Definition - What does Double Flower mean?
A double flower is any blossom that has more than one layer of petals, resulting in a fuller appearance. Double flowers are usually sterile, as the reproductive elements are not easily accessible to insects or pollinators in double flowers. In other cases, the reproductive parts convert to petals.
Most cultivated rose species are double-flowered, as are camellias and carnations. Semi-double flowered plants also exist.
MaximumYield explains Double Flower
Because double-flowers usually end up being sterile, they are often propagated through plant cuttings. This is why they aren't commonly seen growing out in the wild. They are, however, highly prized and bred by horticulturists for their beauty and market value.
Double-flowers are said to be in bloom longer than regular flowers, and the reason is because they are waiting to be pollinated. In other words, with double flowers the stamens have converted to extra petals, creating a fuller, showier bloom. Without pollen, the flower stays open and in bloom for a longer period, waiting for pollen that has nothing to pollinate.
The 'discovery' of double flowers dates back around 2,000 years ago, when they were documented as a form of floral abnormality.
Not sure if a flower is a double flower? Plants labeled with the ‘flore pleno’ are considered to be ‘with a full flower’, and this is most certainly a sign the flower is a double.