Ludwigia decurrens (willow primrose, "Hiretagobo")

Japanese name: ヒレタゴボウ （鰭田牛蒡）
Scientific name: Ludwigia decurrens

[Overview]
An annual 80–100 cm tall, found in wetlands along rivers, at the edges of ponds and marshes, and in rice paddies.
The stems and leaves are often tinged with purple.
The stem is erect, has 3–4 ridges, and branches well.
Leaves are simple and alternate, lanceolate to narrowly elliptic, 5–12 cm long and 1.5–3 cm wide.
The leaves are almost stalkless, and the leaf base joins the ridges of the stem to form wings (fins).
Flowers are solitary in the leaf axils and are yellow, 4-petaled, about 3 cm across.
The flowers open horizontally and fall easily.
The fruit is a capsule about 1–2 cm long, with 4 ridges, forming a square column, with the 4 sepals remaining at the top.

[History]
It was collected in Tokushima in 1954 and in Matsuyama in 1955, and has since spread as a naturalized plant.

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