Parsnips look somewhat like white carrots. They have a sweet, floral flavor. I recommend using water as the base for this soup instead of stock so that the flavor of the parsnips is at the forefront.
Like its root vegetable brother horseradish root, parsnips can last in the garden through the winter or until you get a super hard freeze. The ground serves as their own personal refrigerator.
As for the sweet potato leaves, make sure to pick them before the first frost. They keep in the fridge for a couple weeks. (If you'd rather, you could substitute spinach in this recipe.)
For a special holiday treat, finish the soup with fresh oysters. A quick note if you're planning to cook oysters: cook them until they just start to smile or crinkle around the edges. Don't bring them to a boil!