bullrush, zizania latifolia
苽 centers on a specific aquatic plant: bullrush or zizania latifolia, a type of wild rice. The meanings are direct plant names without a broader abstract core.
苽 combines the grass radical 艹 with 瓜 (melon/gourd), likely as a phono-semantic compound where 瓜 hints at the sound and 艹 indicates a plant. The exact historical development is uncertain.
The grass radical 艹 sits above 瓜, like a melon-shaped plant growing in water. Picture a bullrush with a gourd-like seed head rising from the grass.
For コ, imagine a small child (子, こ) playing among the tall bullrushes by the water: 子 -> コ, and the child hides in the zizania.