Howard Edley saw the head of the bright yellow flower continue to expand until it reached a diameter of 11 inches.
The gardening enthusiast told the Sheffield Star how he had purchased six dahlia tubers at the same time, but while five grew heads measuring three to four inches, one just kept going.
“I’ve never seen one as big in my life,” he told the publication.
The flowers first originated in parts of central and South America and the 63-year-old – who has two greenhouses as well as a range of different plants – has attempted to grow dahlias before.
“I like gardening – I’m not an expert but I enjoy the garden,” he added.
Mr Edley also admitted to having “40 or 50” different clematis plants, which TV presenter Sarah Raven recently told the Daily Telegraph are good to have all year round