
Blue: Nature’s Rarest Flower Color
Director Lew Feldman
Fewer than 10% of flowers are blue, or even bluish, making this color the most uncommon shade in flowers. The majority of flowering plants lack blue pigments, so how do blue-colored flowers arise?
The Role of Pigments
Flower color is determined by pigments which are broadly classified as anthocyanins. Blue color can occur in flower petals by mixing red and yellow anthocyanins and sometimes by combining with other compounds called flavins. But having the “right” anthocyanins is not enough; often certain other conditions must be met. For some flowers development of a blue hue depends on the anthocyanins complexing with specific metals, including iron, magnesium, aluminum, copper or tin, thereby forming colored molecules known as metalloanthocyanins.
The pH Factor
Flower color can also be influenced by other environmental factors, the most common being pH – a measure of how acidic or basic a solution is – for example within the cells where pigments are stored. The effect of pH on petal color is seen in morning glory, Ipomoea tricolor. In the early stages of flower bud formation, petals are reddish-purple in color, but as they unfold their color changes to blue. This change in color is due to an increase in pH from 6.6 (slightly acidic) to 7.7 (slightly basic) within the cells of the petals.*
What the Color Change Means
What might be the adaptive value in this change in flower color? The most likely explanation is that the change is a signal to pollinators that a reward, nectar, is available. However, from the flower’s perspective, the development of blue petals likely correlates with the production of pollen and/or the receptivity of the flower to being pollinated. Flower color is often associated with the type of pollinator; blue flowers are primarily visited by insects like bees and butterflies, but may also attract wasps and flies. For some flowers, their blue color may be part of their strategy to attract the most effective pollinators.
Breeding the Elusive Blue
Producing blue flowers has been an often intense (and unsuccessful) pursuit of flower breeders, with much effort devoted to breeding blue roses, carnations and chrysanthemums. Breeders have focused on a naturally occurring blue anthocyanin, delphinidin; however, this pigment must be biochemically altered by plant-produced enzymes in order to express the blue color. Breeders have focused on understanding the genetics of promoting or inhibiting these enzymes in order to mediate the production of blue coloring. The application of new molecular techniques will surely aid the breeders in the development of a wide range of spectacularly blue-hued flowers.
A few examples of blue flowers in the Garden: Blooming in spring in the Tropical House, look for the spectacular blue-colored flowers of Strongylodon macrobotrys, a vine endemic to the Philippines; Anchusa capensis from Southern Africa (pictured above), or the crowd-pleaser Puya alpestris from Chile.

Strongylodon macrobotrys in the Virginia Haldan Tropical House

Puya alpestris in the South American Area