In a medium saucepan, combine scallions, garlic, ginger, and 3 cups water. Bring to a boil and cook, partially covered, for 10 minutes. Add chicken broth and mushrooms, cook until the mushrooms begin to soften, about 3 minutes.
Add dumplings and baby corn; boil until dumplings are tender, about 4 minutes. Add watercress, stir until wilted, about 30 seconds. Spoon into bowls, and serve immediately.