Alligator weed
Amaranthraceae - Alternanthera philoxeroides
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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, or can 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, and is resistant to flooding/submergence. It prefers full sun but is more shade tolerant than typical pasture grasses, therefore 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. 

Control Methods

Freshwater weeds are difficult to eradicate once established, but are possible to control. Before you start thooughly check the waterway's adjacent areas and inflows to see if the infestation has spread.

As most aquatic weeds grow from fragments, start control at the upstream end of the infestation. In narrow waterways you can reduce the growth of aquatic weeds with riparian planting to reduce light levels.

Small infestations

  • Physical control: Remove the weeds by digging and rakig them up. Dispose of them on land so they dry up and die. Follow up regularly to remove growth.
  • Bottom lining (small ponds): If possible, lower the water level and cover the infestation with black polythene or weedmat (weighted down) for about three months.

Large infestations

  • Herbicide: These are available for some freshwater weeds, and can be an effective control option. Check with your local council's biosecurity team before spraying, as spraying in or over water may require resource consent. CAUTION: When using herbicide, PLEASE READ THE LABEL THOROUGHLY and follow ALL instructions and safety requierments.
  • Biological control: Grass carp can be used as a weed controll agent in some cases. This requires a permit - contact the Department of Conservation for more information.

Aquatic sites

  • Physical removal such as raking and digging - among the most effective control options.
    • ensure the plant material is not left in contact with the ground or it will re-root,
  • Bio-control agents such as alligator weed beetle - among the most effective control options over water, wide spread throughout Northland,
  • Herbicides - all recommendations require consent to use over water contact the consents team at your local regional council,

Land sites (do not use these chemicals over water)

  • Herbicides
    • Metsulfuron-methyl: Spray 5g metsulfuron-methyl (500g/kg e.g. Escort®) + 10ml penetrant per 10 litres water at least three times per growing season for up to five years,
    • Kamba 500 (500g/L Dicamba) 4ml/L + 10ml penetrant per 10 litres water at least three times per growing season for up to five years,

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