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.
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.
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.