50 American Giant Seeds American Giant Hybrid Sunflower was developed especially to respond to your best gardening efforts. American Giant Hybrid Sunflower grows tall and stays standing long after the others fall. In fact, you may have to literally get out a saw and cut it down at the end of the year. It grow as tall as 17 feet! Now, that is a large sunflower. It is a huge (and we mean, huge) improvement on the old garden varieties that you nursed along in the garden only to see them fall to the ground with the first big wind after flowering. American Giant Hybrid grows tall and stays standing long after the others fall.