coarse, crude, plant
楛 centers on something rough and unrefined: coarse or crude in quality. The plant sense likely refers to a specific plant known for its coarse texture or crude appearance.
楛 combines 木 (tree/wood) with 苦, which likely contributes the sound コ and the idea of coarseness or bitterness, suggesting a plant with rough or crude qualities.
A tree 木 that is bitter and rough like 苦: its wood is coarse and crude, not smooth or refined.
For コ, imagine a coarse, crude piece of wood that you would call a "co"-arse log: co -> コ, and the rough tree 木 fits the meaning.
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