Alligator Weed

Also known as:
Aligator Weed
Family:
Amaranthaceae
Origin:
South America

General description

Hairless perennial, roots in <2m water, forms floating mats extending out from banks covering surface of deeper water; also grows on land. White clover-like flowers (summer). Waxy green leaves in opposite pairs with midrib. Green-brown stems often red tinged; root at nodes.

Waterways from 0–30% salinity (eg estuaries, dune lakes, wetlands), pastures & urban areas. Grows best in fertile conditions with warm growing season. Cold winters tolerated but is frost tender.

Stem fragments lodge in stream banks, and establish new colonies. Easily spread by cultivation, boats and nets.

Restricts water flow, increases sedimentation, causes flooding. Impedes access to water. Invades pasture and cultivated land. Can cause photosensitivity in stock that eat it.

Site management

Spraying over water may require resource consent. Requires frequent follow up. Exclude livestock, maintain riparian plantings to lower nutrient runoff and lower light level. Lower water level wherever possible to use terrestrial spray methods.

Recommended approaches

1. Ensure presence of Agasicles hygrophila & Arcola malloi biocontrol agents; very slow acting, only effective late in season, where plants grow over water.
2. Dig out small patches removing all stem fragments; check for regrowth.
3. Cover with weedmat for 6-12 months; check edges for creeping stems.
4. Weed wipe (land) (5g metsulfuron + 1ml penetrant/1L).
5. Spray terrestrial sites spring -autumn (5g metsulfuron + 10ml penetrant/10L). Take extreme care around waterways.
6. Spra

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

Similar species

Senegal tea
Gymnocoronis spilanthoides

Senegal tea has serrated and wavy leaf margins. Its stems are solid as opposed to the hollow stems of alligator weed. Its flowers are clustered rather than singular.

water primrose
Ludwigia peploides ssp. montevidensis

Water primrose has alternate leaves. On its stem there are two swollen stipules at base of leaves. It has bright yellow flowers.

RPMS status

Surveillance - Whole Region
National Pest Plant Accord species - nationwide
alligator weed - Main species image