1. In a medium bowl, whisk together buttermilk, mayonnaise, sour cream, vinegar, mustard, garlic powder and chives until well combined. Season with salt and pepper. Cover and refrigerate until ready to use.
2. Bring a large pot of salted water to boil over high heat. Add peas and boil until tender, about 2 minutes. Drain, run peas under cold water and drain again. In a medium bowl, toss together squash and zucchini.
3. In a large bowl, layer peas, 3 cups greens, red bell peppers, scallions, squash-zucchini mixture, 3 cups greens, cucumber and tomatoes. Pour dressing over top and set aside until layers are soaked through, about 5 minutes. Before serving, top with sunflower seeds.