90-110 Days to maturity. Helianthus annuus. Organic 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 flowers in all of North America. Approximately 375 Seeds / Ounce.