Japanese name: オオブタクサ（大豚草） Scientific name:Ambrosia trifida
[Overview]
An annual 1–3 m tall, growing in colonies on somewhat moist vacant lots and riverbeds.
The stem is erect and branches in the upper part, with coarse hairs.
The leaves are opposite, 20–35 cm long, and usually palmately divided into 3–5 lobes.
The lobes are lanceolate with pointed tips and uneven serrations on the margins, and both surfaces have short hairs and are rough.
The leaf stalks are long.
There are male and female flowers, both forming flower heads.
The male flower heads are yellow, 0.2–0.4 cm across, and many bloom in spike-like inflorescences 5–20 cm long at the branch tips.
The female flower heads grow several in the leaf axils below the male inflorescence.
The fruit is an obovate achene enclosed in the involucre, with one horn-like projection at the tip and 4–8 on the shoulder.
[History]
It was confirmed in 1952 at Shimizu Port in Shizuoka and in Chiba, and spread to various places.
[Notes]
It is designated as an invasive alien species requiring caution under the Invasive Alien Species Act.
It is also a plant that causes hay fever.
