butterbur, bog rhubarb
苳 refers to a specific plant: butterbur or bog rhubarb. It is a plant name with no broader abstract meaning.
苳 combines the plant radical 艹 with 冬, which likely contributes the sound. The character was created to name the butterbur plant.
The grass top 艹 sits above 冬 (winter). Picture butterbur leaves pushing up through the last winter snow: 苳 is the plant that appears at winter's end.
For トウ, imagine a butterbur leaf shaped like a toe: toe -> トウ, and the plant's broad leaf looks like a giant toe pushing through the soil.