated that over 70% of invasive weeds were introduced as ornamental plants, 12% for agriculture, horticulture or forestry, and only 11% accidentally.

Esler3 estimated that, on average, one species of plant became naturalised in Auckland every 88 days, or 4.12 species per year, between 1870 and 1970 (Fig. 1).

Figure 1. Rate of naturalisation of accidental, horticultural and agricultural introductions (Esler)

Figure 1. Rate of naturalisation of accidental, horticultural and agricultural introductions

An interesting sideline to the 'deliberate' introduction of weed species is the value of agricultural introductions. Halloy12 analysed such introductions in New Zealand and observed that most introductions contributed little to the economy and that even those that become important had effective life spans of about 10 years. Lonsdale13 reported that of the hundreds of grasses and legumes intentionally introduced to northern Australia, only 5% increased pasture production whilst over 60% of the remaining species became weeds.

New plant species are still being deliberately introduced as garden or useful species. Others may be introduced accidentally on dirty machinery, shipping containers or cars, although their presence may not be noted for some time. In addition genetically engineered plants are now being developed here and introduced from overseas. Keeping out any plants that may become naturalised and weedy is vital, and making such decisions at national level is easier than at regional level14.

This brief review of the history of weeds in New Zealand carries two messages:

  1. Good border control is vital, as pointed out recently by the Parliamentary Commissioner for the Environment15.
  2. Early detection and control of potentially invasive newly naturalised species is important in avoiding future problems.

Weed research

Past studies

A review of past research on weeds in New Zealand can show what kinds of research have been useful, which if any have helped in the prediction of weed behaviour and which have helped in the control of weeds. Such a study may also serve as a guide to the future needs for weed research.

Government Botanist A H Cockayne probably conducted the first scientific observations and experiments on 'weeds' in New Zealand, when he investigated the effects of Californian thistle rust on Californian thistle16.

Experiments and observations on weedy species by Cockayne and other government officials are recorded in the New Zealand Journal of Agriculture, which began publication in 1909. Control methods at that time mostly involved cultivation or use of grazing animals, although experiments were carried out with a range of chemicals, many of which would today be regarded as too toxic or environmentally damaging. Deem17, for example, recommended 'proprietary weedkiller', arsenic plus soda, or salt at 2 tons/acre for the control of pennyroyal.

Many government scientists examined methods of biological control. Northcroft18 listed the insects and fungi attacking blackberry in New Zealand, and also reviewed pests of blackberry in other countries. At that time potential biocontrol agents were being imported to New Zealand and their effects tested on related crop species with any effects on native flora at least being considered. Cunningham19 described fungi attacking Californian thistle, broom, gorse, blackberry, bracken ring fern, goat's rue, and tauhinu.

Wartime development of the selective herbicides 2,4-D and MCPA revolutionised weed control and led to the formation, in 1948, of the NZ Weed Control Society, which later became the NZ Weed and Pest Control Society and then the NZ Plant Protection Society. Since 1953, most research on weeds conducted in New Zealand has been recorded in the proceedings of the annual conference of those societies. In the first 50 years of the Proceedings, 1184 papers were published on weeds and their control. In more recent years the Proceedings of the New Zealand Biosecurity Institute have also become a useful source of relevant information.

Gorse has long been regarded as New Zealand's most serious weed problem, so the concentration of research efforts on this species comes as no surprise. (Table 1). Surprisingly, barley grass is second on the list of most-researched species, partly the result of the emphasis that MAF placed on barley grass in the 1970s, and partly because of the willingness of researchers to report their work. The results of our survey suggest that it is still a very important weed. Another 223 species of weeds were mentioned in between 1 and 19 papers published in the proceedings.

Majority of the research reported in the proceedings has been on chemical weed control. Of the 118 papers that mentioned gorse, 88 were on chemical control, often with 2,4,5-T with and without the addition of other herbicides. Relatively few papers have dealt with non-chemical control methods of this or other species (Fig. 2).

Table 1 Numbers of papers (out of 1184 on weeds in the first 50 Proceedings of the NZ Plant Protection Conference.

No. papers Weed species
118 gorse
110 barley grass
59 Californian thistle
54 fathen
47 black nightshade
40-42 each blackberry, dock, nodding thistle, ragwort, wild oats
32-39 each broom, redroot, sweet brier
20-28 each annual poa, barnyard grass, bracken, chickweed, white clover, couch, grass weeds, ryegrass, shepherd's purse, summer grass, Scotch thistle, twincress, willow weed, wireweed

 

Figure 2. Number of papers on chemical and biological (includes cultural measures) control of weeds in 11 selected Conference Proceedings spanning 50 years.

