Hydroponic and organic watercress tend to be less bitter than some of what's in the supermarket. I have found some batches from the same grower to vary widely in their bitterness, and do not know what causes this. That said, in this recipe we offset the flavor with a mild, sweeter vegetable like Japanese yam. If you cant find the white-fleshed yams, potatoes or parsnips will do, as all of these provide a creamy backdrop for the soup.Leeks and onion add sweetness as well, and the result is a creamy, green cleansing soup.