One of the largest sunflower varieties available, Mammoth Gray Stripe Sunflower produces flowers up to 14-in. across and huge yields of edible seeds. The high oil content in the seeds makes for a great roasted flavor. An heirloom sunflower variety, Mammoth Gray Stripe Sunflower is an open-pollinated, annual sunflower that grows 9-12 ft. tall. It's a favorite for planting in fields, along fences and near buildings. The sunny yellow flowers attract pollinators. Both birds and children love this giant sunflower. 100 DAYS.