Alligator weed
Amaranthraceae - Alternanthera philoxeroides

What does it look like?

Alligator weed is a perennial amphibious (aquatic and terrestrial) plant.  It is typically bottom-rooted at the margins with floating mats which spread across the water surface. It can also be deep-rooted in wet soils. 

The stems are hollow and buoyant; leaves are green, hairless, waxy, with a conspicuous midrib (4 x 10cm long).  The flowers resemble those of white clover but are smaller and each cluster is produced on a long stalk.   

Alligator weed prefers temperate climates and 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/submergence.  It prefers full sun but is more shade tolerant than typical pasture grasses, therefore it may be advantaged by slight shading. 

Alligator weed doesn’t set viable seed in New Zealand, it reproduces vegetatively from stem nodes and root fragments with are easily spread. This means we have a good opportunity to prevent its spread, by ensuring boots/equipment/machinery is clean between sites where this weed is present.

Control Methods

Before starting, thoroughly check the waterway's adjacent areas, including any inflows/outflows. If it has spread further than the area you are able to control, bear in mind that without management of the full extent of the infestation, reinvasion will always be an issue and will require continued management.

As most aquatic weeds grow from fragments, start control at the upstream end of the infestation. 

Physical control on land:

Remove by digging and raking up the infestation. Dispose of them on dry land, away from any water, so they will dry out and die. Follow up regularly to remove and regrowth.

Herbicide control on land:
DO NOT USE THESE CHEMICALS OVER WATER

  • Spray (spring to autumn) with glyphosate 20ml + 2ml penetrant per 1L water.
    OR
  • Spray (spring to autumn) with metsulfuron-methyl 5g + 20ml penetrant per 10L water.

Physical control in water:

  • Remove by digging and raking up the infestation. Dispose of them on dry land, away from any water, so they will dry out and die. Follow up regularly to remove and regrowth.
    OR
  • Bottom lining: if possible, lower the water level and cover the infestation with black polythene or a weed mat (weighted down) for about three months.

 Herbicide control in water:

  • Spray (spring to autumn) with glyphosate 20ml + 2ml penetrant per 10L water.

CAUTION: when using any herbicide or pesticide PLEASE READ THE LABEL THOROUGHLY to ensure that all instructions and safety requirements are followed

Biological control:

Bio-control agents such as alligator weed beetle are among the most effective control options over water. These beetles are widespread throughout Northland.

Grass carp (herbivorous fish) can also be used as a weed control agent in some cases. Contact the Department of Conservation for more information.

More Information

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.

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