90-110 Days to maturity. Helianthus annuus. Mammoth Grey Stripe Sunflower Seeds: Non-GMO. Open Pollinated. Ornamental. Annual. Mammoth sunflower seeds grow one of the tallest, brightest, and most breathtaking flowers in the world! Mammoth sunflower seeds grow an unbelievable 9-12 feet tall and are famous for their stunning 12 inch blooms. Mammoth sunflowers are one of the most resilient and fastest-growing sunflower varieties in the world. Mammoth sunflower seeds are an ideal crop for bird enthusiasts because the countless seeds per head will keep birds well-fed and returning for more all season long. Approximately 375 Seeds / Ounce.