Roasted Rainbow Radishes are a burst of sunlight on a rainy day! Purple daikon and watermelon radishes pair up in this beautiful roasted radish dish. A drizzle of olive oil, and a sprinkle of salt and pepper is all it takes to make this rainbow appear. Purple daikon and watermelon radishes are part of the winter radish family; winter radishes are sowed in late summer or early fall and take 8-10 weeks to mature and are ready to harvest before winter sets in, making them perfect for winter storage.