reed
蒋 refers to a type of reed, specifically the wild rice plant (Zizania latifolia) known as まこも in Japanese.
蒋 is a phono-semantic compound: the grass radical 艹 indicates a plant, while 将 likely contributes the sound. The exact historical development is uncertain.
The top grass radical 艹 shows it's a plant, and the bottom 将 looks like a general standing tall. Picture a general among the reeds, towering over the まこも grass.
For ショウ, imagine a show of reeds swaying in the wind: show -> ショウ, and the general stands among them.