The cornflower, Centaurea cyanus, is a well-known annual plant. Its distinctive vibrant blue petals have given rise to the widely recognized shade known as “cornflower blue.”
The Distinctive Blue and Its Variants
The characteristic “cornflower blue” is a specific shade, often described as a medium-to-light blue with minimal green undertones. This hue stands out in the botanical world as one of the few flowers that naturally exhibit a true blue color. The striking beauty of this blue has led to its recognition in other contexts, such as describing highly valued sapphire gemstones.
While the vibrant blue is the cornflower’s most celebrated attribute, cultivated varieties display a broader spectrum of colors. Through selective breeding, growers have developed cornflowers that bloom in shades of pink, white, purple, red-purple, deep burgundy, and occasionally even yellow or bicolor combinations. This process, also known as artificial selection, involves humans intentionally breeding plants with desirable traits, like petal color, to propagate those characteristics. This practice has been employed for thousands of years to develop diverse flower colors.
Pigments: The Chemistry of Cornflower Colors
The remarkable color of the cornflower stems from complex biochemical interactions within its petals, primarily involving a group of pigments called anthocyanins. These compounds are widely responsible for red, purple, and blue coloration in many plants. Interestingly, the blue cornflower contains the same anthocyanin, known as cyanin, that is also found in the red rose, a fact that puzzled scientists for nearly a century.
The solution to this long-standing puzzle lies in the cornflower’s unique molecular structure, specifically a supramolecular pigment called protocyanin. This complex is composed of six molecules of anthocyanin (cyanidin) and six molecules of flavone glycoside (apigenin), along with four metal ions: one ferric iron (Fe3+), one magnesium (Mg2+), and two calcium (Ca2+) ions. The precise arrangement and interaction of these four metal ions within the complex are what ultimately give the cornflower its distinct bright blue hue.
Beyond the core components, co-pigmentation also contributes to the cornflower’s blue color. This phenomenon involves the interaction of anthocyanins with other colorless or slightly yellow molecules, such as flavones, which stabilize the pigment and can shift its absorption spectrum towards bluer wavelengths. Variations or the absence of these specific pigments and their complex interactions lead to the other colors seen in cornflower cultivars, such as white, pink, or red.