In a large bowl layer cauliflower, broccoli, green onion, radish, and carrot; set aside.
For dressing, in a medium bowl combine mayonnaise, sugar, lemon juice, horseradish, salt, and pepper; spread over vegetables. Sprinkle with bacon. Cover and chill in the refrigerator for at least 4 hours or up to 24 hours. Stir to coat before serving. Makes 8 to 10 side-dish servings.