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Mammoths do not normally growMammoths do not normally grow multiple blooms on one stem. Wild sunflowers do have multiple blooms, so it is possible that your mammoths are being cross-pollinating with nearby wild sunflowers.

2015 Special Edition Garden GuideCooking Fresh with The Old Farmer's AlmanacThe Almanac Monthly Digital MagazineWhat the heck is a Garden Hod?