Kale, like the Red Russian variety pictured here, is most tender and flavorful in cooler months and has much better flavor after a frost. Look for firm, fresh leaves; pass on those that are flaccid or yellowy. Whether you steam, braise, or blanch and then sauté kale, know that it needs thorough cooking: Unlike delicate greens that are ready to eat when heat sets in, kale will be unpleasantly chewy if only barely cooked. Red Russian and Black kale (also called Lacinato or Tuscan kale) are more tender and need less cooking time, while the frillier blue-green varieties are hardier and take longer. Even when fully cooked, kale will be chewy, but pleasantly so. Its flavor and texture benefit from sautéing a few aromatics in the pan first; I especially like Braised Kale with Pancetta.
Watch a video from our Test Kitchen to get good, quick tips for trimming kale.