an edible water, plant-brasenia
蒓 refers to a specific edible aquatic plant, Brasenia schreberi, also known as watershield. Its meaning is narrow and botanical, without broader abstract extensions.
蒓 is a phono-semantic compound: the grass radical 艹 indicates a plant, while 純 (シュン) provides the sound. The character was created specifically for this plant name.
The top grass radical 艹 marks it as a plant, and the bottom 純 (pure) suggests the pure, clear water where this edible plant grows.
For シュン, imagine a pure (純) shoot of the watershield plant emerging soon (soon -> シュン) in a clear pond.