Weed research in New Zealand has often responded to immediate problems, and commonly used 'quick-fix' control methods with herbicides. Relatively few papers have been published on the ecology of weed species (Table 2), and even fewer on the spread of weeds and the reasons for it (Ahmed and Wardle20; Healy21; Macnab22; Molloy23; Stewart et al.24). The NZ Plant Protection Society hosted a workshop in 1993 on Potential Problem Weeds, highlighting some of the issues mentioned in this document. Summaries of papers presented there are available in the published proceedings25.

Table 2. Papers on the ecology of the named weed species published in the Proceedings of the NZ Plant Protection Society.

Weed species

No. papers
barley grass

13
nodding thistle

12
gorse

11
general weeds

8
ragwort

5
Californian thistle

3
broom, summer grasses

2
alligator weed, fathen, Grecian thistle, horehound, Italian arum, lagarosiphon, manuka, matagouri, mouse ear hawkweed, oxalis, poa annua, spiny emex, sweet brier, thistles generally, wild oats, wireweed, yarrow

1

Present research

Crown Research Institutes (Agresearch, Crop & Food Research, Hortresearch, Landcare Research, NIWA) & Massey University conduct most research on pest plants. FRST, Regional Councils and some government departments, especially DoC, fund the research. Chemical companies may conduct their own or contract out research for registration of new herbicides. Many of the research projects listed here are of little direct relevance to hill country pastoral weeds, but are included for the sake of completeness. Topics include:

AgResearch
  • Herbicide resistance studies on nodding thistle & giant buttercup
  • Population dynamics/ecological studies on Californian thistle & nassella tussock
  • Development of bioherbicides for Californian thistle, buttercup & other weeds
  • Risk assessment/modelling studies of weeds & herbicide resistant plants
  • Weed-crop competition, weed seedbank dynamics and seed longevity
  • Weed management in organic farming systems
  • Ground cover species for managing orchard weeds (with Massey University)
  • Reducing environmental impacts of agriculture
  • Fate of pesticides in the environment & strategies for minimising pesticide impact
Crop & Food Research
  • Reducing economic losses caused by weeds
  • Economic utilisation of weeds
  • Policies regarding plant introductions
  • Weed risk assessments
  • Managing weeds in organic cropping systems
  • Pasture-cropping rotations
  • Developing herbicide resistant crops
Department of Conservation
  • The impact of pest animal control on weed dynamics
  • Border control for ecological weeds (with Landcare & NIWA)
  • Impact of exotic pasture species & grasses on threatened plant species (with Waikato University)
  • Development of a protocol for monitoring weed control
  • Development of a national weeds database & an ecological weeds surveillance plan
  • Weed surveillance - how often to search (private contract)
  • Review of biological control for conservation (private contract)
  • Biological control species feasibility study (with Landcare)
  • Weed risk assessment within New Zealand (with Landcare)
  • Impacts of weeds from coastal subdivisions (with Landcare)
  • Effectiveness of weed control and monitoring
  • Impact of pine in tussock grassland, of weeds in shrubland (with Landcare) & in ephemeral wetlands (with NIWA)
  • Boneseed and climbing asparagus
  • Biology and ecology of Celastrus obiculatus & Chilean rhubarb (with Landcare)
  • Pampas grass control strategy and techniques for difficult habitats (private contract)
  • Use of herbicidal gels and loppers (with HortResearch)
  • Scientific review of weed eradication programme on Raoul & Kermadec Islands
  • Legume weed invasion of northern gumland soils (with Waikato University)Ecological effects of spartina eradication with Gallant (with NIWA)
  • Reserve management for weed control (with Landcare)
Forest Research
  • Multidisciplinary team approach focusing on vegetation management
  • Evaluation and screening of pre-commercial herbicides
  • Screening and field evaluation of herbicides for forest weed species
  • Development and enhancement of herbicide efficacy through novel formulations
  • Fundamental studies of mechanisms of herbicide uptake into plant foliage
  • Application technology & equipment development for liquid & granular formulations
  • Aerial spray modelling and decision support systems with GIS & GPS capability
  • Development of plant competition growth models involving light & water
HortResearch
  • Control of environmental weeds
  • Herbicide formulations for ecologically sensitive areas
  • Mycoherbicides for environmental weeds
  • Ground covers/compost for managing weeds in orchards
  • Pesticide residues in plant, soil and water
  • Development of analytical techniques for pesticides
Landcare Research
  • Characteristics of environmental weeds that affect invasion success & impacts:
  • factors facilitating invasion by species in Pinaceae, Rosaceae and Fabaceae
  • impacts of gorse in kanuka, & of wild ginger & tradescantia in forest
  • use of functional groups in predicting invasion success & system impacts
  • new techniques for measuring extent & rate of spread of scrub weeds
  • Improving performance of biocontrol agents, assessing non-target effects on food webs in ecosystems, & predicting down-stream effects of new agents
  • Integrating environmentally-sustainable methods of invasive weed control
  • Importation and release of 5 insect biological control agents for Hieracium species.
  • Finding biocontrol agents for boneseed, woolly nightshade, nassella & blackberry.
National Institute of Water and Atmosphere (NIWA)
  • Development and refinement of a weed risk assessment model to evaluate potential weeds and compare them with existing weeds
  • Investigating the occurrence of existing weed species
  • Species-focussed research on weed control (e.g. Hydrilla, and Zizania latifolia)
  • Evaluation of potential new aquatic herbicides like endothal and triclopyr
  • Comparing the use and application of currently available control options
  • Evaluation of potential aquatic weeds
  • Management strategies for new weed incursions

