Alligator weed (Alternanthera philoxeroides) is one of the world’s worst weeds. Native to South America, alligator weed was accidentally introduced to New Zealand in the 1880s from ballast water discarded from ships. Since then it has spread through much of Northland and is now found in several parts of the Waikato region.
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Why it is a pest plant
Alligator weed (Alternanthera philoxeroides) is fast growing and difficult to eradicate. It can grow on land and in waterways, and can tolerate certain amounts of seawater when growing in flowing water.
In the Waikato region, alligator weed is found in the lower Waikato River, on several farms, and in market gardens and urban properties.
Because it can grow in water, alligator weed also threatens wetlands, lakes, rivers, dams, drains and other waterways by clogging them, increasing sedimentation and the risk of flooding. Access to waterways for recreational purposes (boating, fishing) can be blocked and plants may affect whitebait spawning areas.
Alligator weed does not set seed in New Zealand but spreads aggressively from even the smallest stem fragments. It can double in area in less than two months.
It can out-compete pastures and crops, affecting farm production and profit. In urban areas it is very persistent and can completely dominate lawns. Although stock will eat it, alligator weed is actually toxic and can cause blindness (and possibly other health problems).
Several new sites of alligator weed were discovered in the Waikato region during 2004 and 2005. Most of these sites are terrestrial and have occurred through soil or other aggregate movement, or vehicle or contracting equipment. Alligator weed has become a significant problem in new subdivisions in Hamilton city.
How to recognise this pest plant
Alligator weed is a low-growing, non-woody perennial. Its leaves are generally arranged in opposite pairs or whorls, spaced at intervals along hollow horizontal stems. Stem and leaf sizes vary greatly. They can be very compact in lawns or grazed pasture, or much larger when growing in water.
Identifying features
- Leaves grow in opposite pairs or whorls along a stem.
- Long horizontal stems (stolons), up to 10 metres long.
- Horizontal stems that can float on the surface of water, forming rafts, or grow onto banks, forming closely matted clumps.
- Clover-like, papery flowers, from December to March.
- No seeds are produced.
Responsibility for control
Environment Waikato is responsible for controlling alligator weed. Our Regional Pest Management Strategy declares alligator weed to be a total control plant pest for the whole Waikato region.
We aim to:
- eradicate alligator weed from all sites where it is currently found
- ensure all other high risk areas are kept free of alligator weed
- increase people’s awareness of alligator weed and the problems it causes.
Environment Waikato carries out direct control and monitors all known alligator weed sites.
How to control alligator weed
Do not attempt to control alligator weed yourself.
If you see this weed on your property, do not cut or treat it. Call 0800 BIOSECURITY (0800 246 732) to report it to your nearest biosecurity pest plant officer.
When disturbed, alligator weed breaks up easily into small fragments which can easily re-grow. Take special care not to disturb it or transport it to new sites. It can spread by water movement such as floods or tides, from soil movement, and equipment such as diggers, farm machinery, eel nets and boats. Land owners/occupiers should check drains regularly for alligator weed.
Alligator weed is banned from sale and distribution throughout New Zealand under the National Pest Plant Accord. This means it is an offence to knowingly sell or distribute alligator weed infested animal feeds (such as silage) or aggregates (such as sand or soil). Subdivision and land development properties at risk of spreading alligator weed, as advised by Environment Waikato, will be required to adhere to soil and vegetation movement restrictions as outlined in the RPMS.
What you can do to stop alligator weed spreading
Practice good weed hygiene by making sure you don't transport alligator weed fragments on your vehicle or equipment.
To prevent the spread of alligator weed and other aquatic pests such as didymo, boaties should clean their boats and trailers when leaving the waterway.
Farmers should protect their properties from alligator weed and other serious plant pests by:
- insisting all contractors practice good weed hygiene, cleaning their equipment before entering the farm
- ensuring supplementary feed brought onto the farm is weed free
- ensuring aggregates, soil and sand brought onto the farm is weed free.
More information
For further advice contact your nearest biosecurity pest plant contractor on 0800 BIOSECURITY (0800 246 732).
For more information on what we do phone Environment Waikato's freephone 0800 800 401.
For more information on pest plants check out the Weedbusters website.
Related publications
Read our Waikato Regional Pest Management Strategy.
Take a look at the Environment Waikato technical report Alligator Weed Control 2004/2005.
'What makes a pest a pest? - A guide to Waikato's pest management future' - download (730 kb, 104 seconds to download, 56k modem), order or pick up for free from our offices.
Pick up or order 'Plant me instead - Plants to use in place of common pest plants' for free from our offices.
Pick up or order the 'National Pest Plant Accord Manual' for $10 from our offices, or view it online for free.
Pick up or order 'Poisonous plants and fungi in New Zealand - A guide for parents, schools and child minders' for $15 from our offices.
Disclaimer
Althought this content has been prepared in good faith from a number of sources believed to be reliable, Environment Waikato does not give any guarantee that all information contained is accurate or complete, or that advice given will be appropriate in all circumstances.