Ludwigia peploides subsp. stipulacea ("Mizukinbai")

Japanese name: ミズキンバイ （水金梅）
Scientific name: Ludwigia peploides subsp. stipulacea

[Overview]
An emergent perennial found in swamps and similar places.
The rhizome creeps for a long distance underground.
The stem is hairless, and no respiratory roots form at the nodes in the water.
Leaves are simple and alternate, oblanceolate to oblong-lanceolate, and hairless.
The leaf margins are entire.
Flowers are borne one or two at a time in the leaf axils; they are yellow, 4–6-petaled, 2–3 cm across.
The fruit is a cylindrical capsule.

[Notes]
The Japanese name comes from its growing in water and from likening its flower to that of Kinbaiso.

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