[Kotem (Armenia), B. Nasturtium nasturtium-aquaticum and N. microphyllum]
Watercress, native to Europe and Western Asia, has been cultured and eaten since prehistoric times. Its relatively mild mustardy bite is a welcome addition to green salads and in sandwiches, but it's also used cooked in soups and other recipes. Depending on how it was grown and cut some of the stems can be quite large. All but the largest can be used along with the leaves as they are hollow, tender and have much the same flavor as the leaves.
Watercress has historically been an important green in Ireland and is still fairly widely used there, but Asian immigrants have greatly increased demand in North America.
More on Mustard, Cabbage and Turnip Greens
Buying: Look for bunches that are crisp, with moderate size stems and particularly watch out for yellowing leaves. Here in Los Angeles watercress is sold in banded bunches typically 7 inches long and weighing between 4 and 5-1/2 ounces. Thickness of the thicker stems varies widely. In the Asian markets the bunches tend to be larger and the price lower than in other markets, due to volume.
Storing: A slightly wilted bunch can be revived by cutting off the bottom 1/4 inch of the stems and standing in a cup of cold water in a cool place for 20 minutes or so. Wrap loosely in plastic and store in the refrigerator. Watercress is very perishable and even a good bunch will be yellowing in two or three days.
Cooking: Watercress is very often used raw, but if not, cooking should be minimal. Watercress is typically added to soups and the like in the last minute or two. Watercress is always cooked in Chinese recipes because they don't use raw vegetables there - for reason.