Weed control legislation: historical government intervention

Early intervention by preventing the introduction or eradicating the first few individuals of a new weed can save the country as a whole millions of dollars. That is why both Central and Regional Governments become involved in weed control. MAF Quarantine's border security aims to prevent the introduction of plant species that could become future problems. 

MAF also oversees the control of four weeds of national importance ("notifiable organisms" under the Biosecurity Act 1993) 

  • water hyacinth (Eichhornia crassipes ), 
  • Cape tulip (Homeria collina), 
  • salvinia (Salvinia molesta) and 
  • Johnson grass (Sorghum halepense). 

These are all weeds of very limited distribution with the potential to form very damaging infestations, as shown by their behaviour overseas. A fifth species, water lettuce (Pistia stratiotes), was formerly in this category, but is now believed to have been eradicated.

Apart from those few species, Regional Councils deal with internal biosecurity, trying to eradicate some species that are of limited distribution in their regions, or to limit the spread of other weeds and, at the same time, protecting good farmers from their neighbours' weeds.

Weed control legislation was first introduced in New Zealand between 1854 and 1875 when Provincial Councils saw the problems caused by noxious weeds. This legislation mostly consisted of Acts or Ordinances against individual species or groups of species like gorse, water cress or thistles. From 1876 until 1899 no weed legislation was in place. In 1900 the Noxious Weeds Act was passed. It included schedules of obligatory weeds, optional weeds and noxious seeds, and was followed by several similar acts in 1908, 1928 and the Nassella Tussock Act of 1946. Worth noting is Cockaynes's (1912) remark7 that 'Many settlers perform only the minimum of work necessary to satisfy the Inspector.'

In 1950 a new Noxious Weeds Act became law, and successive acts were introduced to meet the changing patterns of the agricultural industry. In 1973 Fitzharris presented a report to Parliament26 that prepared the ground for the 1978 Noxious Plants Act. His report pointed out that eradication of noxious plants is a long, expensive and difficult process that is difficult to assess.

Under the new Act, the Noxious Plants Council became responsible for gazetting noxious plants, under the guidance of its Technical Classification Committee (TCC). This committee comprised Arthur Healy from DSIR's Botany Division, a farm adviser, a representative from MAF's plant health and diagnostic station and a research specialist. Many of their decisions relied on Arthur's long experience and memory of weeds and their distribution. District Noxious Plant Committees made the case for plants to be declared noxious, and these proposals were assessed by the TCC. The emphasis of the Act and of the TCC was on the protection of agriculture from invasive weeds.

In 1982 amendments to the Noxious Plants Act made the sale and distribution of all declared species illegal. According to Clayton27, this proved to be a cost effective and largely self-regulating means of reducing risk of spread of aquatic plants.

