What does it look like?
Alligator weed is a perennial aquatic and terrestrial plant. It is typically bottom-rooted at the margins of waterways and can form floating mats which spread across the water surface. It can also be found deep-rooted in wet soils.
The stems are hollow and buoyant; leaves are green, hairless and waxy, with a conspicuous midrib (4 x 10cm long) and arranged opposite on stems. The papery flowers resemble those of white clover but are smaller and each cluster is produced on a long stalk. Plants grow up to one metre tall around the edges of water.
Alligator weed is found in still or slow-moving water bodies, including lakes, streams and drainage channels (also tolerating brackish water). Terrestrial habitats including pasture, cropland and gardens.
Why is it a problem?
Alligator weed is very hardy and is a strong competitor, tolerating a wide range of environmental conditions and disturbance regimes, including flooding and submergence. It prefers full sun but is more shade tolerant than typical pasture grasses. It can reduce farmland and crops, causes sediment build-up and flooding, threatens native plants by taking up nutrients space and light.
Alligator weed doesn’t set viable seed in New Zealand, it reproduces vegetatively from stem nodes and root fragments which are easily spread. This means we have a good opportunity to prevent its spread, by ensuring boots/equipment/machinery are cleaned when leaving sites where this weed is present.
Control methods
If you think you've found alligator weed, please get in touch with our Biosecurity Team at biosecurity@hbrc.govt.nz or call us on 0800 108 838.
More information
Alligator weed is designated an Exclusion Pest under the Hawke’s Bay Regional Council Regional Pest Management Plan 2018-2038.
In narrow waterways you can prevent the return of both land and aquatic weeds with riparian/streamside planting to increase the shade levels. This has a negative impact on the weeds as they often require high light to thrive. It also improves water quality, as shade is an important environmental factor for native freshwater fish & invertebrates and a healthy ecosystem.
Similar species:
Alternanthera denticulata (lesser joy weed) is known to be found in the North Island, mainly in northern areas.
Alternanthera nahui (nahui) is known throughout the North Island and in the Canterbury region, and is indigenous to New Zealand.
Alternanthera sessilis (mukunuwenna, or, sessile joyweed) is not known to be present in New Zealand. This species is commonly grown in Sri Lanka as a vegetable and has many uses. In New Zealand, alligator weed is often mistaken for mukunuwenna, so it is very important to be certain of what you are growing as alligator weed can be toxic and is an Unwanted Organism.
Related links
Notify Council
If you think you've found this pest, please get in touch with our Biosecurity Team at biosecurity@hbrc.govt.nz or call us on 0800 108 838.
Management Programme
Exclusion Programme
To prevent the establishment of the subject, or an organism being spread by the subject, that is present in New Zealand but not yet established in an area.
National Pest Plant Accord (NPPA)
The NPPA is designed to prevent the sale, distribution and propagation of a set list of pest plants (the Accord list) within New Zealand. If allowed to spread further, these pest plants could seriously damage the New Zealand economy and environment.
More informationUnwanted Organism
An unwanted organism is any organism that's capable of causing harm to natural or physical resources (like forests and waterways) or human health. A number of introduced pests in New Zealand are classed as unwanted.
Rules
Sections 52 and 53
Sections 52 and 53 of the Biosecurity Act 1993, which prevent the communication, release, spread, sale and propagation of pests, must be complied with.
Taxonomies
FamilyAmaranthraceae
TypePlants
GroupGroundcover
HabitatFreshwaterLand
Management ProgrammeExclusion ProgrammeNational Pest Plant Accord (NPPA)Unwanted Organism
RulesSections 52 and 53
