Bring potatoes, 1 tsp. salt, and water to cover to a boil in a large Dutch oven over high heat; cook 20 to 25 minutes or until potatoes are tender. Drain and toss with spinach.
Stir together mayonnaise, next 5 ingredients, and remaining 1/4 tsp salt. Place potato mixture in a large mixing bowl; add celery and mayonnaise mixture, stirring gently until blended. Cover and chill at least 1 hour or until ready to serve.