Under the Noxious Plants Act, and until 1st April 1989, half the salaries and travelling expenses of Noxious Plant Officers employed by local governments was met by the Central Government. At this time Central Government also paid subsidies on the control of several noxious plants, including Australian sedge, gorse, broom, barberry, blackberry and sweet brier.

Several innovations were introduced by the Noxious Plants Council. One was the separation of weeds into 'widespread' and 'target'. Widespread weeds were already widely distributed, and legislation demanded that farmers keep their boundaries clear of weeds to protect weed-free neighbours. Target weeds were those whose spread could still be limited by timely action, and were therefore given a high profile by Noxious Plant Officers. Another was the idea of 'surveillance' weeds - those that needed watching in case they suddenly started to increase in numbers or distribution. The Noxious Plants Council also funded Special Projects allowing research to be carried out on species thought by weed researchers to have potential as future weeds - like horse nettle, Mexican marigold and certain aquatic species.

A mapping scheme was begun by the Noxious Plants Council, with Noxious Plant Officers maintaining records of weed distribution and control efforts. This was discontinued as a national scheme when noxious plants, or pest plants as they then became known, fell under the control of Regional Councils. However, regional councils continue to map some of their weed species, often using sophisticated technology.

Most subsidies were discontinued in 1985, Australian sedge control funding was terminated in 1986-87, and in 1989 the subsidy on operational funding of the two Nassella Tussock Boards was stopped. In July 1991, Central Government subsidies for Special Projects, the Nassella Tussock County Scheme, and the Special Nassella Tussock Ranger were all terminated.

In 1993 the Biosecurity Act replaced the Noxious Plants Act. This new legislation devolved the bulk of responsibility for weed legislation from Central to Regional Government. Regional Councils and Unitary Authorities may now develop Plant Pest Management Strategies. Four notifiable weeds remain a Central Government responsibility, but each council can develop control strategies for weeds in their region which are considered to be current or potential threats.

Initiatives from Regional Council staff, backed by advocacy from the Royal Forest and Bird Protection Society and the Department of Conservation resulted in the preparation of a list of species banned from sale and distribution (National Surveillance Pest Plant Initiative). This initiative is consistent throughout the country, although individual regions have declared some plants on this list as Plant Pests. The initiative is now being reviewed by MAF and is likely to be replaced by a National Plant Pest Accord. DoC has looked at using the National Surveillance list as a basis for their unwanted organisms list.

Of the 16 Regional Councils and Unitary Authorities, 14 had Plant Pest Strategies in place by 1998. Each strategy has to be reviewed every 5 years, and some Councils and Authorities are carrying out reviews this year (2001). Only the West Coast and the Chatham Islands do not have current strategies, although the latter recently published a proposed strategy. The West Coast, with extensive conservation property, has special problems with plant pests.

Councils employ Biosecurity Officers to maintain a watching brief on weeds and emerging weed problems, and to enforce legislation. The restructuring of legislation has meant that the number of Biosecurity Officers has dwindled to half of what it used to be.

Plants declared as Noxious Weeds in 1973 under the 1950 Act, as Noxious Plants under the 1978 Act in 1993, or as Plant Pests under the Biosecurity Act 1993 in January 2001 are shown in Appendix A.

Weed control legislation: current strategies for pest plants

DoC's Strategy11

DoC has adopted three over-riding objectives for managing invasive weeds:

  1. Preventing, as far as possible, the introduction of new and potentially dangerous species.
  2. Minimising numbers or containing the distribution of significant new invasive weeds.
  3. Protecting land, freshwater and marine sites that are important to New Zealand's national heritage from the impact of invasive weeds.

To achieve these objectives DoC has actively sought the inclusion of environment-threatening weeds in MAF's list of plant species forbidden entry into the country. It has worked with Regional Councils to develop lists of naturalised plants that could spread or increase in numbers, in order to encourage their control. It has developed site-specific programmes to control weeds on the areas it controls. It has also sponsored extensive studies and research on invasive species and on which species are likely to become invasive. The increased number of environmental species in the latest Regional Council Plant Pest Strategy reviews indicates how successful DoC has been in these areas.

Regional Council Plant Pest Strategies

The regional councils have taken on the task of administering plant pests with enthusiasm. They have continued to pursue 'target plants', with lesser efforts on widespread weeds. Regional Council staff visit most farm properties, and are active in recording the occurrence of new plant pests and in assessing their potential importance, in conjunction with technical expertise from MAF, DoC and the Crown Research Institutes. The regions have added new species to their proposed strategies.

