In a 10-12 quart (10-12 L) heavy kettle bring 4 quarts (4 L) water to a boil and add carrot tops and assorted greens. Simmer greens, stirring occasionally, until just tender, about 15 minutes. Transfer greens with tongs to a large shallow baking dish, letting excess cooking liquid drip into kettle and reserving all cooking liquid. Cool greens until they can be handled and chop coarse.
Stir beans and remaining 3 quarts (3 L) water into reserved cooking liquid and simmer beans, covered partially, until tender, about 1 hour (older beans may take longer to cook). Add roux mixture by large spoonfuls, stirring well after each addition, and stir in greens, garlic, and salt and pepper to taste. Simmer gumbo, partially covered, stirring occasionally, 30 minutes.
Gumbo may be made 2 days ahead, cooled completely, uncovered, and chilled, covered.
Serve gumbo ladled over rice in large soup plates.
Makes 24 cups, serving 10 to 12 as a main course.
posted by Donna Schakelaar