Alternanthera philoxeroides

Alligator weed

Family: Amaranthaceae

Origin: South America

Alligator weed leaves and flowers.
Alligator weed can cause silt accumulation.

Regional Pest Management Plan (RPMP) status

  • Aotea — Exclusion
  • Parkland with Significant Ecological Areas — Site-led
  • Whole region — Sustained control
  • National Pest Plant Accord Species
  • Waitākere Ranges Heritage Area priority status

View more about the RPMP statuses

General description

Emergent aquatic or terrestrial perennial herb. Stems are hollow and buoyant. Leaves are opposite, < 10 x 4 cm and waxy. Flowers are white, clover-like and borne on stalks.

What you need to know

To help protect our environment:

  • You must not breed, distribute, release or sell alligator weed. As alligator weed is a National Pest Plant Accord species, these restrictions apply within the Auckland region and across the whole of New Zealand.
  • You must not plant alligator weed within the Auckland region.
  • You must destroy any alligator weed on land that you occupy if it has been planted in breach of the above rules and you are directed to do so by an authorised person.


If you see alligator weed anywhere on Aotea/Great Barrier Island group, please report it to Auckland Council at pestfree@aucklandcouncil.govt.nz.

Habitats

Still or slow-moving water bodies, pasture, cropland, gardens.

Dispersal

Vegetative spread from stem nodes and root fragments, dispersed by water movement. Human-mediated dispersal through the movement of contaminated soil and machinery, deliberate plantings.

Impact on environment

Alters aquatic habitat structure and invertebrate community composition, reduces native plant cover and diversity in wetlands and water body margins. Displaces valuable pasture species, blocks drainage channels and exacerbates flooding on farmland.

Control

Site Management

Follow up treated areas 3 times per year. Encourage natural regeneration of native plants or replant treated areas where possible after 2-3 treatments to establish dense ground cover and minimise reinvasion.

Recommended approaches

Do not attempt to undertake control of this species on Aotea/Great Barrier Island group. Please report to Auckland Council if seen on Aotea/Great Barrier Island group.

Physical control

Method: Dig out or cover terrestrial sites with black plastic or weedmat for 6 to 12 months.

Plant parts requiring disposal: Stems and roots.

Disposal options: Compost stems and roots in a composting weed bag or remove to greenwaste or landfill.

Biocontrol

Check for the presence of biocontrol agents:

  • Alligator weed flea beetle (Agasicles hygrophila)
  • Alligator weed stem borer (Arcola malloi)

For more information about how biocontrol works, see What is biocontrol?

Community agrichemical control recommendations

Basic Growsafe certified:

For small infestations on terrestrial sites: Foliar spray with 0.5g metsulfuron-methyl per 1L of water.

For small infestations on aquatic sites: Foliar spray with 20ml glyphosate green per 1 L of water.

Certified Handler/Experienced agrichemical user:

For medium to large infestations on terrestrial sites: Foliar spray with 5g metsulfuron-methyl per 10L of water and 20ml penetrant or foliar spray with 200ml glyphosate green per 10L of water.

Caution: When using any herbicide or pesticide please read the label thoroughly to ensure that all instructions and safety requirements are followed.

Kayaker in waterway full of alligator weed.
Alligator weed can be spread by water movement and boats.
Photo of dense alligator weed taken from a kayak.
Alligator weed can dramatically alter wetlands, small lakes, rivers, dams and drains.
Close up photo of leaves on a bench.
Leaves are dark green and arranged in opposite pairs along red hollow stems.
Alligator weed in bloom.
White clover-like flowers appear from December to February.
Two branches of alligator weed on a table top.
Stem sections break and root easily.
Close up photo of alligator weeds flower heads.
Alligator weed has small white papery flower heads.
Photo from above of alligator weed with white flowers.
It is tolerant of high temperatures and low water quality.