Grow mega-tall sunflowers with American Giant Hybrid Sunflower! This fast-growing sunflower grows 8-15-ft tall with huge flower heads that can span 1-ft across! Its extremely sturdy stems make it less likely to topple over during summer storms than other giant types--a great ...Grow mega-tall sunflowers with American Giant Hybrid Sunflower! This fast-growing sunflower grows 8-15-ft tall with huge flower heads that can span 1-ft across! Its extremely sturdy stems make it less likely to topple over during summer storms than other giant types--a great choice for growing fast and reaching record heights. Win the prize for the tallest sunflower at your next county fair or let children experience the wonder and awe of growing such a magnificent flower! 70 DAYS.