90 - 110 days. AKA: Mammoth Sunflower, Giant Gray Stripe Sunflower. Mammoth sunflower seeds grow one of the tallest, brightest, and most breathtaking flowers in the world! Mammoth sunflower seeds grow an unbelievable 9-12' tall and world famous for their stunning 12" blooms. Mammoth sunflowers are one of the most resilient and fastest growing flowers in all of North America and mature in as quickly as 80 days. Mammoth sunflower seeds are an ideal grow for bird enthusiasts because the countless seeds per head will keep birds well-fed and coming back for more all season long.