T.E.R:R.A.I.N - Taranaki Educational Resource: Research, Analysis and Information Network

Ludwigia peploides ssp. montevidensis (Floating primrose-willow)

Kingdom: Plantae
(unranked): Angiosperms
(unranked): Eudicots
(unranked): Rosids
Order: Myrtales
Family: Onagraceae
Genus: Ludwigia
Species: L. peploides
Binomial name: Ludwigia peploides
Subspecies: Ludwigia peploides ssp. montevidensis
Common name: Floating primrose-willow, Water primrose, Primrose Willow

Ludwigia peploides subsp. montevidensis is a low-growing aquatic herbaceous, perennial plant native to South America and the eastern states of Australia. It was introduced to New Zealand as an ornamental pond plant. It is now locally naturalised in Northland, Auckland, Waikato and Manawatu. Its habitat is the moist margins of still and slow flowing, shallow water bodies. Due to its fast growth, it smothers other vegetation and clogs waterways. It is a National Pest Plant Accord species - nationwide.

Floating primrose-willow is a <3m high, fast growing, a low-growing, aquatic herb that has sprawling, branched stems that root at the nodes. The stems spread out over the water surface with their roots hanging in the water. 
The dark green, glossy obtuse leaves (>6 m long) are alternately arranged. The bright yellow, five-petaled, 2 cm flowers which arise on stalks from the leaf axils appear during November to February.
The woody endocarp is an erect, cylindrical capsule (>3 cm long) containing numerous tiny seeds.

Primrose-willow along a bank of wetlands, sending horizontal shoots over the waterbody

Thanks to Wikipedia for text and information: https://creativecommons.org/licenses/by-sa/3.0/