The Regional Councils have also made progress in the objective classification of plant pests. For their first strategies, many used weed risk assessment techniques (based on the work of Esler, Liefting and Champion48 and Champion49) and their second strategies, currently being prepared, incorporate improved methods of estimating the costs and benefits of plant pests, based on the work of Harris50. The Councils have also carried out weed control trials and monitored the spread of plant pests and the success of control efforts.

All the Regional Councils have similar strategies - for total control, containment, boundary clearance, and surveillance plant pests, although the exact group names may differ. In general, the stated aim for total control of weeds is eradication, even if it takes a long time. This is highly appropriate since it gives the councils and their operatives a defined goal, although eradication is unlikely for some of the weeds listed. The aim of containment is to prevent spread to new areas and to gradually reduce the area or density of the weed. Boundary clearance involves neighbour protection and limiting the spread of weeds to clear areas. Surveillance covers a range of purposes, including gathering information on distribution and importance, watching in case the species starts to spread, and allowing for biocontrol agents to be introduced. In the case of some surveillance plants, we wonder if DoC's precautionary principle should be brought into play more - if in doubt, eradicate it.

Regional councils have developed their own systems for handling groups of plant pests. Northland and Auckland Regional Councils have special management strategies for aquatic weeds and forest invasive plants. Wellington has a similar Key Native Ecosystem Programme. Some Regions have an 'unwanted plants' category for plants thought to be eradicated or which would pose significant threats if they reached the Region. Several Regions co-operate with DoC in controlling weeds of ecosystems. Some regions have special strategies for handling particular plant pests in particular areas. Environment Bay of Plenty, for example, has a special category for nut grass and yellow nutsedge, and Canterbury has rules governing weedy gravel supply sites.

Because of the extensive use the Regional councils have made of Weed Risk Assessments in determining which weeds should be included in different categories of plant pests, their strategies are, for most plant pests, cost effective and sensible. We could quibble with the classification by some regional councils of long-established and relatively harmless annual or biennial weeds like burdock, Bathhurst bur or thorn apple, all of which tend to come and go according to seasons and land use. But these examples are few and not serious.

The plant pest species listed by all the Regional Councils in their current or proposed plant pest strategies are shown in Appendix B.

Future changes to Plant Pest Administration

Until the Biosecurity Act 1993, the emphasis of noxious plant legislation was strongly on weeds of agriculture (in the broad sense, including forestry), along with aquatic plants that affect, directly or indirectly, various aspects of agriculture. Clematis vitalba was the first non-agricultural weed to be gazetted under the Noxious Plants Act. Under the present Plant Pest legislation, the emphasis has swung towards environmental weeds, with 66 of the 166 plant pests listed in council strategies or in the National Surveillance Plant Pest list being of environmental interest only.

Buddenhagen28 questioned the lack of a national overview on surveillance weeds and plant pests. He approved the idea of a National Surveillance List, but queried its effectiveness. A plan of this kind needs, he suggested:

  • Regular updating
  • To list a larger number of threatening species
  • A consistent national approach
  • Greater impact on politicians, growers and the public
  • Centralised information gathering
  • A means of acting on aberrant species

In the same paper, Budenhagen suggested the formation of a Biosecurity Technical Advisory Group to provide a national overview. Similar concerns are expressed in MAF Biosecurity's The National Surveillance Pest Plant Initiative Discussion Paper of October 200029. MAF's concerns include:

  • inconsistencies between councils with respect to 'pest' status for plants. These may become more pronounced as pest plant strategy reviews proceed at different times.
  • incomplete national coverage. Two councils do not yet have plant pest management strategies, and others have not included the complete list of pest plants in their strategies.
  • A more robust analysis for each plant listed in a pest management strategy is needed. Amendments to section 72 and the inclusion of appeal provisions may have increased the risk of legal challenge during strategy reviews.

A National Plant Pest Accord is now being developed to replace the National Surveillance Pest Plant List. Development of this accord, between regional councils and biosecurity departments seeking to become a party to the accord, is being co-ordinated by MAF. Plants listed in the Accord are to be classified as "unwanted organisms", and their sale or distribution prohibited throughout NZ.

A national overview of pest plants, similar to that already administered by DoC, could be economic to operate and offers several advantages, including improved inter-Regional consistency and co-operation, improved weed risk assessment techniques, better tracking of potential problems, and integrated research programmes.

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