艹 is the three-stroke grass cap, the compressed form of 艸. It marks 茄 as a plant, fitting for a garden vegetable. The same radical groups it with 花 flower, 菜 vegetable, 茶 tea.
加 at the bottom supplies the sound: jiā drifts to qié through a well-attested j/q palatalization, while the borrowed-word reading keeps the original jiā. Same phonetic family: 驾 drive, 架 frame, 嘉 excellent.
