Bluemink紫花藿香薊 · Ageratum houstonianumMill.

Also known as: 紫花霍香薊、熊耳草、墨西哥藍薊、勝紅薊、紫花毛麝香、臭草仔、斷血草

It is not native to Taiwan, and here it is an invasive species.