hemp, flax
苧 centers on fibrous plants used for making thread and cloth: hemp and flax. The character refers to the plant itself and the material derived from it.
苧 combines the plant radical 艹 with 宁, which likely contributes a phonetic element. The character specifically denotes hemp or flax plants used for fiber.
The grass radical 艹 sits above 宁, like a plant growing tall and straight. Picture a field of hemp plants with their fibrous stalks reaching upward.
For チョ, imagine a hemp stalk being chopped: the sharp 'cho' sound of the blade cues チョ as the plant is cut for fiber.