sow thistle; milk thistle
Refers to the plant Sonchus oleraceus, a common weed with yellow flowers. The name literally means 'spring field mustard'.
See also: ノゲシ
Sow thistles were blooming by the roadside.
野芥子 is a more general term for sow thistle, while 春の野芥子 specifically refers to the spring-blooming species Sonchus oleraceus.
Kanji spelling; the word is usually written in kana, so this form is less common in practice.
Katakana spelling; sometimes used for plant names or emphasis.
Compound of 春 (spring) + の (possessive particle) + 野芥子 (sow thistle). The name distinguishes this spring-blooming species from other sow thistles.