芥 has two readings: jiè (mustard) and gài (Chinese kale, 芥蓝). The idiom 心存芥蒂 (xīn cún jiède) means to harbor a grievance — 芥蒂 literally refers to a tiny mustard seed stuck in the chest.
艹 is the grass radical 艸 in compressed top form. It tags 芥 as a plant — specifically mustard greens and the tiny mustard seed — and groups it with 菜 (vegetable), 草 (grass), and 花 (flower).
介 supplies the reading jiè directly with no shift. The same phonetic appears in 界 (boundary) and 价 (price), all clustered around the jie syllable. Its faint sense of something small pressed between also fits a tiny mustard seed.
