EPPO Global Database

EPPO Reporting Service no. 09 - 2008 Num. article: 2008/188

Entry pathways of aquatic weeds in New Zealand

A large number of freshwater aquatic plants have already been introduced into New Zealand. Some of these are now naturalized and cause detrimental impacts. However, there are many more potential weed species reported as present in New Zealand, but not naturalized, and an even greater number of species which if introduced could be a threat to aquatic ecosystems, and the risk they represent needs to be assessed. Known pathways of entry are described below, based on historical accounts and on common knowledge.

Natural spread
Leaving aside human activities, wind-blown seed and migratory birds represent the two most common or likely pathways of entry. These pathways are clearly observed between Australia and New Zealand. Among the Australian aquatic species which have not yet been recorded in New Zealand, there are still a few species that have the potential to be introduced. For example, additional Typha species (Typhaceae) may have the potential to enter as wind-blown seed. Migratory birds may feed on the seed capsules of aquatic plants such as Potamogeton spp. (Potamogetonaceae) and Myriophyllum spp. (Haloragaceae) or may transport seeds in mud on their legs and feet and spread the plants. Gratiola pedunculata (Scrophulariaceae) is a recent example of natural introduction.

Ship ballast
Zizania latifolia (Poaceae), Alternanthera philoxeroides (Amaranthaceae) and Schoenoplectus californicus (Cyperaceae) are suspected to have been introduced through ship ballast. Ballast shipping is now more carefully regulated in New Zealand, and the current risk of new weed species arriving in ballast appears minimal.

Forage plants
Glyceria maxima (Poaceae) and Paspalum distichum (Poaceae) were introduced as cattle fodder in wet areas, and have proven problematic. Stringent checks for weediness in new imports of forage species should prevent further imports of new weeds by this means.

Industrial purposes
Phragmites australis (Poaceae) was used for the treatment of wastewater. Industrial use of aquatic plants should be considered a potential risk for the entry of new species and any application for import for this purpose should be considered carefully.

Acclimatisation societies
Elodea canadensis (Hydrocharitaceae) was the first aquatic weed to be introduced into New Zealand. The further risk from new imports by acclimatisation societies is unlikely.

Pre-legislative ‘colonisation’
To create a mirror image of Europe, numerous plants were brought into colonial New Zealand. Flowers included lilies, irises and other similar plants which were introduced with contaminants such as Hydrilla verticillata (Hydrocharitaceae), or Chara foetida (Characeae).

Research purposes
Hydrodictyon reticulatum (Hydrodictyaceae) and Marsilea hirsuta (Marsileaceae) have been introduced for teaching purposes. There is a risk that specimens may contain viable seeds that could escape, or that inappropriate practices are implemented with respect to the culture, containment and subsequent disposal of material. Despite the potential scientific value or justification for importing foreign specimens, research personnel are recognized by Border Control Authorities as a high-risk profession for facilitating the entry of unwanted organisms.

Culinary and medicinal purposes
Rorippa officinale (Brassicaceae) was introduced for culinary purposes by the French in 1840, and became a major weed problem within a few years. Eutrema wasabi (Brassicaceae), Ipomoea aquatica (Convolvulaceae) and Eleocharis dulcis (Cyperaceae) were also introduced for culinary purposes. Alternanthera philoxeroides (Amaranthaceae) has recently been found cultivated as a culinary crop by some members of the Sri Lankan and Somalian communities, mistaking the species for the traditional vegetable A. sessilis.

Approved importation
Legislation will now require any organism or new genetic variety not already present in New Zealand to undergo a risk assessment to determine its safety and suitability for entry.

Incorrectly identified import
Examples of incorrectly identified plants of the aquarium and pond plant trade in New Zealand include: the now declared pest plant Gymnocoronis spilanthoides (Asteraceae) which was distributed as Hygrophila costata (Acanthaceae), and Hydrocotyle leucocephala (Apiaceae), which is still sold as Cardamine lyrata (Brassicaceae).

Contaminants with legal imports
The greatest risk of unwanted contaminants would be from pond plants grown in outdoor facilities or collected from natural water bodies. Seeds or rhizomes of Nymphoides peltata (Menyanthaceae) are thought to have arrived as contaminants on imported water lily rhizomes.

Contaminated products
Used drainage machinery from overseas and packaging can be contaminated with unwanted seeds, but the risk of aquatic species being transported by this means is low.

Mail order plants
Aquatic plants are known to be dispatched around the world by mail order, and a lot of species are likely to be traded without approval. This applies to the research, business and private sectors. It includes incorporation of seeds into private letters. The Internet provides another venue for accessing mail order companies from around the world.

Pocket plants
‘Accidental’ (deliberate or genuine) contamination of baggage (e.g. sports equipment used in water) may occur, but it is not possible to prove the intent and to obtain reliable information on the extent of the practice.

Based on enquiries to date, there are only thought to be 2 importers in New Zealand of live aquatic plants and these import plants from Denmark, Singapore and South America. This is due to the small size of the aquatic plant market in New Zealand and the costs of importing plants through legal channels. Additionally, there would appear to be a concern from persons importing plants that the industry must be self-regulating to prevent problematic plants becoming established within the country, as well as to help avoid further prohibitive restrictions being imposed by regulatory authorities. In this regard, the likelihood of new species entering the country by responsible traders would appear limited. On the other hand, prohibitions are reported to encourage the use of illegal entry pathways. The greatest risk of new species entering New Zealand would appear to be through “pocket plants” for aquatic plants.


Champion PD, Clayton JS (2000) Border control for potential aquatic weeds. Stage 1. Weed risk model. Science for conservation 141. Department of conservation, New Zealand. 48 p.