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The first question has to be:The first question has to be: are the sunflowers in the same spot they were last year? If so, that's the source of your problem. Like edible plants, sunflowers benefit from crop rotation. This results in fewer bugs and diseases, and more nutritious soil.

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