rush plant
苕 refers to a rush plant, a type of grass-like wetland plant. Its meaning is narrow and botanical, with no clear extended senses.
苕 combines the plant radical 艹 with 召, which likely provides the sound. The exact historical development is uncertain, but the structure suggests a phono-semantic compound for a plant name.
The grass top 艹 over 召 looks like a plant being called forth from the ground: imagine a rush plant sprouting when summoned.
For チョウ, picture a long rush plant named Chou growing by the water: Chou -> チョウ, and the plant stands tall like a summoned spirit.