The Aster flower, belonging to the genus Symphyotrichum or the broader Aster sensu lato, is not defined by a single color. Instead, it describes a diverse group of plants within the daisy family (Asteraceae) that exhibit a wide and vibrant palette across species and cultivated varieties. This diversity reflects complex biological mechanisms and plays a significant role in their natural ecology. The spectrum of hues displayed is a direct result of specialized chemical compounds produced within the plant cells.
The Full Spectrum of Aster Hues
The most prevalent colors found in wild Aster species are concentrated in the cool end of the visible spectrum, specifically violet, purple, and blue shades. Species like the New England Aster (Symphyotrichum novae-angliae) are famous for their rich, deep purple ray florets. Others, such as the Blue Wood Aster (Symphyotrichum cordifolium), display pale blue to lavender hues, which are often associated with the natural, uncultivated forms of the plant.
Cultivation has expanded this natural range, introducing varieties that showcase shades of pink and pure white. White asters, such as the Calico Aster (Symphyotrichum lateriflorum), offer a clean contrast to the more dominant purples. While ray florets—the petal-like structures—rarely exhibit true yellow, the central disk florets of nearly all Aster species begin as a distinct yellow.
This central yellow color often changes dramatically as the flower matures and is pollinated, frequently darkening to reddish-purple, brown, or deep red. True orange and saturated red in the ray florets are rare or non-existent in the genus, with pinks being the closest shade to red.
The Science Behind Aster Pigmentation
The colors in Aster flowers are primarily due to anthocyanins, a class of water-soluble pigments stored in the plant cell’s vacuoles. These pigments are responsible for the blue, purple, pink, and red coloration seen in the ray florets. The specific hue expressed is highly dependent on the cellular environment, particularly the vacuolar pH level.
Anthocyanins tend to appear redder in more acidic conditions and shift toward blue or purple as the pH becomes more alkaline. The precise type of anthocyanin molecule also influences the resulting color; for instance, cyanidin derivatives produce red-to-purple, while delphinidin derivatives lean toward blue. Environmental factors, such as temperature, can also affect pigment accumulation, with cooler temperatures often promoting richer color development.
The initial bright yellow of the central disk florets is due to a different class of pigments, often carotenoids or flavonoids. The subsequent shift of these disk florets to red or purple is a controlled chemical change, often involving the synthesis of additional red-hued anthocyanins. This localized color change signals the flower’s reproductive status to visiting insects.
Color and Pollinator Attraction
The vivid colors of Aster flowers function as targeted signals to attract insect pollinators. Bees, a primary visitor to Asters, possess vision highly sensitive to the blue and ultraviolet (UV) end of the spectrum. Consequently, the prevalence of blue and purple in Asters is an evolutionary adaptation, drawing in these efficient pollinators.
Beyond the colors visible to the human eye, the flowers display distinct patterns of UV reflection and absorption, which act as “nectar guides.” These UV patterns often appear as a bull’s-eye, directing the pollinator precisely toward the reproductive parts and nectar reward. This visual guidance maximizes the efficiency of pollen transfer.
The color change observed in the central disk florets, from yellow to red-purple, is another precise communication tool. Pollinators consistently favor the yellow disk florets because they contain fresh pollen, while the red-purple florets are older and have already shed their pollen. By retaining these older, spent flowers, the plant maintains a larger, more visible floral display to attract distant insects, while the color change guides the insect to only the viable flowers upon landing.