type of rape plant
菘 refers to a specific type of rape plant, a leafy vegetable in the mustard family. Its meaning is narrow and botanical, with no clear extended senses.
菘 combines the grass radical 艹 with 松 (pine), likely as a phonetic component. The exact historical development is uncertain, and the modern form may be a simplified or borrowed shape.
The grass top 艹 over 松 (pine) suggests a plant that grows like a pine in miniature: a leafy green rape plant with a sturdy, upright habit.
For スウ, imagine a cool breeze (すうっと) blowing through the rape plant leaves: すう -> スウ, and the plant sways gently.
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turnip (Brassica rapa)