Combine watercress, mustard greens, and edible flowers.
Remove lengthwise strips of peel from cucumber. Cut cucumbers into 2-1/2-inch lengths. Using a peeler-corer, remove core leaving a 1/4-inch shell. Tuck some of the greens mixture into cored end of each of the cucumber shells.
Stand filled cucumbers, greens end up, on a serving platter. Arrange remaining greens mixture on platter, if desired, or place remaining greens mixture in a serving bowl.
In a screw-top jar combine vinegar, oil, salt, and pepper. Cover; shake well. Drizzle some of the dressing over the cucumber bouquets. Pass remainder with cucumbers and greens. Makes 6 to 8 servings.