type of reed
蒹 refers to a specific type of reed, a tall grass-like plant growing in wet areas.
蒹 combines the plant radical 艹 with 兼, which likely provides the sound. The exact historical development is uncertain, but the structure points to a plant name.
The top 艹 shows it's a plant, and the bottom 兼 suggests a double or combined form, like a reed that grows in dense, combined clusters.
For ケン, imagine Ken, a botanist, carefully identifying this reed: Ken -> ケン, and he knows exactly which type of reed it is.