The tall yellow weed seen at this time of year along river banks and on waste ground is ragweed (Ambrosia artemisiifolia, known in Japanese as ブタクサ （豚草) lit. "pig grass."
This invasive plant is also a major source of allergy from the pollen on its yellow seeds and affects people who also suffer from allergies to the pollen from Japanese cypress (hinoki) and cryptomeria (sugi) trees.
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