Alligator weed

Alligator weed ( Alternanthera philoxeroides) is a hollow-stemmed super-weed with small clover-like flowers and lush, oval leaves. Luckily it doesn’t spread by seed in New Zealand, but every small fragment can form a new plant.

It was accidentally introduced near Dargaville in ship’s ballast in the 1890s and has since become a major problem.

Location: Northland, Auckland, Bay of Plenty, Waikato, including residential areas in Hamilton

Distinctive features: grows in both water and in pasture. Clover-like flowers appear in clusters from December to February. Floats on water in dense mats.

Where will you see it: freshwater, areas where freshwater and seawater meet, estuaries, damp habitats, dune lakes, hollows, wetlands, can also grow in dry pastures, crops and urban areas.

Why is it weedy: can grow in freshwater (with roots from 2-10m long) and on land; tolerant to high temperatures, high pollution levels and grazing; rapidly forms into dense mats, with sections breaking off and floating away to infest new areas.

How does it spread: can spread only via broken stem sections, but unfortunately the stems break easily and are moved about by water, contaminated diggers and farm machinery, fishing nets, boats and trailers.

Damage: is toxic to mammals, impedes drainage and contributes to flooding, displaces native aquatic and marginal plants, stagnates water and kills fish.